Cryptic exon activation by disruption of exon splice enhancer: novel mechanism causing 3-methylcrotonyl-CoA carboxylase deficiency.

3-Methylcrotonyl-CoA carboxylase (MCC) deficiency is an autosomal recessive disorder of leucine catabolism. MCC is a heteromeric mitochondrial enzyme composed of biotin-containing alpha (MCCA) and smaller beta (MCCB) subunits encoded by MCCA and MCCB, respectively. We report studies of the c.1054G-->A mutation in exon 11 of MCCB detected in the homozygous state in a patient with MCC deficiency. Sequence analysis of MCCB cDNA revealed two overlapping transcripts, one containing the normal 73 bp of exon 11 including the missense mutation c.1054G-->A (p.G352R), the other with exon 11 replaced by a 64-bp sequence from intron 10 (cryptic exon 10a) that maintains the reading frame and is flanked by acceptable splice consensus sites. In expression studies, we show that both transcripts lack detectable MCC activity. Western blot analysis showed slightly reduced levels of MCCB using the transcript containing the missense mutation, whereas no MCCB was detected with the transcript containing the cryptic exon 10a. Analysis of the region harboring the mutation revealed that the c.1054G-->A mutation is located in an exon splice enhancer sequence. Using MCCB minigene constructs to transfect MCCB-deficient fibroblasts, we demonstrate that the reduction in utilization of exon 11 associated with the c.1054G-->A mutation is due to alteration of this exon splice enhancer. Further, we show that optimization of the weak splice donor site of exon 11 corrects the splicing defect. To our knowledge, this is the first demonstration of a point mutation disrupting an exon splice enhancer that causes exon skipping along with utilization of a cryptic exon.

Novel splice site mutations in the gamma glutamyl carboxylase gene in a child with congenital combined deficiency of the vitamin K-dependent coagulation factors (VKCFD).

Congenital combined deficiency of the vitamin K-dependent coagulation factors is a rare bleeding disorder caused by either a defect in the gamma-glutamyl carboxylase or the vitamin K epoxide reductase enzyme complex. The diagnosis should be considered when vitamin-K dependent factor activities are decreased and liver dysfunction, vitamin K deficiency, and factitious coumarin ingestion have been excluded. We report a case of VKCFD in a child resulting from compound heterozygosity for two novel splice site mutations of the gamma-glutamyl carboxylase gene. Oral vitamin K supplementation resulted in partial resolution of proteins and complete resolution of bleeding.

[Infant boy with propionic acidemia: anesthetic implications]. / Implicaciones anestésicas en un niño afecto de acidemia propiónica.

A 12-month-old boy diagnosed with propionic acidemia underwent gastrostomy. The patient's general state was good and he was alert, but with reduced muscular tone (unstable when seated with support, floppy head) and with dystonic movements in all extremities. An electroencephalogram showed slightly slowed brain activity. The patient was being treated with a low protein diet, phenobarbital, L-carnitine, L-isoleucine, and biotin. Surgery was carried out in satisfactory conditions with general anesthesia without opioids combined with infiltration of the surgical wound with local anesthetic. Recovery from anesthesia was rapid and free of complications. Propionic acidemia is caused by mitochondrial propionyl coenzyme carboxylase deficiency. Most patients have episodes of severe metabolic ketoacidosis as a result of excessive protein intake, delayed development, vomiting, gastroesophageal reflux, lethargy, hypotonia, and convulsions. The anesthetic approach involves avoiding triggers of metabolic acidosis (such as fasting, dehydration, hypoxemia, and hypotension) and preventing airway complications. Agents that metabolize propionic acid (such as succinylcholine, benzylisoquinoline neuromuscular blocking agents, and propofol) are not used, as they can exacerbate acidemia. We also believe that using local or regional anesthesia in combination with general anesthesia without opiates is safe and effective for controlling pain during surgery and postoperative recovery, as that combination avoids respiratory depression in these patients, who are highly sensitive to opiates.

Implicaciones anestésicas en un niño afecto de acidemia propiónica / Infant boy with propionic acidemia: anesthetic implications

Un niño de 12 meses de edad con diagnóstico de acidemia propiónica fue intervenido para gastrostomía. El paciente presentaba buen estado general y sensorio despejado, tono muscular disminuido, sedestación inestable con apoyo, sostén cefálico incompetente y movimientos distónicos de las cuatro extremidades. El EEG mostraba una actividad bioeléctrica cerebral discretamente enlentecida. El paciente estaba siendo tratado con dieta hipoproteica, fenobarbital, L-carnitina, L-isoleucina y biotina. La cirugía fue realizada bajo anestesia general sin opioides combinada con infiltración de la herida quirúrgica con anestésico local, que proporcionó condiciones quirúrgicas satisfactorias y una recuperación de la anestesia rápida y sin complicaciones. La acidemia propiónica se produce como consecuencia de la actividad deficiente de la enzima mitocondrial propionilCo-A carboxilasa. La mayoría de los pacientes presentan episodios de cetoacidosis metabólica severa secundaria a la excesiva ingesta proteica, retraso del desarrollo, vómitos, reflujo gastroesofágico, letargia, hipotonía y crisis convulsivas. La conducta anestésica se dirige a evitar los precipitantes de acidosis metabólica (ayuno, deshidratación, hipoxemia e hipotensión arterial) y las complicaciones de la vía aérea, así como no utilizar agentes anestésicos que se metabolizan a ácido propiónico como la succinilcolina, bloqueantes neuromusculares bencilisoquinoleínicos y el propofol, ya que pueden contribuir a la acidemia. Además, consideramos que el empleo de anestesia locorregional combinada con anestesia general sin opioides es segura y efectiva para el control del dolor durante la intervención y en el postoperatorio, ya que evitaría la depresión respiratoria en estos pacientes con elevada sensibilidad a los opioides

A 12-month-old boy diagnosed with propionic acidemia underwent gastrostomy. The patient's general state was good and he was alert, but with reduced muscular tone (unstable when seated with support, floppy head) and with dystonic movements in all extremities. An electroencephalogram showed slightly slowed brain activity. The patient was being treated with a low protein diet, phenobarbital, L-carnitine, L-isoleucine, and biotin. Surgery was carried out in satisfactory conditions with general anesthesia without opioids combined with infiltration of the surgical wound with local anesthetic. Recovery from anesthesia was rapid and free of complications. Propionic acidemia is caused by mitochondrial propionyl coenzyme carboxylase deficiency. Most patients have episodes of severe metabolic ketoacidosis as a result of excessive protein intake, delayed development, vomiting, gastroesophageal reflux, lethargy, hypotonia, and convulsions. The anesthetic approach involves avoiding triggers of metabolic acidosis (such as fasting, dehydration, hypoxemia, and hypotension) and preventing airway complications. Agents that metabolize propionic acid (such as succinylcholine, benzylisoquinoline neuromuscular blocking agents, and propofol) are not used, as they can exacerbate acidemia. We also believe that using local or regional anesthesia in combination with general anesthesia without opiates is safe and effective for controlling pain during surgery and postoperative recovery, as that combination avoids respiratory depression in these patients, who are highly sensitive to opiates

Consanguineous 3-methylcrotonyl-CoA carboxylase deficiency: early-onset necrotizing encephalopathy with lethal outcome.

A patient with a severe neonatal variant of 3-methylcrotonyl-CoA carboxylase (MCC) deficiency is reported. The first child of healthy consanguineous Turkish parents presented on the second day of life with dehydration, cyanosis, no sucking, generalized muscular hypotonia, encephalopathy, respiratory depression requiring mechanic ventilation, macrocephaly, severe acidosis and hypoglycaemia. Elevated C5-OH-carnitine in dried blood spot by tandem MS and elevated urinary excretion of 3-hydroxyisovaleric acid and 3-methylcrotonylglycine suggested MCC deficiency, confirmed by enzyme analysis in cultured fibroblasts. Cerebral ultrasonography and cranial CT findings revealed progressive changes such as disseminated encephalomalacia, cystic changes, ventricular dilatation and cerebral atrophy. Treatment with high-dose biotin and protein-restricted diet was ineffective and the patient died at the age of 33 days with progressive neurological deterioration. Mutation analysis revealed a homozygous mutation in the splice acceptor site of intron 15 in the MCC beta-subunit. Early-onset severe necrotizing encephalopathy should be included in the differential diagnosis of isolated MCC deficiency.

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.

Functional characterization of PCCA mutations causing propionic acidemia.

Propionic acidemia (PA, MIM 232000 and 232050) is caused by a deficiency of mitochondrial biotin-dependent propionyl-CoA carboxylase (PCC, EC 6.4.1.3), a heteropolymeric enzyme composed of alpha and beta subunits, which are encoded by the PCCA and PCCB genes, respectively. The PCCA protein (alpha subunit) is responsible for the formation of carboxybiotin upon hydrolysis of ATP and contains a C-terminal biotin-binding domain and a biotin carboxylase domain, defined by homology with other biotin-dependent carboxylases, some of them characterized structurally. More than 24 mutations have been found in the PCCA gene in patients with PA, among them 14 missense mutations and one in-frame deletion, for which the precise molecular effect is unknown. In this study, we have established the pathogenicity of 11 PCCA mutations (10 missense and an in-frame deletion) by expression studies in deficient fibroblasts and in a cell-free in vitro system, and analyzed the effect of each mutation on PCC activity, protein stability and domain structure. The results show that most mutant proteins show an increased turnover and are functionally deficient, suggesting that the structural alterations they cause are incompatible with normal assembly to produce a stable, functional PCC oligomer. These results are discussed in the context of the genotype-phenotype correlations in PCCA-deficient PA patients.

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.

Cloning of the human MCCA and MCCB genes and mutations therein reveal the molecular cause of 3-methylcrotonyl-CoA: carboxylase deficiency.

3-Methylcrotonyl-CoA: carboxylase (EC 6.4.1.4; MCC) deficiency is an inborn error of the leucine degradation pathway (MIM *210200) characterized by increased urinary excretion of 3-hydroxyisovaleric acid and 3-methylcrotonylglycine. The clinical phenotypes are highly variable ranging from asymptomatic to profound metabolic acidosis and death in infancy. Sequence similarity with Glycine max and Arabidopsis thaliana genes encoding the two subunits of MCC permitted us to clone the cDNAs encoding the alpha- and beta-subunits of human MCC. The 2580 bp MCCA cDNA encodes the 725 amino acid biotin-containing alpha-subunit. The MCCA gene is located on chromosome 3q26-q28 and consists of 19 exons. The 2304 bp MCCB cDNA encodes the non-biotin-containing beta-subunit of 563 amino acids. The MCCB gene is located on chromosome 5q13 and consists of 17 exons. We have sequenced both genes in four patients with isolated biotin-unresponsive deficiency of MCC. In two of them we found mutations in the MCCA gene. Compound heterozygosity for a missense mutation (S535F) and a nonsense mutation (V694X) were identified in one patient. One heterozygous mutation (S535F) was found in another patient. The remaining two patients had mutations in the MCCB gene. One consanguineous patient was homozygous for a missense mutation (R268T). In the other we identified a missense mutation in one allele (E99Q) and allelic loss of the other. Mutations were correlated with an almost total lack of enzyme activity in fibroblasts. These data provide evidence that human MCC deficiency is caused by mutations in either the MCCA or MCCB gene.

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.