Medium-chain acyl-CoA dehydrogenase deficiency: genotype-biochemical phenotype correlations

Case report: Diagnosis of a patient with Sifrim-Hitz-Weiss syndrome, development and epileptic encephalopathy-14, and medium chain acyl-CoA dehydrogenase deficiency

Background

It is generally recognized that genetic metabolic disorders can result in neurological symptoms such as seizures, developmental delay, and intellectual disability. Heterogeneous clinical presentations make the diagnosis challenging.

Case presentation

In this case report, we present a unique and complex genetic disorder observed in a female patient who exhibited three pathogenic gene variants in the KCNT1, ACADM, and CHD4 genes. The convergence of these variants resulted in a multifaceted clinical presentation characterized by severe seizures of combined focal and generalized onset, metabolic dysfunction, and neurodevelopmental abnormalities. The identification and functional characterization of these gene variants shed light on the intricate interplay between these genes and the patient's phenotype. EEG revealed an epileptiform abnormality which presented in the inter-ictal period from the left frontal-central area and in the ictal period from the left mid-temporal area. The brain MRI revealed volume loss in the posterior periventricular area and parietal parenchyma, myelin destruction with no sign of hypoxic involvement, and left dominant enlargement of the lateral ventricles secondary to loss of central parenchyma. The patient was diagnosed through exome sequencing with Sifrim-Hitz-Weiss syndrome, development and epileptic encephalopathy-14, and medium-chain acyl-CoA dehydrogenase deficiency. An antiseizure medication regimen with valproic acid, levetiracetam, phenobarbital, and clonazepam was initiated. However, this led to only partial control of the seizures.

Conclusion

Clinical follow-up of the patient will further define the clinical spectrum of KCNT1, ACADM, and CHD4 gene variants. It will also determine the long-term efficacy of the treatment of seizures and the development of precision medicine for epilepsy syndromes due to gain-of-function variants. Special emphasis should be put on the role and importance of large-scale genomic testing in understanding and diagnosing complex phenotypes and atypical epileptic syndromes.

Free carnitine concentrations and biochemical parameters in medium-chain acyl-CoA dehydrogenase deficiency: Genotype-phenotype correlation

Biallelic variants in the ACADM gene cause medium-chain acyl-CoA dehydrogenase deficiency (MCADD). This study reports on differences in the occurrence of secondary free carnitine (C0) deficiency and different biochemical phenotypes related to genotype and age in 109 MCADD patients followed-up at a single tertiary care center during 22 years. C0 deficiency occurred earlier and more frequently in c.985A>G homozygotes (genotype A) compared to c.985A>G compound heterozygotes (genotype B) and individuals carrying variants other than c.985A>G and c.199C>T (genotype D) (median age 4.2 vs. 6.6 years; p < 0.001). No patient carrying c.199C>T (genotype C) developed C0 deficiency. A daily dosage of 20-40 mg/kg carnitine was sufficient to maintain normal C0 concentrations. Compared to genotype A as reference group, octanoylcarnitine (C8) was significantly lower in genotypes B and C, whereas C0 was significantly higher by 8.28 μmol/L in genotype C (p < 0.05). In conclusion, C0 deficiency is mainly found in patients with pathogenic genotypes associated with high concentrations of presumably toxic acylcarnitines, while individuals carrying the variant c.199C>T are spared and show consistently mild biochemical phenotypes into adulthood. Low-dose carnitine supplementation maintains normal C0 concentrations. However, future studies need to evaluate clinical benefits on acute and chronic manifestations of MCADD.

Lymphocyte Medium-Chain Acyl-CoA Dehydrogenase Activity and Its Potential as a Diagnostic Confirmation Tool in Newborn Screening Cases

The determination of acylcarnitines (AC) in dried blood spots (DBS) by tandem mass spectrometry in newborn screening (NBS) programs has enabled medium-chain acyl-coA dehydrogenase deficiency (MCADD) to be identified in presymptomatic newborns. Nevertheless, different confirmatory tests must be performed to confirm the diagnosis. In this work, we have collected and analyzed the NBS results and confirmatory test results (plasma AC, molecular findings, and lymphocyte MCAD activity) of forty individuals, correlating them with clinical outcomes and treatment, with the aim of obtaining useful diagnostic information that could be applied in the follow-up of the patients. Our results led us to classify patients into two groups. The first group (14 cases) had high increased octanoylcarnitine (C8) levels, biallelic pathogenic variants, and severe impaired enzyme activity (<10% of the intra-assay control (IAC)); all of these cases received nutritional therapy and required carnitine supplementation during follow-up, representing the most severe form of the disease. The second group (16 patients) was a heterogeneous group presenting moderate increases in C8, biallelic likely pathogenic/pathogenic variants, and intermediate activity (<41% IAC). All of them are currently asymptomatic and could be considered as having a milder form of the disease. Finally, eight cases presented a normal−mild increase in plasma C8, with only one pathogenic variant detected, and high−intermediate residual activity (15−100%). Based on our results, we confirm that combined evaluation of acylcarnitine profiles, genetic findings, and residual enzyme activities proves useful in predicting the risk of future metabolic decompensation, in making decisions regarding future treatment or follow-up, and also in confirming the clinical effects of unknown clinical variants.

Diagnosis of inborn errors of metabolism within the expanded newborn screening in the Madrid region

We present the results of our experience in the diagnosis of inborn errors of metabolism (IEM) since the Expanded Newborn Screening was implemented in our Region. Dried blood samples were collected 48 h after birth. Amino acids and acylcarnitines were quantitated by mass spectrometry (MS)/MS. Newborns with alterations were referred to the clinical centers for follow-up. Biochemical and molecular genetic studies for confirmation of a disease were performed. In the period 2011 to 2019, 592 822 children were screened: 902 of them were referred for abnormal results. An IEM was confirmed in 222 (1/2670): aminoacidopathies: 89 hyperphenylalaninemia (HPA) (51 benign HPA, 32 phenylketonuria, 4 DNAJC12 defect, and 2 primapterinuria), 6 hypermethioninemia, 3 tyrosinemia type 1 (TYR-1), 1 TYR-3, 4 maple syrup urine disease (MSUD), 2 branched-chain amino acid transferase 2 deficiency, 2 homocystinuria, 1 cystinuria, 2 ornithine transcarbamylase (OTC) deficiency, 2 citrullinemia type I (CTLN1); FAO defects: 43 medium-chain acyl-CoA dehydrogenase deficiency (MCADD), 13 very long-chain acyl-CoA dehydrogenase deficiency, 2 long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD), 1 multiple acyl-coA dehydrogenation deficiency, 11 systemic primary carnitine deficiency, 2 carnitine palmitoyltransferase type 2 (CPT-II) deficiency, 1 CPT-I deficiency; organic acidurias: 12 glutaric aciduria type 1 (GA-1), 4 methylmalonic acidemia (MMA), 7 MMA including combined cases with homocystinuria (MMAHC), 6 propionic acidemia (PA), 7 3-methylcrotonyl-CoA carboxylase, 1 3-hydroxy-3-methylglutaryl-CoA lyase deficiency lyase deficiency. Only 19 infants (8.5%) were symptomatic at newborn screening result (1 LCHADD, 5 PA, 1 CPT-II deficiency, 1 MMA, 3 MMAHC, 2 MSUD, 2 OTC deficiency, 1 CTLN1, 1 MCADD, 2 TYR-1). No false negative cases were identified. Genetic diagnosis was conclusive in all biochemically confirmed cases, except for two infants with HPA, identifying pathogenic variants in 32 different genes. The conditions with the highest incidence were HPA (1/6661) and MCAD deficiencies (1/13 787).

Vulnerable exons, like ACADM exon 5, are highly dependent on maintaining a correct balance between splicing enhancers and silencers

It is now widely accepted that aberrant splicing of constitutive exons is often caused by mutations affecting cis-acting splicing regulatory elements, but there is a misconception that all exons have an equal dependency on splicing regulatory elements and thus a similar susceptibility to aberrant splicing. We investigated exonic mutations in ACADM exon 5 to experimentally examine their effect on splicing and found that 7 out of 11 tested mutations affected exon inclusion, demonstrating that this constitutive exon is particularly vulnerable to exonic splicing mutations. Employing ACADM exon 5 and 6 as models, we demonstrate that the balance between splicing enhancers and silencers, flanking intron length, and flanking splice site strength are important factors that determine exon definition and splicing efficiency of the exon in question. Our study shows that two constitutive exons in ACADM have different inherent vulnerabilities to exonic splicing mutations. This suggests that in silico prediction of potential pathogenic effects on splicing from exonic mutations may be improved by also considering the inherent vulnerability of the exon. Moreover, we show that single nucleotide polymorphism that affect either of two different exonic splicing silencers, located far apart in exon 5, all protect against both immediately flanking and more distant exonic splicing mutations.

Biochemical Markers for the Diagnosis of Mitochondrial Fatty Acid Oxidation Diseases

Mitochondrial fatty acid β-oxidation (FAO) contributes a large proportion to the body’s energy needs in fasting and in situations of metabolic stress. Most tissues use energy from fatty acids, particularly the heart, skeletal muscle and the liver. In the brain, ketone bodies formed from FAO in the liver are used as the main source of energy. The mitochondrial fatty acid oxidation disorders (FAODs), which include the carnitine system defects, constitute a group of diseases with several types and subtypes and with variable clinical spectrum and prognosis, from paucisymptomatic cases to more severe affectations, with a 5% rate of sudden death in childhood, and with fasting hypoketotic hypoglycemia frequently occurring. The implementation of newborn screening programs has resulted in new challenges in diagnosis, with the detection of new phenotypes as well as carriers and false positive cases. In this article, a review of the biochemical markers used for the diagnosis of FAODs is presented. The analysis of acylcarnitines by MS/MS contributes to improving the biochemical diagnosis, both in affected patients and in newborn screening, but acylglycines, organic acids, and other metabolites are also reported. Moreover, this review recommends caution, and outlines the differences in the interpretation of the biomarkers depending on age, clinical situation and types of samples or techniques.

Use of Molecular Genetic Analyses in Danish Routine Newborn Screening

Historically, the analyses used for newborn screening (NBS) were biochemical, but increasingly, molecular genetic analyses are being introduced in the workflow. We describe the application of molecular genetic analyses in the Danish NBS programme and show that second-tier molecular genetic testing is useful to reduce the false positive rate while simultaneously providing information about the precise molecular genetic variant and thus informing therapeutic strategy and easing providing information to parents. When molecular genetic analyses are applied as second-tier testing, valuable functional data from biochemical methods are available and in our view, such targeted NGS technology should be implemented when possible in the NBS workflow. First-tier NGS technology may be a promising future possibility for disorders without a reliable biomarker and as a general approach to increase the adaptability of NBS for a broader range of genetic diseases, which is important in the current landscape of quickly evolving new therapeutic possibilities. However, studies on feasibility, sensitivity, and specificity are needed as well as more insight into what views the general population has towards using genetic analyses in NBS. This may be sensitive to some and could have potentially negative consequences for the NBS programme.

Genotype and residual enzyme activity in medium-chain acyl-CoA dehydrogenase (MCAD) deficiency: Are predictions possible?

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is the most common defect of mitochondrial β-oxidation. Confirmation diagnostics after newborn screening (NBS) can be performed either by enzyme testing and/or by sequencing of the ACADM gene. Here, we report the results from enzyme testing in lymphocytes with gene variants from molecular analysis of the ACADM gene and with the initial acylcarnitine concentrations in the NBS sample. From April 2013 to August 2019, in 388 individuals with characteristic acylcarnitine profiles suggestive of MCADD the octanoyl-CoA-oxidation was measured in lymphocytes. In those individuals with residual activities <50%, molecular genetic analysis of the ACADM gene was performed. In 50% of the samples (195/388), MCADD with a residual activity ranging from 0% to 30% was confirmed. Forty-five percent of the samples (172/388) showed a residual activity >35% excluding MCADD. In the remaining 21 individuals, MCAD residual activity ranged from 30% to 35%. The latter group comprised both heterozygous carriers and individuals carrying two gene variants on different alleles. Twenty new variants could be identified and functionally classified based on their effect on enzyme function. C6 and C8 acylcarnitine species in NBS correlated with MCAD activity and disease severity. MCADD was only confirmed in half of the cases referred suggesting a higher false positive rate than expected. Measurement of the enzyme function in lymphocytes allowed fast confirmation diagnostics and clear determination of the pathogenicity of new gene variants. There is a clear correlation between genotype and enzyme function underlining the reproducibility of the functional measurement in vitro.

DeepCLIP: predicting the effect of mutations on protein-RNA binding with deep learning

Nucleotide variants can cause functional changes by altering protein-RNA binding in various ways that are not easy to predict. This can affect processes such as splicing, nuclear shuttling, and stability of the transcript. Therefore, correct modeling of protein-RNA binding is critical when predicting the effects of sequence variations. Many RNA-binding proteins recognize a diverse set of motifs and binding is typically also dependent on the genomic context, making this task particularly challenging. Here, we present DeepCLIP, the first method for context-aware modeling and predicting protein binding to RNA nucleic acids using exclusively sequence data as input. We show that DeepCLIP outperforms existing methods for modeling RNA-protein binding. Importantly, we demonstrate that DeepCLIP predictions correlate with the functional outcomes of nucleotide variants in independent wet lab experiments. Furthermore, we show how DeepCLIP binding profiles can be used in the design of therapeutically relevant antisense oligonucleotides, and to uncover possible position-dependent regulation in a tissue-specific manner. DeepCLIP is freely available as a stand-alone application and as a webtool at http://deepclip.compbio.sdu.dk.

Clinical and genetic characteristics of patients with fatty acid oxidation disorders identified by newborn screening

BACKGROUND

Fatty acid oxidation disorders (FAODs) include more than 15 distinct disorders with variable clinical manifestations. After the introduction of newborn screening using tandem mass spectrometry, early identification of FAODs became feasible. This study describes the clinical, biochemical and molecular characteristics of FAODs patients detected by newborn screening (NBS) compared with those of 9 patients with symptomatic presentations.

METHODS

Clinical and genetic features of FAODs patients diagnosed by NBS and by symptomatic presentations were reviewed.

RESULTS

Fourteen patients were diagnosed with FAODs by NBS at the age of 54.8 ± 4.8 days: 5 with very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency, 5 with medium chain acyl-CoA dehydrogenase (MCAD) deficiency, 1 with primary carnitine deficiency, 1 with carnitine palmitoyltransferase 1A (CPT1A) deficiency, 1 with long-chain 3-hydroxyacyl-CoA dehydrogenase or mitochondrial trifunctional protein (LCAHD/MTP) deficiency, and 1 with short chain acyl-CoA dehydrogenase (SCAD) deficiency. Three patients with VLCAD or LCHAD/MTP deficiency developed recurrent rhabdomyolysis or cardiomyopathy, and one patient died of cardiomyopathy. The other 10 patients remained neurodevelopmentally normal and asymptomatic during the follow-up. In 8 patients with symptomatic presentation, FAODs manifested as LCHAD/MTP deficiencies by recurrent rhabdomyolysis or cadiomyopathy (6 patients), and VLCAD deficiency by cardiomyopathy (1 patient), and CPT1A deficiency by hepatic failure (1 patient). Two patients with LCHAD/MTP deficiencies died due to severe cardiomyopathy in the neonatal period, and developmental disability was noted in CPT1A deficiency (1 patient).

CONCLUSIONS

NBS helped to identify the broad spectrum of FAODs and introduce early intervention to improve the clinical outcome of each patient. However, severe clinical manifestations developed in some patients, indicating that careful, life-long observation is warranted in all FAODs patients.

Unveiling the Pathogenic Molecular Mechanisms of the Most Common Variant (p.K329E) in Medium-Chain Acyl-CoA Dehydrogenase Deficiency by in Vitro and in Silico Approaches

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is the most common genetic disorder affecting the mitochondrial fatty acid β-oxidation pathway. The mature and functional form of human MCAD (hMCAD) is a homotetramer assembled as a dimer of dimers (monomers A/B and C/D). Each monomer binds a FAD cofactor, necessary for the enzyme's activity. The most frequent mutation in MCADD results from the substitution of a lysine with a glutamate in position 304 of mature hMCAD (p.K329E in the precursor protein). Here, we combined in vitro and in silico approaches to assess the impact of the p.K329E mutation on the protein's structure and function. Our in silico results demonstrated for the first time that the p.K329E mutation, despite lying at the dimer-dimer interface and being deeply buried inside the tetrameric core, seems to affect the tetramer surface, especially the β-domain that forms part of the catalytic pocket wall. Additionally, the molecular dynamics data indicate a stronger impact of the mutation on the protein's motions in dimer A/B, while dimer C/D remains similar to the wild type. For dimer A/B, severe disruptions in the architecture of the pockets and in the FAD and octanoyl-CoA binding affinities were also observed. The presence of unaffected pockets (C/D) in the in silico studies may explain the decreased enzymatic activity determined for the variant protein (46% residual activity). Moreover, the in silico structural changes observed for the p.K329E variant protein provide an explanation for the structural instability observed experimentally, namely, the disturbed oligomeric profile, thermal stability, and conformational flexibility, with respect to the wild-type.

Development of a Tandem Mass Spectrometry Method for Rapid Measurement of Medium- and Very-Long-Chain Acyl-CoA Dehydrogenase Activity in Fibroblasts

Mitochondrial fatty acid oxidation is a vital biochemical process for energy metabolism. Among the known fatty-acid metabolism disorders, very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency and medium-chain acyl-CoA dehydrogenase (MCAD) deficiency count among the most frequent. Both are potentially very serious diseases as they carry a risk of severe neurological post-crisis sequelae, and even sudden death. Diagnosis relies on plasma acylcarnitine profile analysis and urine organic acid analysis, followed by genetic testing to confirm diagnosis. However, in some cases, it is crucial to run a specific diagnostic assay for enzyme activity, which is generally performed in leukocytes or fibroblasts. The aim of this study was to address this need, first by developing a MCAD and VLCAD enzyme activity-specific diagnostic assay in fibroblasts (by measuring the reaction products, i.e. enoyl-CoA) via a rapid LC-MS/MS-based technique, and then by testing MCAD-deficient patients (n = 6), VLCAD-deficient patients (n = 10), and control patients (n = 12). MCAD activity was significantly different in the MCAD-deficiency (MCADD) group (mean = 0.07 nmol C8:1 formed/min/mg protein) compared to the control group (mean = 0.36 nmol C8:1 formed/min/mg protein). All MCADD patients showed less than 35% residual MCAD activity. VLCAD activity was significantly decreased in the VLCADD group (mean = 0.06 nmol C16:1 formed/min/mg protein) compared to the control group (mean = 0.86 nmol C16:1 formed/min/mg protein, respectively). All VLCADD patients showed less than 35% residual VLCAD activity. This technique allowed also to confirm that a novel ACADVL gene mutation (c.1400T>C) is responsible for a defective VLCAD activity (residual activity at 10%).

Screening of MCAD deficiency in Japan: 16years' experience of enzymatic and genetic evaluation

BACKGROUND

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is a representative disorder of fatty acid oxidation and is one of the most prevalent inborn errors of metabolism among Caucasian populations. In Japan, however, it was as late as 2000 when the first patient was found, and enzymatic and genetic evaluation of MCAD deficiency began.

METHODS

We measured octanoyl-CoA dehydrogenase activity in lymphocytes of symptomatic children and newborn screening (NBS)-positive subjects who showed elevated levels of C8-acylcarnitine in blood. The results were further confirmed by direct sequencing of the ACADM gene.

RESULTS

The disease was diagnosed in 9 out of 18 symptomatic children. The affected patients showed residual activities from 0% to 3% of the normal average value, except for one patient with 10% activity. Concerning 50 NBS-positive subjects, 18 with enzymatic activities around 10% or lower and 14 with activities ranging from 13% to 30% were judged to be affected patients, and biallelic variants were detected in most of the cases tested. Newborns with higher enzymatic activities were estimated to be heterozygous carriers or healthy subjects, though biallelic variants were detected in 5 of them. Genetic analysis detected 22 kinds of variant alleles. The most prevalent was c.449_452delCTGA (p.T150Rfs), which was followed by c.50G>A (p.R17H), c.1085G>A (p.G362E), c.157C>T (p.R53C), and c.843A>T (p.R281S); these five variants accounted for approximately 60% of all the alleles examined.

CONCLUSION

Our study has revealed the unique genetic backgrounds of MCAD deficiency among Japanese, based on the largest series of non-Caucasian cases. A continuous spectrum of severity was also observed in our series of NBS-positive cases, suggesting that it is essential for every nation and ethnic group to accumulate its own information on gene variants, together with their enzymatic evaluation, in order to establish an efficient NBS system for MCAD deficiency.

221 newborn-screened neonates with medium-chain acyl-coenzyme A dehydrogenase deficiency: Findings from the Inborn Errors of Metabolism Collaborative

INTRODUCTION

There is limited understanding of relationships between genotype, phenotype and other conditions contributing to health in neonates with medium-chain acyl-coenzyme A dehydrogenase deficiency (MCADD) identified through newborn screening.

METHODS

Retrospective analysis of comprehensive data from a cohort of 221 newborn-screened subjects identified as affected with MCADD in the Inborn Errors of Metabolism - Information System (IBEM-IS), a long term follow-up database of the Inborn Errors of Metabolism Collaborative, was performed.

RESULTS

The average age at notification of first newborn screen results to primary care or metabolic providers was 7.45days. The average octanoylcarnitine (C8) value on first newborn screen was 11.2μmol/L (median 8.6, range 0.36-43.91). A higher C8 level correlated with an earlier first subspecialty visit. Subjects with low birth weight had significantly lower C8 values. Significantly higher C8 values were found in symptomatic newborns, in newborns with abnormal lab testing in addition to newborn screening and/or diagnostic tests, and in subjects homozygous for the c.985A>G ACADM gene mutation or compound heterozygous for the c.985A>G mutation and deletions or other known highly deleterious mutations. Subjects with neonatal symptoms, or neonatal abnormal labs, or neonatal triggers were more likely to have at least one copy of the severe c.985A>G ACADM gene mutation. C8 and genotype category were significant predictors of the likelihood of having neonatal symptoms. Neonates with select triggers were more likely to have symptoms and laboratory abnormalities.

CONCLUSIONS

This collaborative study is the first in the United States to describe health associations of a large cohort of newborn-screened neonates identified as affected with MCADD. The IBEM-IS has utility as a platform to better understand the characteristics of individuals with newborn-screened conditions and their follow-up interactions with the health system.

Medium-chain acyl-CoA dehydrogenase deficiency associated with a novel splice mutation in the ACADM gene missed by newborn screening

Background

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common disorder of mitochondrial fatty acid β-oxidation and a target disease of newborn screening in many countries.

Case presentation

We report on two siblings with mild MCAD deficiency associated with a novel splice site mutation in the ACADM gene. The younger sibling was detected by newborn screening, while the older sister was missed, but diagnosed later on by genetic family testing. Both children were found to be compound heterozygous for the common c.985A > G (p.K329E) mutation and a novel splice site mutation, c.600-18G > A, in the ACADM gene. To determine the biological consequence of the c.600-18G > A mutation putative missplicing was investigated at RNA level in granulocytes and monocytes of one of the patients. The splice site mutation was shown to lead to partial missplicing of the ACADM pre-mRNA. Of three detected transcripts two result in truncated, non-functional MCAD proteins as reflected by the reduced octanoyl-CoA oxidation rate in both patients. In one patient a decrease of the octanoyl-CoA oxidation rate was found during a febrile infection indicating that missplicing may be temperature-sensitive.

Conclusions

Our data indicate that the c.600-18G > A variant activates a cryptic splice site, which competes with the natural splice site. Due to only partial missplicing sufficient functional MCAD protein remains to result in mild MCADD that may be missed by newborn screening.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-015-0199-5) contains supplementary material, which is available to authorized users.

Medium-Chain Acyl-CoA Dehydrogenase Deficiency: Evaluation of Genotype-Phenotype Correlation in Patients Detected by Newborn Screening

BACKGROUND

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is included in many newborn screening programmes worldwide. In addition to the prevalent mutation c.985A>G in the ACADM gene, potentially mild mutations like c.199T>C are frequently found in screening cohorts. There is ongoing discussion whether this mutation is associated with a clinical phenotype.

METHODS

In 37 MCADD patients detected by newborn screening, biochemical phenotype (octanoylcarnitine (C8), ratios of C8 to acetylcarnitine (C2), decanoylcarnitine (C10) and dodecanoylcarnitine (C12) at screening and confirmation) and clinical phenotype (inpatient emergency treatment, metabolic decompensations, clinical assessments, psychometric tests) were assessed in relation to genotype.

RESULTS

16 patients were homozygous for c.985A>G (group 1), 11 compound heterozygous for c.199T>C and c.985A>G/another mutation (group 2) and 7 compound heterozygous for c.985A>G and mutations other than c.199T>C (group 3) and 3 carried neither c.985A>G nor c.199T>C but other known homozygous mutations (group 4). At screening C8/C2 and C8/C10, at confirmation C8/C2, C8/C10 and C8/C12 differed significantly between patients compound heterozygous for c.199T>C (group 2) and other genotypes. C8, C10 and C8/C2 at screening were strongly associated with time of sampling in groups 1 + 3 + 4, but not in group 2. Clinical phenotype did not differ between genotypes. Two patients compound heterozygous for c.199T>C and a severe mutation showed neonatal decompensation with hypoglycaemia.

CONCLUSION

Biochemical phenotype differs between MCADD patients compound heterozygous for c.199T>C with a severe mutation and other genotypes. In patients detected by newborn screening, clinical phenotype does not differ between genotypes following uniform treatment recommendations. Neonatal decompensation can also occur in patients with the presumably mild mutation c.199T>C prior to diagnosis.

Targeted metabolomics in the expanded newborn screening for inborn errors of metabolism

This paper highlights the importance of metabolic profiling by LC-MS/MS and GC-MS of biological fluids for diagnosis of inborn errors of metabolism and confirms a high incidence of these disorders.

The domain-specific and temperature-dependent protein misfolding phenotype of variant medium-chain acyl-CoA dehydrogenase

The implementation of expanded newborn screening programs reduced mortality and morbidity in medium-chain acyl-CoA dehydrogenase deficiency (MCADD) caused by mutations in the ACADM gene. However, the disease is still potentially fatal. Missense induced MCADD is a protein misfolding disease with a molecular loss-of-function phenotype. Here we established a comprehensive experimental setup to analyze the structural consequences of eight ACADM missense mutations (p.Ala52Val, p.Tyr67His, p.Tyr158His, p.Arg206Cys, p.Asp266Gly, p.Lys329Glu, p.Arg334Lys, p.Arg413Ser) identified after newborn screening and linked the corresponding protein misfolding phenotype to the site of side-chain replacement with respect to the domain. With fever being the crucial risk factor for metabolic decompensation of patients with MCADD, special emphasis was put on the analysis of structural and functional derangements related to thermal stress. Based on protein conformation, thermal stability and kinetic stability, the molecular phenotype in MCADD depends on the structural region that is affected by missense-induced conformational changes with the central β-domain being particularly prone to structural derangement and destabilization. Since systematic classification of conformational derangements induced by ACADM mutations may be a helpful tool in assessing the clinical risk of patients, we scored the misfolding phenotype of the variants in comparison to p.Lys329Glu (K304E), the classical severe mutation, and p.Tyr67His (Y42H), discussed to be mild. Experiments assessing the impact of thermal stress revealed that mutations in the ACADM gene lower the temperature threshold at which MCAD loss-of-function occurs. Consequently, increased temperature as it occurs during intercurrent infections, significantly increases the risk of further conformational derangement and loss of function of the MCAD enzyme explaining the life-threatening clinical courses observed during fever episodes. Early and aggressive antipyretic treatment thus may be life-saving in patients suffering from MCADD.

Medium-chain acyl-CoA deficiency: outlines from newborn screening, in silico predictions, and molecular studies

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is a disorder of fatty acid oxidation characterized by hypoglycemic crisis under fasting or during stress conditions, leading to lethargy, seizures, brain damage, or even death. Biochemical acylcarnitines data obtained through newborn screening by liquid chromatography-tandem mass spectrometry (LC-MS/MS) were confirmed by molecular analysis of the medium-chain acyl-CoA dehydrogenase (ACADM) gene. Out of 324.000 newborns screened, we identified 14 MCADD patients, in whom, by molecular analysis, we found a new nonsense c.823G>T (p.Gly275∗) and two new missense mutations: c.253G>C (p.Gly85Arg) and c.356T>A (p.Val119Asp). Bioinformatics predictions based on both phylogenetic conservation and functional/structural software were used to characterize the new identified variants. Our findings confirm the rising incidence of MCADD whose existence is increasingly recognized due to the efficacy of an expanded newborn screening panel by LC-MS/MS making possible early specific therapies that can prevent possible crises in at-risk infants. We noticed that the “common” p.Lys329Glu mutation only accounted for 32% of the defective alleles, while, in clinically diagnosed patients, this mutation accounted for 90% of defective alleles. Unclassified variants (UVs or VUSs) are especially critical when considering screening programs. The functional and pathogenic characterization of genetic variants presented here is required to predict their medical consequences in newborns.

A synonymous polymorphic variation in ACADM exon 11 affects splicing efficiency and may affect fatty acid oxidation

In recent studies combining genome-wide association and tandem-MS based metabolic profiling, a single-nucleotide polymorphism (SNP), rs211718C>T, located far upstream of the MCAD gene (ACADM) was found to be associated with serum concentrations of medium-chain acylcarnitines indicating improved beta-oxidation of medium-chain fatty acids. We examined the functional basis for this association and identified linkage between rs211718 and the intragenic synonymous polymorphic variant c.1161A>G in ACADM exon 11 (rs1061337). Employing minigene studies we show that the c.1161A allele is associated with exon 11 missplicing, and that the c.1161G allele corrects this missplicing. This may result in production of more full length MCAD protein from the c.1161G allele. Our analysis suggests that the improved splicing of the c.1161G allele is due to changes in the relative binding of splicing regulatory proteins SRSF1 and hnRNP A1. Using publicly available pre-aligned RNA-seq data, we find that the ACADM c.1161G allele is expressed at significantly higher levels than the c.1161A allele across different tissues. This supports that c.1161A>G is a functional SNP, which leads to higher MCAD expression, perhaps due to improved splicing. This study is a proof of principle that synonymous SNPs are not neutral. By changing the binding sites for splicing regulatory proteins they can have significant effects on pre-mRNA splicing and thus protein function. In addition, this study shows that for a sequence variation to have an effect, it might need to change the balance in the relative binding of positive and negative splicing factors.

Newborn screening for medium-chain acyl-CoA dehydrogenase deficiency: regional experience and high incidence of carnitine deficiency

Background

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is the most common inherited defect in the mitochondrial fatty acid oxidation pathway, resulting in significant morbidity and mortality in undiagnosed patients.

Newborn screening (NBS) has considerably improved MCADD outcome, but the risk of complication remains in some patients. The aim of this study was to evaluate the relationship between genotype, biochemical parameters and clinical data at diagnosis and during follow-up, in order to optimize monitoring of these patients.

Methods

We carried out a multicenter study in southwest Europe, of MCADD patients detected by NBS. Evaluated NBS data included free carnitine (C0) and the acylcarnitines C8, C10, C10:1 together with C8/C2 and C8/C10 ratios, clinical presentation parameters and genotype, in 45 patients. Follow-up data included C0 levels, duration of carnitine supplementation and occurrence of metabolic crises.

Results

C8/C2 ratio and C8 were the most accurate biomarkers of MCADD in NBS. We found a high number of patients homozygous for the prevalent c.985A > G mutation (75%). Moreover, in these patients C8, C8/C10 and C8/C2 were higher than in patients with other genotypes, while median value of C0 was significantly lower (23 μmol/L vs 36 μmol/L).

The average follow-up period was 43 months. To keep carnitine levels within the normal range, carnitine supplementation was required in 82% of patients, and for a longer period in patients homozygotes for the c.985A>G mutation than in patients with other genotypes (average 31 vs 18 months). Even with treatment, median C0 levels remained lower in homozygous patients than in those with other genotypes (14 μmol/L vs 22 μmol/L).

Two patients died and another three suffered a metabolic crisis, all of whom were homozygous for the c.985 A>G mutation.

Conclusions

Our data show a direct association between homozygosity for c.985A>G and lower carnitine values at diagnosis, and a higher dose of carnitine supplementation for maintenance within the normal range. This study contributes to a better understanding of the relationship between genotype and phenotype in newborn patients with MCADD detected through screening which could be useful in improving follow-up strategies and clinical outcome.

Regulation of mitochondrial fatty acid β-oxidation in human: what can we learn from inborn fatty acid β-oxidation deficiencies?

The mitochondrial fatty acid β-oxidation (FAO) pathway plays a crucial role in ATP production in many tissues with high-energy demand. This is highlighted by the diverse and possibly severe clinical manifestations of inborn fatty acid β-oxidation deficiencies. More than fifteen genetic FAO enzyme defects have been described to date, forming a large group of rare diseases. Inborn FAO disorders are characterized by a high genetic heterogeneity, with a variety of gene mutations resulting in complete or partial loss-of-function of the corresponding enzyme. The panel of observed phenotypes varies from multi-organ failure in the neonate with fatal outcome, up to milder late onset manifestations associated with significant disabilities. Diagnosis of FAO disorders has markedly improved over the last decades, but few treatments are available. The clinical, biochemical, and molecular analysis of these disorders provided new, and sometimes unexpected, data on the organization and regulation of mitochondrial FAO in humans, in various tissues, and at various stages of development. This will be illustrated by examples of FAO defects affecting enzymes of long-chain fatty acid import into the mitochondria, or Lynen helix enzymes. The involvement of the transcriptional network regulating FAO gene expression, in particular the PGC-1α/PPAR axis, as a target for pharmacological therapy of these genetic disorders, will also be discussed.

Ammonium metabolism in humans

Free ammonium ions are produced and consumed during cell metabolism. Glutamine synthetase utilizes free ammonium ions to produce glutamine in the cytosol whereas glutaminase and glutamate dehydrogenase generate free ammonium ions in the mitochondria from glutamine and glutamate, respectively. Ammonia and bicarbonate are condensed in the liver mitochondria to yield carbamoylphosphate initiating the urea cycle, the major mechanism of ammonium removal in humans. Healthy kidney produces ammonium which may be released into the systemic circulation or excreted into the urine depending predominantly on acid-base status, so that metabolic acidosis increases urinary ammonium excretion while metabolic alkalosis induces the opposite effect. Brain and skeletal muscle neither remove nor produce ammonium in normal conditions, but they are able to seize ammonium during hyperammonemia, releasing glutamine. Ammonia in gas phase has been detected in exhaled breath and skin, denoting that these organs may participate in nitrogen elimination. Ammonium homeostasis is profoundly altered in liver failure resulting in hyperammonemia due to the deficient ammonium clearance by the diseased liver and to the development of portal collateral circulation that diverts portal blood with high ammonium content to the systemic blood stream. Although blood ammonium concentration is usually elevated in liver disease, a substantial role of ammonium causing hepatic encephalopathy has not been demonstrated in human clinical studies. Hyperammonemia is also produced in urea cycle disorders and other situations leading to either defective ammonium removal or overproduction of ammonium that overcomes liver clearance capacity. Most diseases resulting in hyperammonemia and cerebral edema are preceded by hyperventilation and respiratory alkalosis of unclear origin that may be caused by the intracellular acidosis occurring in these conditions.

Sudden unexpected infant death (SUDI) in a newborn due to medium chain acyl CoA dehydrogenase (MCAD) deficiency with an unusual severe genotype

Medium chain acyl CoA dehydrogenase deficiency (MCAD) is the most common inborn error of fatty acid oxidation. This condition may lead to cellular energy shortage and cause severe clinical events such as hypoketotic hypoglycemia, Reye syndrome and sudden death. MCAD deficiency usually presents around three to six months of life, following catabolic stress as intercurrent infections or prolonged fasting, whilst neonatal-onset of the disease is quite rare. We report the case of an apparently healthy newborn who suddenly died at the third day of life, in which the diagnosis of MCAD deficiency was possible through peri-mortem blood-spot acylcarnitine analysis that showed very high concentrations of octanoylcarnitine. Genetic analysis at the ACADM locus confirmed the biochemical findings by demonstrating the presence in homozygosity of the frame-shift c.244dup1 (p.Trp82LeufsX23) mutation, a severe genotype that may explain the unusual and very early fatal outcome in this newborn. This report confirms that inborn errors of fatty acid oxidation represent one of the genetic causes of sudden unexpected deaths in infancy (SUDI) and underlines the importance to include systematically specific metabolic screening in any neonatal unexpected death.

Clinical and molecular aspects of Japanese children with medium chain acyl-CoA dehydrogenase deficiency

We report the outcome of 16 Japanese patients with medium chain acyl-CoA dehydrogenase deficiency. Of them, 7 patients were diagnosed after metabolic crisis, while 9 were detected in the asymptomatic condition. Of the 7 symptomatic cases, 1 died suddenly, and 4 cases had delayed development. All 9 patients identified by neonatal or sibling screening remained healthy. Of 14 mutations identified, 10 were unique for Japanese, and 4 were previously reported in other nationalities. Presymptomatic detection including neonatal screening obviously improves quality of life of Japanese patients, probably regardless of the genotypes.

MCAD deficiency in Denmark

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is the most common defect of fatty acid oxidation. Many countries have introduced newborn screening for MCADD, because characteristic acylcarnitines can easily be identified in filter paper blood spot samples by tandem mass spectrometry (MS/MS), because MCADD is a frequent disease, and because of the success of early treatment initiated before clinical symptoms have emerged. In Denmark we have screened 519,350 newborns for MCADD by MS/MS and identified 58 affected babies. The diagnosis of MCADD was confirmed in all 58 newborns by mutation analysis. This gives an incidence of MCADD detected by newborn screening in Denmark of 1/8954. In sharp contrast to this we found that the incidence of clinically presenting MCADD in Denmark in the 10 year period preceding introduction of MS/MS-based screening was only 1 in 39,691. This means that four times more newborns with MCADD are detected by screening than what is expected based on the number of children presenting clinically in an unscreened population. The mutation spectrum in the newborns detected by screening is different from that observed in clinically presenting patients with a much lower proportion of newborns being homozygous for the prevalent disease-causing c.985A>G mutation. A significant number of the newborns have genotypes with mutations that have not been observed in patients detected clinically. Some of these mutations, like c.199T>C and c.127G>A, are always associated with a milder biochemical phenotype and may cause a milder form of MCADD with a relatively low risk of disease manifestation, thereby explaining part of the discrepancy between the frequency of clinically manifested MCADD and the frequency of MCADD determined by screening. In addition, our data suggest that some of this discrepancy can be explained by a reduced penetrance of the c.985A>G mutation, with perhaps only 50% of c.985A>G homozygotes presenting with disease manifestations. Interestingly, we also report that the observed number of newborns identified by screening who are homozygous for the c.985A>G mutation is twice that predicted from the estimated carrier frequency. We therefore redetermined the carrier frequency in a new sample of 1946 blood spots using a new assay, but this only confirmed that the c.985A>G carrier frequency in Denmark is approximately 1/105. We conclude that MCADD is much more frequent than expected, has a reduced penetrance and that rapid genotyping using the initial blood spot sample is important for correct diagnosis and counseling.

Risk stratification by residual enzyme activity after newborn screening for medium-chain acyl-CoA dehyrogenase deficiency: data from a cohort study

Background

Since the introduction of medium-chain acyl coenzyme A dehydrogenase (MCAD) deficiency in population newborn bloodspot screening (NBS) programs, subjects have been identified with variant ACADM (gene encoding MCAD enzyme) genotypes that have never been identified in clinically ascertained patients. It could be hypothesised that residual MCAD enzyme activity can contribute in risk stratification of subjects with variant ACADM genotypes.

Methods

We performed a retrospective cohort study of all patients identified upon population NBS for MCAD deficiency in the Netherlands between 2007–2010. Clinical, molecular, and enzymatic data were integrated.

Results

Eighty-four patients from 76 families were identified. Twenty-two percent of the subjects had a variant ACADM genotype. In patients with classical ACADM genotypes, residual MCAD enzyme activity was significantly lower (median 0%, range 0-8%) when compared to subjects with variant ACADM genotypes (range 0-63%; 4 cases with 0%, remainder 20-63%). Patients with (fatal) neonatal presentations before diagnosis displayed residual MCAD enzyme activities <1%. After diagnosis and initiation of treatment, residual MCAD enzyme activities <10% were associated with an increased risk of hypoglycaemia and carnitine supplementation. The prevalence of MCAD deficiency upon screening was 1/8,750 (95% CI 1/7,210–1/11,130).

Conclusions

Determination of residual MCAD enzyme activity improves our understanding of variant ACADM genotypes and may contribute to risk stratification. Subjects with variant ACADM genotypes and residual MCAD enzyme activities <10% should be considered to have the same risks as patients with classical ACADM genotypes. Parental instructions and an emergency regimen will remain principles of the treatment in any type of MCAD deficiency, as the effect of intercurrent illness on residual MCAD enzyme activity remains uncertain. There are, however, arguments in favour of abandoning the general advice to avoid prolonged fasting in subjects with variant ACADM genotypes and >10% residual MCAD enzyme activity.

Expanded newborn screening: articulating the ontology of diseases with bridging work in the clinic

Population screening follows the logic of secondary prevention: a population is screened to detect disease early and to initiate treatment before symptoms emerge. However, not all population screening is justifiable under all circumstances. In this article, we unpack Wilson and Jungner's requirement that knowledge about the natural history of a disease must be 'adequate' for screening to proceed. We argue that any prior understanding of disease is inevitably found to be insufficient once population screening is instituted. Drawing upon ethnographic observations of clinical consultations and staff meetings conducted in a California regional clinical centre for metabolic-genetic disorders, we introduce the notion of bridging work to draw attention to the collective activities of the genetics team to revise the ontological nature of conditions unsettled by population-based newborn screening. Bridging work refers to the many activities required to reconcile the promise of technologies with the realities of their implementation. We illustrate how clinicians bridge the gap between what was known about a disease prior to screening and anomalous screening results, leading to an ontological transformation of disease categories.

Newborn screening for medium chain acyl-CoA dehydrogenase deficiency in England: prevalence, predictive value and test validity based on 1.5 million screened babies

BACKGROUND

Medium chain acyl-CoA dehydrogenase deficiency (MCADD) is a rare, life-threatening condition. Early diagnosis by screening asymptomatic newborns may improve outcome, but the benefit to newborns identified with variants not encountered clinically is uncertain.

OBJECTIVE

To estimate, overall and by ethnic group: screen-positive prevalence and predictive value (PPV); MCADD prevalence; proportion MCADD variants detected of predicted definite or uncertain clinical importance.

SETTING

All births in areas of high ethnic minority prevalence in England.

METHODS

Prospective multicentre pilot screening service; testing at age five to eight days; standardized screening, diagnostic and management protocols; independent expert review of screen-positive cases to assign MCADD diagnosis and predicted clinical importance (definite or uncertain).

RESULTS

Approximately 1.5 million babies (79% white; 10% Asian) were screened. MCADD was confirmed in 147 of 190 babies with a positive screening result (screen-positive prevalence: 1.20 per 10,000; MCADD prevalence: 0.94 per 10,000; PPV 77% [95% CI 71-83]), comprising 103 (70%) with MCADD variants of definite clinical importance (95 white [95%]; 2 Asian [2%]) and 44 (30%) with variants of uncertain clinical importance (29 white [67%]; 12 Asian [28%]).

CONCLUSION

One baby in every 10,000 born in England is diagnosed with MCADD by newborn screening; around 60 babies each year. While the majority of MCADD variants detected are predicted to be of definite clinical importance, this varies according to ethnic group, with variants of uncertain importance most commonly found in Asian babies. These findings provide support for MCADD screening but highlight the need to take account of the ethnic diversity of the population tested at implementation.

Clinical, biochemical and genetic analyses in two Korean patients with medium-chain acyl-CoA dehydrogenase deficiency

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is an autosomal recessive hereditary metabolic disorder of mitochondrial fatty acid β-oxidation. It is characterized by hypoketotic hypoglycemia, hyperammonemia, seizure, coma, and sudden infant death syndrome-like illness. The most frequently isolated mutation in the acyl-CoA dehydrogenase, medium-chain (ACADM) gene of Caucasian patients with MCADD is c.985A>G, but ethnic variations exist in the frequency of this mutation. Here, we describe 2 Korean pediatric cases of MCADD, which was detected during newborn screening by tandem mass spectrometry and confirmed by molecular analysis. The levels of medium-chain acylcarnitines, including octanoylcarnitine (C8), hexanoylcarnitine (C6), and decanoylcarnitine (C10), were typically elevated. Molecular studies revealed that Patient 1 was a compound heterozygote for c.449_452delCTGA (p.Thr150ArgfsX4) and c.461T>G (p.L154W) mutations, and Patient 2 was a compound heterozygote for c.449_452delCTGA (p.Thr150ArgfsX4) and c.1189T>A (p.Y397N) mutations. We detected asymptomatic patients with MCADD by using a newborn screening test and confirmed it by ACADM mutation analysis. This report presents evidence of the biochemical and molecular features of MCADD in Korean patients and, to the best of our knowledge, this is the first report of the c.461T>G mutation in the ACADM gene.

Relevance of expanded neonatal screening of medium-chain acyl co-a dehydrogenase deficiency: outcome of a decade in galicia (Spain)

Neonatal screening of medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is of major importance due to the significant morbidity and mortality in undiagnosed patients. MCADD screening has been performed routinely in Galicia since July 2000, and until now 199,943 newborns have been screened. We identified 11 cases of MCADD, which gives an incidence of 1/18,134. During this period, no false negative screens have been detected. At diagnosis, all identified newborns were asymptomatic. Our data showed that octanoylcarnitine (C8) and C8/C10 ratio are the best markers for screening of MCADD. C8 was increased in all patients and C8/C10 was increased in all but one patient.The common mutation, c.985A > G, was found in homozygosity in seven newborns and in compound heterozygosity in three, while one patient did not carry the common mutation at all. In addition, two novel mutations c.245G > C (p.W82S) and c.542A > G (p.D181G) were identified. Ten of the 11 identified newborns did not experience any episodes of decompensation. The patient with the highest level of medium chain acylcarnitines at diagnosis, who was homozygous for the c.985A > G mutation, died at the age of 2 years due to a severe infection.This is the first report of the results from neonatal screening for MCADD in Spain. Our data provide further evidence of the benefits of MCADD screening and contribute to better understanding of this disease.

Medium-chain acyl-CoA dehydrogenase deficiency in Saudi Arabia: incidence, genotype, and preventive implications

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD), caused by mutated ACADM gene, is a potentially fatal fatty acid oxidation defect. Detection of MCADD is now part of tandem mass spectrometry (MS-MS)-based newborn screening programs worldwide. To date, more than 67 mutations have been reported to cause MCADD with a single allele, c.985A>G, being the most common in patients of northwestern European descent. In Saudi Arabia, the Newborn Screening Program, officially launched in 2005, screens for 16 disorders including MCADD. Over a period of 3 years, 237,812 newborns were screened; 13 were identified to have MCADD giving an incidence of 1:18,293. Since the introduction of MS-MS to our institution, however, a total of 30 patients were detected to have MCADD. These cases were either newborns, at high-risk family members, or clinically suspected. The C8-carnitine levels (median 3.31, range 0.81-16.33 µM) were clearly diagnostic in all analyzed samples. Sequencing ACADM in 20 DBS revealed two novel mutations: c.362C>T (p.T121I) and c.347G>A (p.C116Y) substitutions, neither of which were detected in 300 chromosomes from controls. Eighteen (90%) patients were homozygous for the T121I mutation and two (10%) were compound heterozygous (T121I/C116Y). Our molecular data lend further support to MS-MS biochemical screening for MCADD and provide evidence for the relatively high incidence of MCADD in the Arab population. The identification of a founder mutation for MCADD has important implications for the preventive screening programs not only in Saudi Arabia but potentially also in other countries in the region.

The first three years of screening for medium chain acyl-CoA dehydrogenase deficiency (MCADD) by newborn screening ontario

Background

Medium chain acyl-CoA dehydrogenase deficiency (MCADD) is a disorder of mitochondrial fatty acid oxidation and is one of the most common inborn errors of metabolism. Identification of MCADD via newborn screening permits the introduction of interventions that can significantly reduce associated morbidity and mortality. This study reports on the first three years of newborn screening for MCADD in Ontario, Canada.

Methods

Newborn Screening Ontario began screening for MCADD in April 2006, by quantification of acylcarnitines (primarily octanoylcarnitine, C8) in dried blood spots using tandem mass spectrometry. Babies with positive screening results were referred to physicians at one of five regional Newborn Screening Treatment Centres, who were responsible for diagnostic evaluation and follow-up care.

Results

From April 2006 through March 2009, approximately 439 000 infants were screened for MCADD in Ontario. Seventy-four infants screened positive, with a median C8 level of 0.68 uM (range 0.33-30.41 uM). Thirty-one of the screen positive infants have been confirmed to have MCADD, while 36 have been confirmed to be unaffected. Screening C8 levels were higher among infants with MCADD (median 8.93 uM) compared to those with false positive results (median 0.47 uM). Molecular testing was available for 29 confirmed cases of MCADD, 15 of whom were homozygous for the common c.985A > G mutation. Infants homozygous for the common mutation tended to have higher C8 levels (median 12.13 uM) relative to compound heterozygotes for c.985A > G and a second detectable mutation (median 2.01 uM). Eight confirmed mutation carriers were identified among infants in the false positive group. The positive predictive value of a screen positive for MCADD was 46%. The estimated birth prevalence of MCADD in Ontario is approximately 1 in 14 000.

Conclusions

The birth prevalence of MCADD and positive predictive value of the screening test were similar to those identified by other newborn screening programs internationally. We observed some evidence of correlation between genotype and biochemical phenotype (C8 levels), and between C8 screening levels and eventual diagnosis. Current research priorities include further examining the relationships among genotype, biochemical phenotype, and clinical phenotype, with the ultimate goal of improving clinical risk prediction in order to provide tailored disease management advice and genetic counselling to families.

[Infant coma in the emergency department: 2 cases of MCAD deficiency]

Medium-chain Acyl-CoA dehydrogenase deficiency (MCAD) is one of the most common fatty acid oxidation disorders. Clinical manifestations can be serious and lead to death if unrecognized. They are not specific and can mimic meningitis or an acute intestinal intussusception in its neurological form. Early recognition of MCAD and presymptomatic treatment of intercurrent illness improve the prognosis over the short- and long-term. MCAD deficiency satisfies the major criteria for newborn screening. We report the cases of 2 patients whose presentation was typical and severe. Early diagnosis of MCAD deficiency helped to start a simple treatment in both patients aimed at preventing further decompensation.

A novel mutation of the ACADM gene (c.145C>G) associated with the common c.985A>G mutation on the other ACADM allele causes mild MCAD deficiency: a case report

A female patient, with normal familial history, developed at the age of 30 months an episode of diarrhoea, vomiting and lethargy which resolved spontaneously. At the age of 3 years, the patient re-iterated vomiting, was sub-febrile and hypoglycemic, fell into coma, developed seizures and sequels involving right hemi-body. Urinary excretion of hexanoylglycine and suberylglycine was low during this metabolic decompensation. A study of pre- and post-prandial blood glucose and ketones over a period of 24 hours showed a normal glycaemic cycle but a failure to form ketones after 12 hours fasting, suggesting a mitochondrial β-oxidation defect. Total blood carnitine was lowered with unesterified carnitine being half of the lowest control value. A diagnosis of mild MCAD deficiency (MCADD) was based on rates of 1-¹⁴C-octanoate and 9, 10-³H-myristate oxidation and of octanoyl-CoA dehydrogenase being reduced to 25% of control values. Other mitochondrial fatty acid oxidation proteins were functionally normal. De novo acylcarnitine synthesis in whole blood samples incubated with deuterated palmitate was also typical of MCADD. Genetic studies showed that the patient was compound heterozygous with a sequence variation in both of the two ACADM alleles; one had the common c.985A>G mutation and the other had a novel c.145C>G mutation. This is the first report for the ACADM gene c.145C>G mutation: it is located in exon 3 and causes a replacement of glutamine to glutamate at position 24 of the mature protein (Q24E). Associated with heterozygosity for c.985A>G mutation, this mutation is responsible for a mild MCADD phenotype along with a clinical story corroborating the emerging literature view that patients with genotypes representing mild MCADD (high residual enzyme activity and low urinary levels of glycine conjugates), similar to some of the mild MCADDs detected by MS/MS newborn screening, may be at risk for disease presentation.

Clinical and biochemical monitoring of patients with fatty acid oxidation disorders

Evidence-based guidelines for monitoring patients with disorders in fatty acid oxidation (FAO) are lacking, and most protocols are based on expert statements. Here, we describe our protocol for Danish patients. Clinical monitoring is the most important measure and has the main aims of checking growth, development and diet and of bringing families to the clinic regularly to remind them of their child's risk and review how they cope and adjust, e.g. to an acute intercurrent illness. Most of these measures are simple and can be carried out during a routine out-patient visit; we seldom do more complicated assessments by a neuropsychologist, speech therapist, or physical and occupational therapists. Paraclinical measurements are not used for short-chain and medium-chain disorders; electrocardiography (including 24 h monitoring) and echocardiography are done for most patients with long-chain and carnitine transporter deficiencies. Eye examination is done in all, and liver ultrasonography in some patients with long-chain 3-hydroxyacyl-coenzyme A dehydrogenase/tri-functional protein (LCHAD/TFP) deficiencies. Biochemical follow-up includes determination of free carnitine and acylcarnitines. Free carnitine is measured to monitor carnitine supplementation in patients with multiple acyl-coenzyme A dehydrogenase deficiency (MADD) and carnitine transporter deficiency (CTD) and to follow metabolic control and disclose deficiency states in other FAO disorders. We are evaluating long-chain acylcarnitines in patients with long-chain disorders; so far there does not seem to be any clear-cut benefit in following these levels. An erythrocyte fatty acid profile is done in patients with long-chain disorders to test for essential fatty acid and docosahexanoic acid (DHA) deficiencies. The measurement of creatine kinase is helpful in long-chain disorders. Ongoing follow-up and education of the patient is important throughout life to prevent disease morbidity or death from metabolic crises.

The clinical manifestation of MCAD deficiency: challenges towards adulthood in the screened population

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is the most common fatty acid oxidation disorder. Typically, undiagnosed individuals are asymptomatic until an episode of increased energy demand and fasting occurs, resulting in metabolic derangement. Phenotypic heterogeneity has been increasingly realized, with reports of both neonates and adults manifesting with life-threatening symptoms including encephalopathy, rhabdomyolysis, and cardiac failure. If diagnosed presymptomatically, outcome is favorable basically by avoidance of fasting. Early detection by newborn screening (NBS) has significantly reduced the incidence of severe adverse events including deaths. In this manuscript we focus on the natural course of the disease in both children and adults. Although NBS for MCADD has been successfully established, continuing efforts need to be made to avoid acute crises and deterioration of outcome in screened patients entering adolescence and adulthood.

Sudden death in medium chain acyl-coenzyme a dehydrogenase deficiency (MCADD) despite newborn screening

INTRODUCTION

Medium chain acyl-CoA dehydrogenase deficiency (MCADD) is the most frequent of the fatty acid oxidation disorders (FAOD), a group caused by defects in the mitochondrial B-oxidation of fatty acids. Fatty acid oxidation is critical in supplying energy during periods when glucose is limited or when energy needs are increased beyond the availability of glucose. In MCADD, this energy shortage can result in acute metabolic episodes or sudden death. The prevention of sudden death from MCADD served as the primary impetus to expand newborn screening. However, we have experienced sudden death in four children with MCADD despite their detection by newborn screening. The purpose of this report is to alert others to the danger of sudden death in MCADD even when it is detected by newborn screening, to identify the clinical symptoms that precede sudden death, and to examine the relationship between the newborn screening result and the risk for sudden death.

METHODS

We describe these children and their metabolic findings with emphasis on their newborn screening octanoylcarnitine (C8) level, the primary marker for newborn detection of MCADD. We also performed a literature search of cases of sudden death in MCADD in which the clinical status preceding death is described.

RESULTS

The newborn screening C8 levels in our four cases were markedly elevated, ranging from 8.4 to 24.8micromol/L (cut off<0.8micromol/L). Only two of the children were homozygous for the common c.985A>G MCAD mutation; the other two were heterozygous for this mutation. Similarly, among the eight reported cases which included MCAD genotypes, five were homozygous for the c.985A>G mutation, while two were heterozygous and one was homozygous for a splice site mutation. Vomiting 12-24h before sudden death was present in all four of our cases, and the review of reported cases of sudden death in MCADD disclosed vomiting as a frequent symptom.

CONCLUSION

We suggest that in MCADD (1) a newborn screening C8 level of 6micromol/L or greater represents particular risk of sudden death; (2) that MCAD genotypes other than homozygosity for the c.985A>G mutation are also associated with sudden death; (3) that vomiting is a frequent symptom preceding sudden death; and (4) social support and medical follow-up of these families are crucial in reducing the occurrence of sudden death.

Pantothenate kinase 1 is required to support the metabolic transition from the fed to the fasted state

Coenzyme A (CoA) biosynthesis is regulated by the pantothenate kinases (PanK), of which there are four active isoforms. The PanK1 isoform is selectively expressed in liver and accounted for 40% of the total PanK activity in this organ. CoA synthesis was limited using a Pank1(-/-) knockout mouse model to determine whether the regulation of CoA levels was critical to liver function. The elimination of PanK1 reduced hepatic CoA levels, and fasting triggered a substantial increase in total hepatic CoA in both Pank1(-/-) and wild-type mice. The increase in hepatic CoA during fasting was blunted in the Pank1(-/-) mouse, and resulted in reduced fatty acid oxidation as evidenced by abnormally high accumulation of long-chain acyl-CoAs, acyl-carnitines, and triglycerides in the form of lipid droplets. The Pank1(-/-) mice became hypoglycemic during a fast due to impaired gluconeogenesis, although ketogenesis was normal. These data illustrate the importance of PanK1 and elevated liver CoA levels during fasting to support the metabolic transition from glucose utilization and fatty acid synthesis to gluconeogenesis and fatty acid oxidation. The findings also suggest that PanK1 may be a suitable target for therapeutic intervention in metabolic disorders that feature hyperglycemia and hypertriglyceridemia.

Lack of genotype-phenotype correlations and outcome in MCAD deficiency diagnosed by newborn screening in New York State

INTRODUCTION

Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is one of the most common inborn errors of metabolism. Affected patients have impaired ability to break down medium chain fatty acids during fasting, and typically present in the early years of life with hypoketotic hypoglycemia, Reye syndrome-like symptoms, brain damage or death. The development of newborn screening (NBS) for MCAD deficiency has greatly improved outcome, but some patients still appear at risk for severe complications. We reviewed the outcome of patients identified with MCAD deficiency by the New York State NBS process to identify biochemical or genotypic markers which might predict outcome.

METHOD

All eight NBS follow-up centers in New York State contributed the cases of MCAD deficiency diagnosed by newborn screen, who received diagnostic and follow-up care in their clinic. Data reviewed included gender, age, birthweight, initial NBS octanoylcarnitine level (C8) and C8/C2 ratio, follow-up C8 and hexanoylglycine, race/ethnicity, and presence of neonatal or later symptoms.

RESULTS

We identified 53 cases of MCAD deficiency. More than one quarter of patients had a post-neonatal symptomatic admission (predominantly lethargy associated with an intercurrent illness). No genotype or C8 level was protective for neonatal or later symptoms. There was a relationship between initial C8 level or C8/C2 ratio and occurrence of later symptoms (7.3 micromol/L in the asymptomatic vs. 19.1 micromol/L in the symptomatic, p<0.0002 for C8, and 0.26 vs. 0.6, respectively, for C8/C2 ratio, p<0.012). Four infants had initial C8 level >30 micromol/L; these infants had a high rate of symptomatic or multiple symptomatic episodes or a history of sibling death from "SIDS", and typically had deletion, nonsense or splice sites mutations. Infants having a history of a symptomatic episode were more likely to have higher initial C8 on NBS and a genotype predicted to strongly affect protein function. In our ethnically diverse group of patients, the c.985A>G mutation was rarely found in non-Caucasians.

DISCUSSION

No genotype or metabolite profile is protective from symptoms. The strong relationship between initial C8 level and outcome suggests that in at least some cases neonates having high initial C8 levels may be demonstrating an increased susceptibility to catabolic stress, and may merit additional precautions. Our data also suggest that these infants are more likely to carry severe mutations including homozygosity for the common mutation, deletions, nonsense or splice site mutations. The reports of significant lethargy or hypoglycemia during intercurrent illness in over one quarter of cases even when early medical intervention is recommended (and even when initial C8 is not profoundly elevated) underscores the importance of continued vigilance to prevent stressful fasting in this disorder.

Acylcarnitine analysis by tandem mass spectrometry

Carnitine plays an essential role in fatty acid metabolism, as well as modulation of intracellular concentrations of free coenzyme A by esterification of acyl residues. Acylcarnitine analysis of various biological fluids is a sensitive method to detect >20 inborn errors of metabolism that result in abnormal accumulation of acylcarnitine species due to several organic acidemias and most fatty acid beta-oxidation disorders. In addition, acylcarnitine analysis may aid in monitoring treatment of known patients affected with these inborn errors of metabolism. This unit describes protocols that can be used to measure acylcarnitine species of various carbon chain lengths in several biological specimen types including plasma, dried blood and bile spots, and urine, by derivatization to butylesters and flow-injection electrospray ionization tandem mass spectrometry (ESI-MS/MS).

Dissection of biochemical borderline phenotypes in carriers and genetic variants of medium-chain acyl-CoA dehyrogenase deficiency: implications for newborn screening [corrected]

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) represents a potentially fatal fatty acid beta-oxidation disorder. Newborn screening (NBS) by tandem mass spectrometry (MS/MS) has been implemented worldwide, but is associated with unresolved questions regarding population heterogeneity, burden on healthy carriers, cut-off policies, false-positive and negative rates. In a retrospective case-control study, 333 NBS samples showing borderline acylcarnitine patterns but not reaching recall criteria were genotyped for the two most common mutations (c.985A>G/c.199C>T) and compared with genotypes and acylcarnitines of 333 controls, 68 false-positives, and 34 patients. c.985A>G was more frequently identified in the study group and false-positives compared to controls (1:4.3/1:2.3 vs. 1:42), whereas c.199C>T was found more frequently only within the false-positives (1:23). Biochemical criteria were devised to differentiate homozygous (c.985A>G), compound heterozygous (c.985A>G/c.199C>T), and heterozygous individuals. Four false-negatives were identified because our initial algorithm required an elevation of octanoylcarnitine (C(8)) and three secondary markers in the initial and follow-up sample. The new approach allowed a reduction of false-positives (by defining high cut-offs: 1.4 micromol/l for C(8); 7 for C(8)/C(12)) and false-negatives (by sequencing the ACADM gene of few suspicious samples). Our validation strategy is able to differentiate healthy carriers from patients doubling the positive predictive value (42-->88%) and to target NBS to MCADD-subsets with potentially higher risk of adverse outcome. It remains controversial, if NBS programs should aim at identifying all subsets of all diseases included. Because the natural course of milder variants cannot be assessed by observational studies, our strategy could serve as a general model for evaluation of MS/MS-based NBS.

Revisiting the possibility of serious adverse events from the whole cell pertussis vaccine: were metabolically vulnerable children at risk?

In the early 1980's concerns about the safety of the whole cell pertussis vaccine in the United States resulted in declining vaccination rates and the withdrawal of multiple vaccine providers from the market. While the possibility of inflammation and febrile reactions to the vaccine were acknowledged by public health authorities, parents also claimed the vaccine was associated with sudden infant death syndrome and encephalopathy. Epidemiological studies examining this question, however, consistently failed to identify an association. We argue that these reactions may have occurred in metabolically vulnerable children, specifically those with defects in fatty acid oxidation. In these children the combination of anorexia and fever that could be caused by the vaccine may have resulted in hypoglycemic episodes and possibly death. We believe that this association was not detected because these conditions were not recognized at the time and because these conditions are uncommon. Nevertheless, at a population level, enough events could have occurred to cause concern amongst parents.

Protein misfolding is the molecular mechanism underlying MCADD identified in newborn screening

Newborn screening (NBS) for medium-chain acyl-CoA dehydrogenase deficiency (MCADD) revealed a higher birth prevalence and genotypic variability than previously estimated, including numerous novel missense mutations in the ACADM gene. On average, these mutations are associated with milder biochemical phenotypes raising the question about their pathogenic relevance. In this study, we analyzed the impact of 10 ACADM mutations identified in NBS (A27V, Y42H, Y133H, R181C, R223G, D241G, K304E, R309K, I331T and R388S) on conformation, stability and enzyme kinetics of the corresponding proteins. Partial to total rescue of aggregation by co-overexpression of GroESL indicated protein misfolding. This was confirmed by accelerated thermal unfolding in all variants, as well as decreased proteolytic stability and accelerated thermal inactivation in most variants. Catalytic function varied from high residual activity to markedly decreased activity or substrate affinity. Mutations mapping to the beta-domain of the protein predisposed to severe destabilization. In silico structural analyses of the affected amino acid residues revealed involvement in functionally relevant networks. Taken together, our results substantiate the hypothesis of protein misfolding with loss-of-function being the common molecular basis in MCADD. Moreover, considerable structural alterations in all analyzed variants do not support the view that novel mutations found in NBS bear a lower risk of metabolic decompensation than that associated with mutations detected in clinically ascertained patients. Finally, the detailed insight into how ACADM missense mutations induce loss of MCAD function may provide guidance for risk assessment and counseling of patients, and in future may assist delineation of novel pharmacological strategies.

A novel molecular aspect of Japanese patients with medium-chain acyl-CoA dehydrogenase deficiency (MCADD): c.449-452delCTGA is a common mutation in Japanese patients with MCADD

We studied 11 Japanese patients with medium-chain acyl-CoA dehydrogenase deficiency (MCADD) and found a common mutation, c.449-452delCTGA, which accounted for 45% of the mutations. Seven of 10 independent patients carried at least one copy of this mutation. Phenotypes of homozygous patients with the c.449-452delCTGA mutation varied from asymptomatic to life-threatening metabolic decompensation in Japanese patients with MCADD, similar to the phenotypic variations in Caucasians. This study suggests the genotypic difference between those of Caucasians and Japanese regarding MCADD.