Identification of a novel mutation in patients with medium-chain acyl-CoA dehydrogenase deficiency

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.

Adult presentations of medium-chain acyl-CoA dehydrogenase deficiency (MCADD)

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is an autosomal recessive disorder of mitochondrial fatty acid oxidation which is usually diagnosed in infancy or through neonatal screening. In the absence of population screening, adults with undiagnosed MCADD can be expected. This review discusses 14 cases that were identified during adulthood. The mortality of infantile patients is approximately 25% whereas in this adult case series it was shown it to be 50% in acutely presenting patients and 29% in total. Therefore, undiagnosed individuals are at risk of sudden fatal metabolic decompensation with high mortality. This review illustrates the need to consider the possibility of a fatty acid oxidation defect in an adult who presents with unexplained sudden clinical deterioration, particularly if precipitated by fasting or alcohol consumption. A history of unexplained sibling death may also raise the index of suspicion. There also needs to be appropriate clinical support for those patients identified clinically or as a result of family studies (sibling or parent).

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.

Spectrum of medium-chain acyl-CoA dehydrogenase deficiency detected by newborn screening

OBJECTIVE

Our goal was to describe the clinical spectrum of medium-chain acyl-CoA dehydrogenase deficiency detected by routine newborn screening and assess factors associated with elevations of octanoylcarnitine in newborns and characteristics associated with adverse clinical consequences of medium-chain acyl-CoA dehydrogenase deficiency.

METHODS

The first 47 medium-chain acyl-CoA dehydrogenase deficiency cases detected by the New England Newborn Screening Program were classified according to initial and follow-up octanoylcarnitine values, octanoylcarnitine-decanoylcarnitine ratios, medium-chain acyl-CoA dehydrogenase genotype, follow-up biochemical parameters, and feeding by breast milk or formula.

RESULTS

All 20 patients who were homozygous for 985A-->G had high initial octanoylcarnitine values (7.0-36.8 microM) and octanoylcarnitine-decanoylcarnitine ratios (7.0-14.5), whereas the 27 patients with 0 to 1 copy of 985A-->G exhibited a wide range of octanoylcarnitine values (0.5-28.6 microM) and octanoylcarnitine-decanoylcarnitine ratios (0.8-12.7). Initial newborn octanoylcarnitine values decreased by days 5 to 8, but the octanoylcarnitine-decanoylcarnitine ratio generally remained stable. Among 985A-->G homozygotes, breastfed newborns had higher initial octanoylcarnitine values than newborns who received formula. Adverse events occurred in 5 children, 4 985A-->G homozygotes and 1 compound heterozygote with a very high initial octanoylcarnitine: 2 survived severe neonatal hypoglycemia, 1 survived a severe hypoglycemic episode at 15 months of age, and 2 died as a result of medium-chain acyl-CoA dehydrogenase deficiency at ages 11 and 33 months.

CONCLUSION

Newborn screening for medium-chain acyl-CoA dehydrogenase deficiency has detected cases with a wide range of genotypes and biochemical abnormalities. Although most children do well, adverse outcomes have not been entirely avoided. Assessment of potential risk and determination of appropriate treatment remain a challenge.

Population spectrum of ACADM genotypes correlated to biochemical phenotypes in newborn screening for medium-chain acyl-CoA dehydrogenase deficiency

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is the most frequent inherited defect of fatty acid oxidation, with a significant morbidity and mortality in undiagnosed patients. Adverse outcomes can effectively be prevented by avoiding metabolic stress and following simple dietary measures. Therefore, prospective newborn screening (NBS) is being proposed for this condition. However, technical validation of MCADD population screening and assessment of its overall benefit require broadening of the as-yet-scarce knowledge of the MCADD genetic heterogeneity unraveled by NBS and its phenotypic consequences. Here, we describe the entire spectrum of sequence variations occurring in newborns with MCADD in the population of Bavaria, Germany, in relation to the biochemical phenotype. Among 524,287 newborns, we identified 62 cases of MCADD, indicating a birth incidence of 1 in 8,456. In all of the 57 newborns available for analysis, two alterations within the MCADD gene (ACADM) were identified. The most prevalent alteration c.985A>G (Lys329Glu) occurred in 27 (47%) newborns in the homozygous and in 18 (32%) in the heterozygous state (63% of defective alleles). The mild folding variant c.199T>C (Tyr67His) was identified in nine individuals, six of them being compound heterozygous with c.985A>G (Lys329Glu). Neither of the prevalent alterations were found in the remaining nine newborns. A total of 18 sequence variations were identified; 13 of them were novel: eight missense mutations, one nonsense mutation, two splice variants, and two small deletions. The remaining five were previously reported in MCADD patients. The ACADM heterogeneity uncovered was larger as anticipated from previous c.985A>G (Lys329Glu) carrier screening data. In addition, we show that MCADD appears to occur as frequently in Turkish newborns as in the native German population. Our data validate that biochemical NBS for MCADD is a highly specific procedure for disease detection, with the identification of a significant share of milder biochemical phenotypes, such as c.199T>C (Tyr67His). These show statistically lower acylcarnitine markers, allowing us to distinguish subgroups within the spectrum of ACADM sequence variations that correlate to biochemical MCADD disease expression. Our data might provide technical and medical guidance for decision making in the worldwide efforts to introduce MCADD population screening.

The natural history of medium-chain acyl CoA dehydrogenase deficiency in the Netherlands: clinical presentation and outcome

OBJECTIVES

To describe the clinical presentation and long-term follow-up of a large cohort of patients with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency.

STUDY DESIGN

A nationwide, retrospective analysis of clinical presentation and follow-up in 155 Dutch patients with MCAD deficiency.

RESULTS

Most patients presented between 3 months and 5.1 years of age; 13% had symptoms as neonates not exclusively related to breast-feeding. An acute presentation before the diagnosis was made resulted in a mortality of 22% (25/114), whereas 21% (19/89) developed disabilities after the diagnosis. On follow-up, a total of 44 patients reported fatigue (35%; 28/80), muscle pain (31%; 25/80), and/or reduced exercise tolerance (39%; 31/80). Cardiac evaluation in 11 adult patients revealed no abnormalities in cardiac function explaining these complaints. Children with MCAD deficiency readily become overweight.

CONCLUSIONS

Mortality and morbidity were high in undiagnosed children with MCAD deficiency; establishment of the diagnosis significantly improves outcome. Strikingly, after the diagnosis and initiation of treatment, overweight and chronic complaints (fatigue, muscle pain, and reduced exercise tolerance) were prominent.

Expanded newborn screening of inherited metabolic disorders by tandem mass spectrometry: Clinical and laboratory aspects

Newborn screening started in the 1960s for the purpose of identifying phenylketonuric patients to begin early intervention and to prevent mental retardation in these patients. Soon thereafter, screening programs expanded to include additional genetic disorders added individually one at a time. In the 1980s, tandem mass spectrometry (MS/MS) was introduced in clinical laboratories, and in the 1990s, the technique was used for newborn screening. Unlike measuring one analyte at a time, MS/MS allows measurement of >40 analytes, in a few minutes with the use of a single assay. Currently, MS/MS is being used for the identification of several amino acid, organic acid and fatty acid disorders. Several states in the United States and many other countries are using MS/MS in newborn screening. However, there is a significant disparity among different newborn screening programs for disorders being screened by MS/MS and many other challenges are faced by the expanded newborn screening. It is anticipated that in the future the use of MS/MS in newborn screening will expand both at the analyte and geographic levels. Clinicians and laboratory scientists should become familiar with MS/MS, disorders being screened in their patients' population and the future of this emerging technology.

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

The fatty acid oxidation disorder most commonly identified by tandem mass spectrometry newborn screening is the potentially fatal medium-chain acyl-CoA dehydrogenase deficiency (MCAD). In clinically presenting cases, 80% are homozygous for the common mutation, c.985A > G and 18% heterozygous. We screened 592,785 babies and identified 34 with MCAD, 17 homozygous for c.985A > G, 14 with one copy, and 3 with no copy. We sequenced the exons of 19 patients, the 17 carrying one or no copy of c.985A > G, and two with marginal findings, and examined correlations between groups of mutations and biochemical markers. We found two known or putative pathogenic mutations in 18 of the 19 patients. Two mutations appeared more than once: c.199T > C, not recorded in clinically presenting cases (n = 4), and c.583G > A (n = 2). Patients homozygous for c.985A > G had the highest levels of neonatal octanoylcarnitine, plasma octanoylcarnitine when asymptomatic, and urinary acylglycines. Compound heterozygotes of c.985A > G and other mutations had intermediate levels, and those without c.985A > G, or heterozygous for that and c.199T > C had the lowest levels of these analytes. There was overlap in all values. The c.985A > G and c.583G > A mutations appear to have functional effects towards the severe end of the spectrum, and the c.199T > C mutation a smaller effect, as has been previously postulated. If these results are confirmed and extended, this could influence the advice given to parents of babies with MCAD detected by newborn screening, and make management more specific. In the meantime, all MCAD patients identified by newborn screening have, by definition, a functional defect and require careful clinical management.

Enzymatic diagnosis of medium-chain acyl-CoA dehydrogenase deficiency by detecting 2-octenoyl-CoA production using high-performance liquid chromatography: a practical confirmatory test for tandem mass spectrometry newborn screening in Japan

Many of the previously described enzymatic assay methods for the diagnosis of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency have been dependent upon the measurement of radioisotope-labeled co-products or reduction of electron acceptors. We have developed a direct assay method to detect 2-enoyl-CoA production using high-performance liquid chromatography (HPLC). Crude cell lysate prepared from lymphocytes were incubated with n-octanoyl-CoA and ferrocenium hexafluorophosphate. The detection of 2-octenoyl-CoA was significantly reproducible. We applied the assay to samples from four infants suspected to have MCAD deficiency by tandem mass spectrometry (MS/MS) newborn screening conducted in the Hiroshima area of Japan. Three of them were proved to have pathologically reduced residual enzyme activities, although they were associated with various clinical and biochemical phenotypes. In addition, another symptomatic Japanese patient and her presymptomatic sibling who were detected by MS/MS selective screening were successfully diagnosed by our enzymatic assay. These results indicate that the method can be a useful confirmatory test for MS/MS screening of MCAD deficiency.

The Y42H mutation in medium-chain acyl-CoA dehydrogenase, which is prevalent in babies identified by MS/MS-based newborn screening, is temperature sensitive

Medium-chain acyl-CoA dehydrogenase (MCAD) is a homotetrameric flavoprotein which catalyses the initial step of the beta-oxidation of medium-chain fatty acids. Mutations in MCAD may cause disease in humans. A Y42H mutation is frequently found in babies identified by newborn screening with MS/MS, yet there are no reports of patients presenting clinically with this mutation. As a basis for judging its potential consequences we have examined the protein phenotype of the Y42H mutation and the common disease-associated K304E mutation. Our studies of the intracellular biogenesis of the variant proteins at different temperatures in isolated mitochondria after in vitro translation, together with studies of cultured patient cells, indicated that steady-state levels of the Y42H variant in comparison to wild-type were decreased at higher temperature though to a lesser extent than for the K304E variant. To distinguish between effects of temperature on folding/assembly and the stability of the native enzyme, the thermal stability of the variant proteins was studied after expression and purification by dye affinity chromatography. This showed that, compared with the wild-type enzyme, the thermostability of the Y42H variant was decreased, but not to the same degree as that of the K304E variant. Substrate binding, interaction with the natural electron acceptor, and the binding of the prosthetic group, FAD, were only slightly affected by the Y42H mutation. Our study suggests that Y42H is a temperature sensitive mutation, which is mild at low temperatures, but may have deleterious effects at increased temperatures.

Mutation analysis in mitochondrial fatty acid oxidation defects: Exemplified by acyl-CoA dehydrogenase deficiencies, with special focus on genotype-phenotype relationship

Mutation analysis of metabolic disorders, such as the fatty acid oxidation defects, offers an additional, and often superior, tool for specific diagnosis compared to traditional enzymatic assays. With the advancement of the structural part of the Human Genome Project and the creation of mutation databases, procedures for convenient and reliable genetic analyses are being developed. The most straightforward application of mutation analysis is to specific diagnoses in suspected patients, particularly in the context of family studies and for prenatal/preimplantation analysis. In addition, from these practical uses emerges the possibility to study genotype-phenotype relationships and investigate the molecular pathogenesis resulting from specific mutations or groups of mutations. In the present review we summarize current knowledge regarding genotype-phenotype relationships in three disorders of mitochondrial fatty acid oxidation: very-long chain acyl-CoA dehydrogenase (VLCAD, also ACADVL), medium-chain acyl-CoA dehydrogenase (MCAD, also ACADM), and short-chain acyl-CoA dehydrogenase (SCAD, also ACADS) deficiencies. On the basis of this knowledge we discuss current understanding of the structural implications of mutation type, as well as the modulating effect of the mitochondrial protein quality control systems, composed of molecular chaperones and intracellular proteases. We propose that the unraveling of the genetic and cellular determinants of the modulating effects of protein quality control systems may help to assess the balance between genetic and environmental factors in the clinical expression of a given mutation. The realization that the effect of the monogene, such as disease-causing mutations in the VLCAD, MCAD, and SCAD genes, may be modified by variations in other genes presages the need for profile analyses of additional genetic variations. The rapid development of mutation detection systems, such as the chip technologies, makes such profile analyses feasible. However, it remains to be seen to what extent mutation analysis will be used for diagnosis of fatty acid oxidation defects and other metabolic disorders.

Medium-chain acyl-CoA dehydrogenase (MCAD) mutations identified by MS/MS-based prospective screening of newborns differ from those observed in patients with clinical symptoms: identification and characterization of a new, prevalent mutation that results in mild MCAD deficiency

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most frequently diagnosed mitochondrial beta-oxidation defect, and it is potentially fatal. Eighty percent of patients are homozygous for a common mutation, 985A-->G, and a further 18% have this mutation in only one disease allele. In addition, a large number of rare disease-causing mutations have been identified and characterized. There is no clear genotype-phenotype correlation. High 985A-->G carrier frequencies in populations of European descent and the usual avoidance of recurrent disease episodes by patients diagnosed with MCAD deficiency who comply with a simple dietary treatment suggest that MCAD deficiency is a candidate in prospective screening of newborns. Therefore, several such screening programs employing analysis of acylcarnitines in blood spots by tandem mass spectrometry (MS/MS) are currently used worldwide. No validation of this method by mutation analysis has yet been reported. We investigated for MCAD mutations in newborns from US populations who had been identified by prospective MS/MS-based screening of 930,078 blood spots. An MCAD-deficiency frequency of 1/15,001 was observed. Our mutation analysis shows that the MS/MS-based method is excellent for detection of MCAD deficiency but that the frequency of the 985A-->G mutant allele in newborns with a positive acylcarnitine profile is much lower than that observed in clinically affected patients. Our identification of a new mutation, 199T-->C, which has never been observed in patients with clinically manifested disease but was present in a large proportion of the acylcarnitine-positive samples, may explain this skewed ratio. Overexpression experiments showed that this is a mild folding mutation that exhibits decreased levels of enzyme activity only under stringent conditions. A carrier frequency of 1/500 in the general population makes the 199T-->C mutation one of the three most prevalent mutations in the enzymes of fatty-acid oxidation.