Juvenile polyposis, hereditary hemorrhagic telangiectasia, and early onset colorectal cancer in patients with SMAD4 mutation.

BACKGROUND: Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant disorder most often caused by mutation in the endoglin or ALK1 genes. A distinct syndrome combines the clinical features of HHT and juvenile polyposis (JP) and has been associated with SMAD4 mutation. The aim of this study was to describe the phenotype of patients with JP-HHT and SMAD4 mutations and to compare this phenotype with HHT or JP patients with mutations other than SMAD4. METHODS: Patients prospectively enrolled in the Toronto HHT and JP databases who underwent genotyping were included. The phenotypic characteristics of JP-HHT patients with SMAD4 mutations and patients with mutations other than SMAD4 were analyzed and compared. RESULTS: Three hundred and fifty-eight patients underwent genetic testing (HHT, n = 332; JP, n = 26). Among fourteen patients identified with SMAD4 mutations, ten met the clinical diagnostic criteria for both JP and HHT (71%). Patients with SMAD4 mutations had 100% penetrance of the polyposis phenotype. All patients with JP and SMAD4 mutation had features of HHT. Three JP-HHT patients developed early onset colorectal cancer (CRC) (mean age 28 years). JP-HHT patients with SMAD4 mutation had a significantly higher rate of anemia than HHT patients with mutations other than SMAD4. CONCLUSIONS: Patients with HHT and SMAD4 mutations are at significant risk of JP and CRC. The gastrointestinal phenotype is similar to JP patients without SMAD4 mutation. It is essential for HHT patients to undergo genetic testing to determine if they have SMAD4 mutations so that appropriate gastrointestinal screening and surveillance for JP and CRC can be completed.

Atypical methylmalonic aciduria: frequency of mutations in the methylmalonyl CoA epimerase gene (MCEE).

Methylmalonic aciduria is known to result from defects in the enzyme methylmalonyl CoA mutase (MCM) (mut complementation group) and from defects in the synthesis of the MCM cofactor adenosylcobalamin (cblA, cblB, cblC, cblD, and cblF groups). Two patients who excrete methylmalonic acid have recently been shown to have a homozygous nonsense mutation in the gene coding for methylmalonyl CoA epimerase (MCEE). To further understand the cause of methylmalonic acid excretion, the MCEE gene was sequenced in 229 patients with elevations of methylmalonic acid excretion for which no cause was known. Mutations in MCEE were detected in five patients: two patients homozygous for c.139C>T, p.R47X, one patient homozygous for c.178A>C, p.K60Q, and two patients heterozygous for c.427C>T, p.R143C. Fusion of fibroblast lines from two patients homozygous for c.139C>T, p.R47X did not result in correction of [(14)C]propionate incorporation toward control values while the defect in these fibroblasts was complemented by mut, cblA, and cblB fibroblasts. Infection with wild-type MCEE cDNA resulted in correction of the biochemical phenotype in cells from both patients.

Homozygous nonsense mutation in the MCEE gene and siRNA suppression of methylmalonyl-CoA epimerase expression: a novel cause of mild methylmalonic aciduria.

Methylmalonyl-CoA epimerase (MCE) catalyzes the interconversion of D- and L-methylmalonyl-CoA in the pathway responsible for the degradation of branched chain amino acids, odd chain-length fatty acids, and other metabolites. Despite the occurrence of metabolic disorders in the enzymatic step occurring immediately upstream of MCE (propionyl-CoA carboxylase) and downstream of MCE (adenosylcobalamin-dependent methylmalonyl-CoA mutase), no disease-causing mutations have been described affecting MCE itself. A patient, formerly identified as belonging to the cblA complementation group of vitamin B12 disorders but lacking mutations in the affected gene, MMAA, was tested for mutations in the MCEE gene. The patient's fibroblasts had normal levels of adenosylcobalamin compared to controls, whereas other cblA cell lines typically had reduced levels of the cofactor. As well, this patient had a milder form of methylmalonic aciduria than usually observed in cblA patients. The patient was found to be homozygous for a c.139C>T (p.R47X) mutation in MCEE by sequence analysis that was confirmed by restriction digestion of PCR products. One sibling, also with mild methylmalonic aciduria, was homozygous for the mutation. Both parents and one other sibling were heterozygous. A nearby insertion polymorphism, c.41-160_161insT, heterozygous in both parents, showed the wild-type configuration on the mutant alleles. To assess the impact of isolated MCE deficiency in cultured cells, HeLa cells were transfected with a selectable vector containing MCEE-specific small interfering RNA (siRNA) to suppress gene expression. The reduced level of MCEE mRNA resulted in the reduction of [14C]-propionate incorporation into cellular macromolecules. However, siRNA only led to a small reduction in pathway activity, suggesting that previously postulated non-enzymatic conversion of D- to L-methylmalonyl-CoA may contribute to some flux through the pathway. We conclude that the patient's MCEE defect was responsible for the mild methylmalonic aciduria, confirming a partial requirement for the enzymatic activity in humans.

Mutation and biochemical analysis of patients belonging to the cblB complementation class of vitamin B12-dependent methylmalonic aciduria.

Methylmalonic aciduria, cblB type (OMIM 251110) is an inborn error of vitamin B(12) metabolism that occurs due to mutations in the MMAB gene. MMAB encodes the enzyme ATP:cobalamin adenosyltransferase, which catalyzes the synthesis of the coenzyme adenosylcobalamin required for the activity of the mitochondrial enzyme methylmalonyl CoA mutase (MCM). MCM catalyzes the isomerization of methylmalonyl CoA to succinyl CoA. Deficient MCM activity results in methylmalonic aciduria and a susceptibility to life-threatening acidotic crises. The MMAB gene was sequenced from genomic DNA from a panel of 35 cblB patients, including five patients previously investigated. Nineteen MMAB mutations were identified, including 13 previously unknown mutations. These included 11 missense mutations, two duplications, one deletion, four splice-site mutations, and one nonsense mutation. None of these mutations was identified in 100 control alleles. Most of the missense mutations (9/11) were clustered in exon 7; many of these affected amino acid residues that are part of the probable active site of the enzyme. One previously described mutation, c.556C >T (p.R186W), was particularly common, accounting for 33% of pathogenic alleles. It was seen almost exclusively in patients of European background and was typically associated with presentation in the first year of life.