3-Methylcrotonyl-coenzyme A carboxylase deficiency in Amish/Mennonite adults identified by detection of increased acylcarnitines in blood spots of their children

An asymptomatic father diagnosed with 3-methylcrotonyl-CoA carboxylase deficiency following his son newborn screening test

3-methylcrotonyl-CoA carboxylase deficiency (3MCCD) is a hereditary disorder of leucine catabolism caused by pathogenetic variants in the MCCC1 or MCCC2 genes. Typically diagnosed through newborn screening (NBS), 3MCCD is characterized by elevation of 3-hydroxyisovalerylcarnitine (C5OH) in blood as well as increased excretion of 3-methylcrotonylglycine (3-MCG) in urine. While most diagnosed children remain asymptomatic, data on adults are scarce. To date, only 39 molecularly confirmed adult individuals have been reported, all being mothers diagnosed subsequent to their child NBS results. Herein, we present a 36-year-old asymptomatic man who was incidentally diagnosed with 3MCCD following his son NBS recall. Molecular analysis revealed compound heterozygosity for two pathogenic variants in the MCCC1 gene. This is the first molecularly confirmed adult man with 3MCCD reported. This case highlights the need for additional longitudinal follow-up data on individuals with 3MCCD to clarify the clinical significance of this condition and guide clinical practice, including NBS strategy.

Maternal 3-methylcrotonyl-coenzyme A carboxylase deficiency with elevated 3-hydroxyisovalerylcarnitine in breast milk

We report here a case of maternal 3-methylcrotonyl-coenzyme A carboxylase (3-MCC) deficiency in a Korean woman. Her 2 infants had elevated 3-hydroxyisovalerylcarnitine (C5-OH) on a neonatal screening test by liquid chromatography-tandem mass spectrometry (LC-MS/MS), but normal results were found on urine organic acid analysis. The patient was subjected to serial testing and we confirmed a maternal 3-MCC deficiency by blood spot and breast milk spot test by LC-MS/MS, serum amino acid analysis, urine organic acid and molecular genetic analysis that found c.838G>T (p.Asp280Tyr) homozygous mutation within exon 9 of the MCCB gene. Especially, we confirmed marked higher levels of C5-OH on breast milk spot by LC-MS/MS, in the case of maternal 3-MCC deficiency vs. controls.

3-Methylcrotonyl-CoA carboxylase deficiency: Mutational spectrum derived from comprehensive newborn screening

The deficiency of 3-methycrotonyl-CoA carboxylase (3-MCC; EC 6.4.1.4) is an autosomal recessive organic aciduria that is included in the newborn screening programs of several countries. This study reports data mainly obtained from the Portuguese newborn screening program collected over a ten-year period. Analysis of the MCCC1 and MCCC2 genes yielded 26 previously unreported mutations and a variant of clinically unknown significance. These mutations are discussed in the context of their likely impact on the function of the 3-MCC enzyme, with a view to exploring whether a phenotype-genotype correlation might be discerned. Further, these mutations were analysed in the context of what is known of the MCCC1 and MCCC2 mutational spectra, information that will be useful in both clinical and laboratory practice.

Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System

INTRODUCTION

3-Methyl crotonyl CoA carboxylase (3MCC) deficiency is an inborn error of leucine metabolism whose detection was increased with the advent of expanded newborn screening. While most NBS-identified infants appear clinically normal, prior studies suggest a possible increased risk for developmental or metabolic abnormalities. As yet, no predictive markers are known that can identify children at risk for biochemical or developmental abnormalities.

METHOD

All available 3-MCC cases diagnosed by newborn screening in the Inborn Errors of Metabolism Information System (IBEM-IS) were reviewed for markers that might be predictive of outcome.

RESULTS

A limited number of cases were identified with traditional biochemical symptoms including acidosis, hyperammonemia or lactic acidosis, and 15% of those with available developmental information had recorded developmental disabilities not clearly attributable to other causes. There was no correlation between newborn screening (NBS) C5OH level and presence of metabolic, newborn, later-life or developmental abnormalities in these cases.

DISCUSSION

This sample, obtained from the IBEM-IS database, attempts to avoid some of the ascertainment bias present in retrospective studies. An increase in developmental abnormalities and in traditionally described metabolic symptoms remains apparent, although no specific biochemical markers appear predictive of outcome. The role that prevention of fasting plays in outcome cannot be ascertained. These data suggest that C5OH level found on newborn screening by itself is not sufficient for diagnostic or predictive purposes.

Primary and maternal 3-methylcrotonyl-CoA carboxylase deficiency: insights from the Israel newborn screening program

BACKGROUND

3-Methylcrotonyl-CoA carboxylase deficiency (3MCCD) is an inborn error of leucine catabolism. Tandem mass spectrometry newborn screening (NBS) programs worldwide confirmed 3MCCD to be the most common organic aciduria and a relatively benign disorder with favorable outcome. In addition, several asymptomatic 3MCCD mothers were initially identified following abnormal screening of their healthy babies and were appropriately termed maternal 3MCCD.

METHODS

This is a retrospective study that summarizes all the clinical, biochemical, and genetic data collected by questionnaires of all 3MCCD individuals that were identified by the extended Israeli NBS program since its introduction in 2009 including maternal 3MCCD cases.

RESULTS

A total of 36 3MCCD subjects were diagnosed within the 50-month study period; 16 were classified primary and 20 maternal cases. Four additional 3MCCD individuals were identified following sibling screening. All maternal 3MCCD cases were asymptomatic except for one mother who manifested childhood hypotonia. Most of the primary 3MCCD individuals were asymptomatic except for two whose condition was also complicated by severe prematurity. Initial dried blood spot (DBS) free carnitine was significantly lower in neonates born to 3MCCD mothers compared with newborns with primary 3MCCD (p = 0.0009). Most of the mutations identified in the MCCC1 and MCCC2 genes were missense, five of them were novel.

CONCLUSIONS

Maternal 3MCCD is more common than previously thought and its presence may be initially indicated by low DBS free carnitine levels. Our findings provide additional confirmation of the benign nature of 3MCCD and we suggest to exclude this disorder from NBS programs.

Abnormal Newborn Screening in a Healthy Infant of a Mother with Undiagnosed Medium-Chain Acyl-CoA Dehydrogenase Deficiency

A neonate with low blood free carnitine level on newborn tandem mass spectrometry screening was evaluated for possible carnitine transporter defect (CTD). The plasma concentration of free carnitine was marginally reduced, and the concentrations of acylcarnitines (including C6, C8, and C10:1) were normal on confirmatory tests. Organic acids in urine were normal. In addition, none of the frequent Faroese SLC22A5 mutations (p.N32S, c.825-52G>A) which are common in the Danish population were identified. Evaluation of the mother showed low-normal free carnitine, but highly elevated medium-chain acylcarnitines (C6, C8, and C10:1) consistent with medium-chain acyl-CoA dehydrogenase deficiency (MCADD). The diagnosis was confirmed by the finding of homozygous presence of the c.985A>G mutation in ACADM.

Asymptomatic maternal 3-methylcrotonylglycinuria detected by her unaffected baby's neonatal screening test

3-methylcrotonyl-coenzyme A carboxylase (3MCC) deficiency is an autosomal recessive disorder in which leucine catabolism is hampered, leading to increased urinary excretion of 3-methylcrotonylglycine. In addition, 3-hydroxyisovalerylcarnitine levels increase in the blood, and the elevated levels form the basis of neonatal screening. 3MCC deficiency symptoms are variable, ranging from neonatal onset with severe neurological abnormality to a normal, asymptomatic phenotype. Although 3MCC deficiency was previously considered to be rare, it has been found to be one of the most common metabolic disorders in newborns after the neonatal screening test using tandem mass spectrometry was introduced. Additionally, asymptomatic 3MCC deficient mothers have been identified due to abnormal results of unaffected baby's neonatal screening test. Some of the 3MCC-deficient mothers show symptoms such as fatigue, myopathy, or metabolic crisis with febrile illnesses. In the current study, we identified an asymptomatic 3MCC deficient mother when she showed abnormal results during a neonatal screening test of a healthy infant.

Screening of a healthy newborn identifies three adult family members with symptomatic glutaric aciduria type I

We report three adult sibs (one female, two males) with symptomatic glutaric acidura type I, who were diagnosed after a low carnitine level was found by newborn screening in a healthy newborn of the women. All three adults had low plasma carnitine, elevated glutaric acid levels and pronounced 3-hydroxyglutaric aciduria. The diagnosis was confirmed by undetectable glutaryl-CoA dehydrogenase activity in lymphocytes and two pathogenic heterozygous mutations in the GCDH gene (c.1060A > G, c.1154C > T). These results reinforce the notion that abnormal metabolite levels in newborns may lead to the diagnosis of adult metabolic disease in the mother and potentially other family members.

Abnormal newborn screens and acylcarnitines in HIV-exposed and ARV-exposed infants

BACKGROUND

Antiretroviral drugs (ARV), specifically nucleoside analogs, are toxic to mitochondrial oxidative phosphorylation. Other metabolic pathways, such as fatty acid oxidation, organic acid metabolism and amino acid metabolism, are dependent on normal oxidative phosphorylation but remain unexamined as potential points of ARV toxicity.

METHODS

We analyzed newborn screening data from New York and compared proportions of abnormal newborn metabolic screens in HIV antibody screen-positive and HIV screen-negative neonates. Subsequently, we compared acylcarnitine levels in ARV-exposed (n = 16) and ARV-unexposed (n = 14) HIV-exposed infants to assess for dysfunctional fatty and organic acid metabolism.

RESULTS

: The rate of abnormal newborn metabolic screens in HIV screen-positive infants was higher than that in the general population (2.2% versus 1.2%; P = 0.00025), most of which were for disorders of mitochondria-related metabolism. Abnormal acylcarnitine levels occurred more frequently in ARV-exposed compared with ARV-unexposed infants (43% versus 0%; P = 0.02).

CONCLUSIONS

A higher proportion of positive metabolic screens in HIV screen-positive neonates suggests that HIV or ARV exposure is associated with dysfunctional intermediary metabolism in newborns. Abnormal acylcarnitine levels were more frequent in ARV-exposed infants, suggesting that ARV may perturb normal fatty acid oxidation in some infants. Studies designed to validate and determine the clinical significance of these findings are warranted.

Outcome of infants diagnosed with 3-methyl-crotonyl-CoA-carboxylase deficiency by newborn screening

INTRODUCTION

3-Methyl CoA carboxylase (3-MCC) deficiency is an inborn error of metabolism in the catabolism of the amino acid leucine. Original reports suggested this disorder was associated with significant neurological and biochemical effects. However newborn screening has identified a higher than expected incidence of this disorder with apparent normal outcome in most cases.

METHOD

A retrospective analysis of thirty-five cases of 3-MCC deficiency identified by newborn screening and diagnosed by enzyme or molecular analysis.

RESULTS

There was a strong inverse correlation between initial C5OH level and residual enzyme activity. A few reports of hypoglycemia, ketosis, poor feeding/failure to thrive or fasting intolerance were reported, but there was no clear relationship between symptoms and residual enzyme activity. Developmental outcome included several children with mental retardation (including one with Down syndrome and one with schizencephaly) and two with Autism Spectrum disorders but there was no apparent relationship to residual enzyme activity. Free carnitine deficiency was relatively common.

DISCUSSION

Although residual enzyme activity was clearly related to metabolite elevation, there was no apparent relationship with other measures of outcome. The number of reports of neurologic abnormalities or metabolic symptoms (poor feeding, hypoglycemia, fasting intolerance, etc.) is concerning, but the significance is unclear in this retrospective sample.

Newborn screening for lysosomal storage diseases: an ethical and policy analysis

The traditional focus of newborn screening (NBS) is testing infants for medical conditions like phenylketonuria (PKU) that may cause significant morbidity or mortality unless treatment is initiated early. Although the Wilson and Jungner criteria were not designed specifically for NBS, the public health screening criteria have been used, with some modifications, to justify what conditions are included in a universal NBS panel. These criteria are being challenged by platform technologies like tandem mass spectrometry (MS/MS) that allow for the identification of numerous conditions on a single sample because they identify many conditions and variants simultaneously, some of which meet and others which fail to meet the criteria. In this manuscript, I evaluate three lysosomal storage diseases included in this multiplex screening test-Pompe disease, Fabry disease, and Krabbe disease. I show that they fail to meet some of the critical Wilson and Jungner criteria and thus are not ready for inclusion in universal NBS panels. Rather, screening for these conditions should only be performed in the research context with institutional review board approval and parental permission.

3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals

Background

Isolated 3-methylcrotonyl-CoA carboxylase (MCC) deficiency is an autosomal recessive disorder of leucine metabolism caused by mutations in MCCC1 or MCCC2 encoding the α and β subunit of MCC, respectively. The phenotype is highly variable ranging from acute neonatal onset with fatal outcome to asymptomatic adults.

Methods

We report clinical, biochemical, enzymatic and mutation data of 88 MCC deficient individuals, 53 identified by newborn screening, 26 diagnosed due to clinical symptoms or positive family history and 9 mothers, identified following the positive newborn screening result of their baby.

Results

Fifty-seven percent of patients were asymptomatic while 43% showed clinical symptoms, many of which were probably not related to MCC deficiency but due to ascertainment bias. However, 12 patients (5 of 53 identified by newborn screening) presented with acute metabolic decompensations. We identified 15 novel MCCC1 and 16 novel MCCC2 mutant alleles. Additionally, we report expression studies on 3 MCCC1 and 8 MCCC2 mutations and show an overview of all 132 MCCC1 and MCCC2 variants known to date.

Conclusions

Our data confirm that MCC deficiency, despite low penetrance, may lead to a severe clinical phenotype resembling classical organic acidurias. However, neither the genotype nor the biochemical phenotype is helpful in predicting the clinical course.

A single mutation in MCCC1 or MCCC2 as a potential cause of positive screening for 3-methylcrotonyl-CoA carboxylase deficiency

Isolated 3-Methylcrotonyl-CoA carboxylase deficiency (MCC deficiency) is an organic aciduria presenting with a highly variable phenotype and has been part of newborn screening programs in various countries, in particular in the US. Here we present enzymatic and genetic characterisation of 22 individuals with increased 3-hydroxyisovalerylcarnitine and/or 3-methylcrotonylglycine suggesting MCC deficiency, but only partially reduced 3-methylcrotonyl-CoA carboxylase activity. Among these, 21 carried a single mutant allele in either MCCC1 (n=20) or MCCC2 (n=1). Our results suggest that heterozygosity for such a single deleterious mutation may lead to misdiagnosis of MCC deficiency.

Positive newborn screen in a normal infant of a mother with asymptomatic very long-chain Acyl-CoA dehydrogenase deficiency

A neonate with elevated tetradecenoylcarnitine (C14:1) on the newborn screen was evaluated for possible very long-chain acyl-CoA dehydrogenase deficiency (VLCADD) and found to be a carrier. However, his symptom-free mother was subsequently diagnosed with VLCADD. This documents maternal VLCADD causing a positive newborn screening result in an offspring.

Maternal medium-chain acyl-CoA dehydrogenase deficiency identified by newborn screening

Prior to the advent of expanded newborn screening, sudden and unexplained death was often the first and only symptom of medium-chain acyl-CoA dehydrogenase deficiency (MCADD). With the use of tandem mass spectrometry, infants can now be identified and treated before a life threatening metabolic decompensation occurs. Newborn screening has also been shown to detect previously undiagnosed maternal inborn errors of metabolism. We have now diagnosed two women with MCADD following the identification of low free carnitine in their newborns. While one of the women reported prior symptoms of fasting intolerance, neither had a history of metabolic decompensation or other symptoms consistent with a fatty acid oxidation disorder. These cases illustrate the importance of including urine organic acid analysis and an acylcarnitine profile as part of the confirmatory testing algorithm for mothers when low free carnitine is identified in their infants.

Four years of expanded newborn screening in Portugal with tandem mass spectrometry

INTRODUCTION

The Portuguese Neonatal Screening Programme (PNSP) was started in 1979 for phenylketonuria (2,590,700 newborns screened; prevalence 1:11,031) and, shortly after, for congenital hypothyroidism (2,558,455 newborns screened; prevalence 1:3,174). In 2004, expanded neonatal screening was implemented in the National Laboratory. The programme is not mandatory and has 99.8% coverage of the country (including Madeira and the Azores islands).

MATERIAL AND METHODS

In the past 4 years, 316,243 neonates were screened with the use of tandem mass spectrometry (MS/MS) to test for selected amino acids and acylcarnitines.

RESULTS

During this time, 132 patients were identified with 24 different inherited metabolic diseases (classic forms and variants). To date, the global frequency for all disorders integrated into the PNSP is estimated to be 1:1,380, with 1:2,396 for metabolic disorders. A total of 379 tests (0.12%) were classified as having false positive results, yielding an overall specificity of 99.9%. Despite the low frequency of several disorders, the positive predictive value of the overall MS/MS screening was found to be 26%, reflecting high diagnostic specificity of the method. Diagnostic sensitivity of extended screening for the different groups of disorders was 100%. Eight cases of maternal disorders [three glutaric aciduria type I, one carnitine transporter defect, and four 3-methylcrotonyl coenzyme A (CoA) carboxylase deficiency] were also detected through newborn screening.

CONCLUSIONS

Our data support the advantage of a centralised laboratory for screening an elevated number of samples and making decisions if relying on a clinical network able to provide fast treatment and a good outcome in the screened cases.

Elevated neonatal 3-OH isovalerylcarnitine due to breast milk sources in maternal 3-MCC deficiency

We report a positive newborn screen for 3-hydroxyisovalerylcarnitine (C(5)OH) with an absence of 3-methylcrotonyl-coenzyme A carboxylase deficiency in the neonate. Subsequent blood tests demonstrated persistently elevated C(5)OH. Serial testing of the mother identified markedly elevated C(5)OH in both maternal blood and breast milk. High C(5)OH milk concentrations provide a significant source of C(5)OH to the nursing neonate and possibly explains its persistent elevation in the neonate, a commonly observed finding in maternal 3-MCC deficiency.

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).

Asymptomatic maternal combined homocystinuria and methylmalonic aciduria (cblC) detected through low carnitine levels on newborn screening

A symptom-free woman gave birth to a girl with a low carnitine level on newborn screening. The baby was unaffected, but the mother had biochemical abnormalities and mutations characteristic of the cblC defect of vitamin B(12) metabolism (late-onset form). This patient with cblC was detected through her infant's newborn screening.

Positive newborn screen in the biochemically normal infant of a mother with treated holocarboxylase synthetase deficiency

Expanded programmes of newborn screening permit early diagnosis in time to prevent serious complications. These programmes have begun to detect patients who might otherwise remain asymptomatic. An additional confounding variable is the positive screen that results from maternal rather than neonatal disease. This was the case in an infant in whom elevated hydroxyisovalerylcarnitine (C(5)OH) in his newborn screen was the result of placental transfer from his mother, whose holocarboxylase synthetase deficiency was being successfully treated with biotin. The mother had been diagnosed and treated with biotin prenatally. She had no phenotypic feature of holocarboxylase synthetase deficiency, most importantly no episodes ever of acute metabolic acidosis. In the infant a repeat screen was also positive. On day 28 the infant's plasma C(5)OH carnitine was 0.05 mumol/L (normal) and urinary organic acids on day 39 were normal. The mother's excretion of 3-hydroxyisovaleric acid was 109 mmol/mol creatinine. These observations indicate that holocarboxylase synthetase deficiency is one more maternal metabolic disease which may lead to a positive screen in her unaffected newborn infant. They also make the point that holocarboxylase synthetase deficiency in an infant should be detectable in programmes of neonatal screening, which was not clear previously.

Outcome of three cases of untreated maternal glutaric aciduria type I

We report, for the first time, the outcome of three children born to two women with untreated glutaric aciduria type I (GA I). Isolated hypocarnitinemia in neonatal screening in one baby allowed the identification of the disease in his mother, who was undiagnosed so far and had had a previous daughter. The other baby was born to an already diagnosed mother who was not treated; newborn screening in the child reflected the metabolic state of the mother. Biochemical abnormalities returned to normal within one week. At the age of 4 months, neuroimaging showed Sylvian enlargement in both infants and bilateral temporal arachnoid cysts in one. Physical and neurological developments were normal for the three patients at ages 2 and 5 years. We conclude that long-term follow up will determine the true impact of GA I in such children.

The consequences of extended newborn screening programmes: do we know who needs treatment?

The development of an evidence base for newborn screening is especially difficult because of the rarity of disorders now detectable. One consequence of expanded newborn screening is that physicians are being called upon to manage asymptomatic babies with persistent biochemical disturbances that indicate likely enzyme deficiencies. Some of these may be very mild. There is not always agreement as to who should be treated. Particular problems are seen with disorders that were previously thought very rare but are now found frequently by newborn screening. Some of these disorders appear benign or nearly so, and in the present state of knowledge should clearly not be included in routine newborn screening panels.

Potential misdiagnosis of 3-methylcrotonyl-coenzyme A carboxylase deficiency associated with absent or trace urinary 3-methylcrotonylglycine

We report 2 patients with isolated 3-methylcrotonyl-coenzyme A carboxylase deficiency whose urine was devoid of, or contained only trace, 3-methylcrotonylglycine, the pathognomonic marker for this disorder. The first patient, a girl with trisomy 21, was detected through newborn screening with an elevated 5 carbon hydroxycarnitine species level, and the second patient came to clinical attention at the age of 5 months because of failure to thrive and developmental delay. Investigation of urinary organic acids revealed an elevated 3-hydroxyisovaleric acid level but no demonstrable 3-methylcrotonylglycine in both patients. Enzyme studies in cultured fibroblasts confirmed isolated 3-methylcrotonyl-coenzyme A carboxylase deficiency with residual activities of 5% to 7% and 12% of the median control value, respectively. Incorporation of 14C-isovaleric acid into intact fibroblasts was essentially normal, showing that the overall pathway was at least partially functional and potentially explaining the absence of 3-methylcrotonylglycine in urine. Mutation analysis of the MCCA and MCCB genes revealed that both patients were compound heterozygous for a missense mutation, MCCB-c.1015G-->A (p.V339M), and a second mutation that leads to undetectable MCCB messenger (poly A+) RNA. Absent or trace 3-methylcrotonylglycine levels in urine raises the potential for misdiagnosis in the clinical biochemical genetics laboratory based solely on urine organic acid analysis using combined gas chromatography-mass spectrometry.

Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency: population heterogeneity ofMCCA andMCCB mutations and impact on risk assessment

New technology enables expansion of newborn screening (NBS) of inborn errors aimed to prevent adverse outcome. In conditions with a large share of asymptomatic phenotypes, the potential harm created by NBS must carefully be weighed against benefit. Policies vary throughout the United States, Australia, and Europe due to limited data on outcome and treatability of candidate screening conditions. We elaborated the rationale for decision making in 3-methylcrotonyl-coenzyme A (CoA) carboxylase deficiency (MCCD), which afflicts leucine catabolism, with reported outcomes ranging from asymptomatic to death. In Bavaria, we screened 677,852 neonates for 25 conditions, including MCCD, based on elevated concentrations of 3-hydroxyisovalerylcarnitine (3-HIVA-C). Genotypes of MCCA (MCCC1) and MCCB (MCCC2) were assessed in identified newborns, their relatives, and in individuals (n = 17) from other regions, and correlated to biochemical and clinical phenotypes. NBS revealed eight newborns and six relatives with MCCD, suggesting a higher frequency than previously assumed (1:84,700). We found a strikingly heterogeneous spectrum of 22 novel and eight reported mutations. Allelic variants were neither related to biochemical nor anamnestic data of our probands showing all asymptomatic or benign phenotypes. Comparative analysis of case reports with NBS data implied that only few individuals (< 10%) develop symptoms. In addition, none of the symptoms reported so far can clearly be attributed to MCCD. MCCD is a genetic condition with low clinical expressivity and penetrance. It largely represents as nondisease. So far, there are no genetic or biochemical markers that would identify the few individuals potentially at risk for harmful clinical expression. The low ratio of benefit to harm was pivotal to the decision to exclude MCCD from NBS in Germany. MCCD may be regarded as exemplary of the ongoing controversy arising from the inclusion of potentially asymptomatic conditions, which generates a psychological burden for afflicted families and a financial burden for health care systems.

Methylcrotonyl-CoA carboxylase (MCC) deficiency associated with severe muscle pain and physical disability in an adult

We present a patient with methylcrotonyl-CoA carboxylase (MCC) deficiency (McKusick 210200) who suffered from severe muscle pain and physical disability, and propose that this disorder be considered in the differential diagnosis of adult patients presenting with muscle pain and weakness.

Isolated 3-methylcrotonyl-CoA carboxylase deficiency: evidence for an allele-specific dominant negative effect and responsiveness to biotin therapy

Deficiency of 3-methylcrotonyl-CoA carboxylase (MCC) results in elevated excretion of 3-methylcrotonylglycine (3-MCG) and 3-hydroxyisovaleric acid (3-HIVA). MCC is a heteromeric mitochondrial enzyme comprising biotin-containing alpha subunits and smaller beta subunits, encoded by MCCA and MCCB, respectively. Mutations in these genes cause isolated MCC deficiency, an autosomal recessive disorder with a variable phenotype that ranges from severe neonatal to asymptomatic adult forms. No reported patients have responded to biotin therapy. Here, we describe two patients with a biochemical and, in one case, clinical phenotype of MCC deficiency, both of whom were responsive to biotin. The first patient presented at 3 months with seizures and progressive psychomotor retardation. Metabolic investigation at 2 years revealed elevated excretion of 3-MCG and 3-HIVA, suggesting MCC deficiency. High-dose biotin therapy was associated with a dramatic reduction in seizures, normalization of the electroencephalogram, and correction of the organic aciduria, within 4 weeks. MCC activity in fibroblasts was 25% of normal levels. The second patient, a newborn detected by tandem-mass-spectrometry newborn screening, displayed the same biochemical phenotype and remained asymptomatic with biotin up to the age of 18 months. In both patients, sequence analysis of the complete open reading frames of MCCA and MCCB revealed heterozygosity for MCCA-R385S and for the known polymorphic variant MCCA-P464H but revealed no other coding alterations. MCCA-R385S is unusual, in that it has a normal amount of MCC alpha protein but confers no MCC activity. We show that MCCA-R385S, but not other MCCA missense alleles, reduces the MCC activity of cotransfected MCCA-wild-type allele. Our results suggest that MCCA-R385S is a dominant negative allele and is biotin responsive in vivo.

Fungal Metabolic Model for 3-Methylcrotonyl-CoA Carboxylase Deficiency

Aspergillus nidulans is able to use Leu as the sole carbon source through a metabolic pathway leading to acetyl-CoA and acetoacetate that is homologous to that used by humans. mccA and mccB, the genes encoding the subunits of 3-methylcrotonyl-CoA carboxylase, are clustered with ivdA encoding isovaleryl-CoA dehydrogenase, a third gene of the Leu catabolic pathway, on the left arm of chromosome III. Their transcription is induced by Leu and other hydrophobic amino acids and repressed by glucose. Phenotypically indistinguishable DeltamccA, DeltamccB, and DeltamccA DeltamccB mutations prevent growth on Leu but not on lactose or other amino acids, formally demonstrating in vivo the specific involvement of 3-methylcrotonyl-CoA carboxylase in Leu catabolism. Growth of mcc mutants on lactose plus Leu is impaired, indicating that Leu metabolite(s) accumulation resulting from the metabolic block is toxic. Human patients carrying loss-of-function mutations in the genes encoding the subunits of 3-methylcrotonyl-CoA carboxylase suffer from methylcrotonylglycinuria. Gas chromatography/mass spectrometry analysis of culture supernatants revealed that fungal Deltamcc strains accumulate 3-hydroxyisovaleric acid, one of the diagnostic compounds in the urine of these patients, illustrating the remarkably similar consequences of equivalent genetic errors of metabolism in fungi and humans. We use our fungal model(s) for methylcrotonylglycinuria to show accumulation of 3-hydroxyisovalerate on transfer of 3-methylcrotonyl-CoA carboxylase-deficient strains to the isoprenoid precursors acetate, 3-hydroxy-3-methylglutarate, or mevalonate. This represents the first reported genetic evidence for the existence of a metabolic link involving 3-methylcrotonyl-CoA carboxylase between isoprenoid biosynthesis and Leu catabolism, providing additional support to the mevalonate shunt proposed previously (Edmond, J., and Popják, G. (1974) J. Biol. Chem. 249, 66-71).

Pediatric medicine and the genetic disorders of the Amish and Mennonite people of Pennsylvania

The Clinic for Special Children in Lancaster County, Pennsylvania, is a community-supported, nonprofit pediatric medical practice for Amish and Mennonite children who have genetic disorders. Over a 14-year period, 1988-2002, we have encountered 39 heritable disorders among the Amish and 23 among the Mennonites. We emphasize early recognition and long-term medical care of children with genetic conditions. In the clinic laboratory we perform amino acid analyses by high-performance liquid chromatography (HPLC), organic acid analyses by gas chromatography/mass spectrometry (GC/MS), and molecular diagnoses and carrier tests by polymerase chain reaction (PCR) amplification and sequencing or restriction digestion. Regional hospitals and midwives routinely send whole-blood filter paper neonatal screens for tandem mass spectrometry and other modern analytical methods to detect 14 of the metabolic disorders found in these populations as part of the NeoGen Inc. Supplemental Newborn Screening Program (Pittsburgh, PA). Medical care based on disease pathophysiology reduces morbidity, mortality, and costs for the majority of disorders. Among our patients who are homozygous for the same mutation, differences in disease severity are not unusual. Clinical problems typically arise from the interaction of the underlying genetic disorder with common infections, malnutrition, injuries, and immune dysfunction that act through classical pathophysiological disease mechanisms to influence the natural history of disease.

Expanded newborn screening

The advent of expanded newborn screening has resulted in huge advances in the ability to detect presymptomatic infants with a large number of inborn errors of metabolism. Widespread implementation of this type of screening will ultimately lead to reduced mortality and morbidity from these diseases and also will increase our knowledge of the frequency and phenotypic variability for each of them.

Screening newborns for inborn errors of metabolism by tandem mass spectrometry

BACKGROUND

The recent development of electrospray tandem mass spectrometry makes it possible to screen newborns for many rare inborn errors of metabolism, but the efficacy and outcomes of screening remain unknown. We examined the effect of the screening of newborns by tandem mass spectrometry on the rates of diagnosis of 31 disorders.

METHODS

We compared the rates of detection of 31 inborn errors affecting the metabolism of the urea cycle, amino acids, and organic acids and fatty-acid oxidation among 362,000 newborns screened by tandem mass spectrometry over a four-year period (April 1998 through March 2002) with the rates in six preceding four-year birth cohorts in New South Wales and the Australian Capital Territory, Australia, where screening, diagnostic, and clinical services were centralized.

RESULTS

The overall prevalence of disorders during the periods when clinical diagnosis was used did not vary between 1982 and 1998. In the cohort screened with tandem mass spectrometry, the prevalence of inborn errors, excluding phenylketonuria, was 15.7 per 100,000 births (95 percent confidence interval, 11.9 to 20.4), as compared with adjusted rates of 8.6 to 9.5 per 100,000 births in the four preceding four-year cohorts. Of the 57 cases diagnosed after the introduction of newborn screening, 15 were diagnosed clinically; 7 of the 15 newborns had a normal result on screening. The rate of detection was increased specifically for medium-chain acyl-coenzyme A dehydrogenase deficiency (P<0.001) and other disorders of fatty-acid oxidation (P=0.007), as compared with the 16-year period before the implementation of neonatal screening for these disorders.

CONCLUSIONS

More cases of inborn errors of metabolism are diagnosed by screening with tandem mass spectrometry than are diagnosed clinically. It is not yet clear which patients with disorders diagnosed by such screening would have become symptomatic if screening had not been performed.

Evaluation of 3-methylcrotonyl-CoA carboxylase deficiency detected by tandem mass spectrometry newborn screening

Since the addition of tandem mass spectrometry (MS/MS) to the North Carolina Newborn Screening Program, 20 infants with two consecutive elevated 3-hydroxyisovalerylcarnitine (C5OH) levels have been evaluated for evidence of inborn errors of metabolism associated with this metabolite. Ten of these 20 infants had significant concentrations of both 3-hydroxyisovaleric acid and 3-methylcrotonylglycine in their urine, suggestive of 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency. Four of these 10 were infants whose abnormal metabolites were found to be of maternal origin. Of 8 patients with probable 3-MCC deficiency, 7 have been tested and found to have the enzyme deficiency confirmed in lymphoblasts or cultured fibroblasts; one of these 7 infants had only marginally decreased 3-MCC activity in lymphocytes but deficient 3-MCC in fibroblasts. We estimate the incidence of 3-MCC deficiency at 1:64000 live births in North Carolina. We conclude that MS/MS newborn screening will detect additional inborn errors of metabolism, such as 3-MCC deficiency, not traditionally associated with newborn screening. The evaluation of newborns with two abnormally elevated C5OH levels on MS/MS newborn screening should include, at least, urine organic acid analysis by capillary GC-MS and a plasma acylcarnitine profile by MS/MS. Long-term follow-up is needed to determine the outcome of presymptomatically diagnosed patients with 3-MCC deficiency by MS/MS newborn screening.

The application of tandem mass spectrometry to neonatal screening for inherited disorders of intermediary metabolism

This review is intended to serve as a practical guide for geneticists to current applications of tandem mass spectrometry to newborn screening. By making dried-blood spot analysis more sensitive, specific, reliable, and inclusive, tandem mass spectrometry has improved the newborn detection of inborn errors of metabolism. Its innate ability to detect and quantify multiple analytes from one prepared blood specimen in a single analysis permits broad recognition of amino acid, fatty acid, and organic acid disorders. An increasing number of newborn screening programs are either utilizing or conducting pilot studies with tandem mass spectrometry. It is therefore imperative that the genetics community be familiar with tandem mass spectrometric newborn screening.

Biotin in metabolism and molecular biology

Biotin is a water-soluble vitamin required by all organisms by virtue of its essential role in carboxylation reactions. Although the metabolism and role of biotin in intermediary metabolism are well established, biotin remains one of the most poorly understood water-soluble vitamins in terms of nutritional requirements and responsiveness to physiological and pharmacological states. Significant advances in the understanding of biotin nutriture have been recently accomplished through the description of the kinetics and regulation of biotin transport and improved methods for biotin status assessment. Additionally, the potential role of biotin in the regulation of gene expression has been strengthened through description of altered gene expression during biotin deficiency and through newly described enzymatic activities of the enzyme biotinidase. Given mounting evidence of suboptimum biotin status, a more complete understanding of these aspects of biotin should lead to a greater appreciation of the ways in which biotin aids in the maintenance of health.

Application of tandem mass spectrometry to biochemical genetics and newborn screening

Tandem mass spectrometry (MS/MS) has become a key technology in the fields of biochemical genetics and newborn screening. The development of electrospray ionisation (ESI) and associated automation of sample handling and data manipulation have allowed the introduction of expanded newborn screening for disorders which feature accumulation of acylcarnitines and certain amino acids in a number of programs worldwide. In addition, the technique has proven valuable in several areas of biochemical genetics including quantification of carnitine and acylcarnitines, in vitro studies of metabolic pathways (in particular beta-oxidation), and diagnosis of peroxisomal and lysosomal disorders. This review covers some of the basic theory of MS/MS and focuses on the practical application of the technique in these two interrelated areas.

Human biotin-containing subunit of 3-methylcrotonyl-CoA carboxylase gene (MCCA): cDNA sequence, genomic organization, localization to chromosomal band 3q27, and expression

3-Methylcrotonyl-CoA carboxylase (MCCase; EC 6.4.1.4) is a mitochondrial biotin enzyme and plays an essential role in the catabolism of leucine and isovalerate in animals, bacterial species, and plants. MCCase consists of two subunits, those that are biotin-containing and non-biotin-containing. The genes responsible for these subunits have been isolated in soybean, Arabidopsis thaliana, and tomatoes, but not in mammals. In humans, MCCase deficiency has been thought to be a rare metabolic disease, but the number of patients with MCCase deficiency appears to be increasing with a wide range of clinical presentations, some that result in a lethal condition and others that are asymptomatic. In this report, we have isolated and carried out chromosomal mapping of the gene for the biotin-containing subunit (A subunit) of the human MCCase gene, MCCA. The cDNA predicts an open reading frame coding for a 725-amino-acid protein with mitochondrial signal peptide, biotin carboxylase, and biotin-carrier domains. The gene is composed of at least 19 exons and covers more than 70 kb of sequence on band q27 of chromosome 3. MCCA was abundantly expressed in mitochondria-rich organs, such as the heart, skeletal muscles, kidney, and liver. In exon 13, we observed a His/Pro polymorphism at codon 464 (an A to C transition at nucleotide position 1391 in the cDNA sequence). Then, we determined the DNA sequences of the 5' untranslated region and entire coding regions in two patients with MCCase deficiency, but no sequence substitution was detected, suggesting that the gene mutations might be in the non-biotin-containing subunit (B subunit) gene, MCCB, in these patients.

The molecular basis of human 3-methylcrotonyl-CoA carboxylase deficiency

Isolated biotin-resistant 3-methylcrotonyl-CoA carboxylase (MCC) deficiency is an autosomal recessive disorder of leucine catabolism that appears to be the most frequent organic aciduria detected in tandem mass spectrometry-based neonatal screening programs. The phenotype is variable, ranging from neonatal onset with severe neurological involvement to asymptomatic adults. MCC is a heteromeric mitochondrial enzyme composed of biotin-containing alpha subunits and smaller beta subunits. Here, we report cloning of MCCA and MCCB cDNAs and the organization of their structural genes. We show that a series of 14 MCC-deficient probands defines two complementation groups, CG1 and 2, resulting from mutations in MCCB and MCCA, respectively. We identify five MCCA and nine MCCB mutant alleles and show that missense mutations in each result in loss of function.

The molecular basis of 3-methylcrotonylglycinuria, a disorder of leucine catabolism

3-Methylcrotonylglycinuria is an inborn error of leucine catabolism and has a recessive pattern of inheritance that results from the deficiency of 3-methylcrotonyl-CoA carboxylase (MCC). The introduction of tandem mass spectrometry in newborn screening has revealed an unexpectedly high incidence of this disorder, which, in certain areas, appears to be the most frequent organic aciduria. MCC, an heteromeric enzyme consisting of alpha (biotin-containing) and beta subunits, is the only one of the four biotin-dependent carboxylases known in humans that has genes that have not yet been characterized, precluding molecular studies of this disease. Here we report the characterization, at the genomic level and at the cDNA level, of both the MCCA gene and the MCCB gene, encoding the MCC alpha and MCC beta subunits, respectively. The 19-exon MCCA gene maps to 3q25-27 and encodes a 725-residue protein with a biotin attachment site; the 17-exon MCCB gene maps to 5q12-q13 and encodes a 563-residue polypeptide. We show that disease-causing mutations can be classified into two complementation groups, denoted "CGA" and "CGB." We detected two MCCA missense mutations in CGA patients, one of which leads to absence of biotinylated MCC alpha. Two MCCB missense mutations and one splicing defect mutation leading to early MCC beta truncation were found in CGB patients. A fourth MCCB mutation also leading to early MCC beta truncation was found in two nonclassified patients. A fungal model carrying an mccA null allele has been constructed and was used to demonstrate, in vivo, the involvement of MCC in leucine catabolism. These results establish that 3-methylcrotonylglycinuria results from loss-of-function mutations in the genes encoding the alpha and beta subunits of MCC and complete the genetic characterization of the four human biotin-dependent carboxylases.