Late-infantile 3-methylcrotonyl-CoA carboxylase deficiency presenting as global developmental delay

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.

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.

Recurrent attacks of status epilepticus as predominant symptom in 3-methylcrotonyl-CoA carboxylase deficiency

A patient with isolated 3-methylcrotonyl-CoA carboxylase (MCC) deficiency with an unusual clinical presentation is described. The patient presented with clusters of seizures with two or three months disease free interval in the first year of life which then evolved into attacks of status epilepticus after the age of 12 months. MCC deficiency was suspected because of elevated C5-OH-carnitine in tandem mass spectrometry and elevated 3-hydroxy-isovaleric acid in urine organic acid analysis. Deficiency of MCC was confirmed in cultured fibroblasts and mutation analysis revealed a novel mutation in MCCB, p.S39F. Attacks of status epilepticus as a predominant symptom have not been described before in isolated MCC deficiency.

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.

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.