Methylmalonic aciduria and homocystinuria, cblC type (OMIM 277400), is the most common inborn error of vitamin B(12) (cobalamin) metabolism, with about 250 known cases. Affected individuals have developmental, hematological, neurological, metabolic, ophthalmologic and dermatologic clinical findings. Although considered a disease of infancy or childhood, some individuals develop symptoms in adulthood. The cblC locus was mapped to chromosome region 1p by linkage analysis. We refined the chromosomal interval using homozygosity mapping and haplotype analyses and identified the MMACHC gene. In 204 individuals, 42 different mutations were identified, many consistent with a loss of function of the protein product. One mutation, 271dupA, accounted for 40% of all disease alleles. Transduction of wild-type MMACHC into immortalized cblC fibroblast cell lines corrected the cellular phenotype. Molecular modeling predicts that the C-terminal region of the gene product folds similarly to TonB, a bacterial protein involved in energy transduction for cobalamin uptake.

Mutations in the MTR gene, which encodes methionine synthase on human chromosome 1p43, result in the methylcobalamin deficiency G (cblG) disorder, which is characterized by homocystinuria, hyperhomocysteinemia, and hypomethioninemia. To investigate the molecular basis of the disorder, we have characterized the structure of the MTR gene, thereby identifying exon-intron boundaries. This enabled amplification of each of the 33 exons of the gene, from genomic DNA from a panel of 21 patients with cblG. Thirteen novel mutations were identified. These included five deletions (c.12-13delGC, c.381delA, c.2101delT, c.2669-2670delTG, and c.2796-2800delAAGTC) and two nonsense mutations (R585X and E1204X) that would result in synthesis of truncated proteins that lack portions critical for enzyme function. One mutation was identified that resulted in conversion of A to C of the invariant A of the 3' splice site of intron 9. Five missense mutations (A410P, S437Y, S450H, H595P, and I804T) were identified. The latter mutations, as well as the splice-site mutation, were not detected in a panel of 50 anonymous DNA samples, suggesting that these sequence changes are not polymorphisms present in the general population. In addition, a previously described missense mutation, P1173L, was detected in 16 patients in an expanded panel of 24 patients with cblG. Analysis of haplotypes constructed using sequence polymorphisms identified within the MTR gene demonstrated that this mutation, a C-->T transition in a CpG island, has occurred on at least two separate genetic backgrounds.

Cultured diploid fibroblasts from a patient with a previously undescribed inborn error of cobalamin metabolism accumulate unmetabolized, nonprotein-bound vitamin B12 in lysosomes. These cells are able to endocytose the transcobalamin II-B12 complex and to release B12 from transcobalamin II. The freed vitamin B12 is not released from lysosomes into the cytoplasm of the cell. This suggests that there is a specific lysosomal transport mechanism for vitamin B12 in the human.

. Homocysteine levels in A/J and C57BL/6J mice: genetic, diet, gender, and parental effects. Physiol Genomics 21: [404][405][406][407][408][409][410] 2005. First published March 1, 2005; 10.1152/physiolgenomics.00199.2004.-Increased levels of homocysteine in the blood have been associated with various birth defects and adult diseases. However, the extent to which genetic factors control homocysteine levels in healthy individuals is unclear. Laboratory mice are valuable models for dissecting the genetic and environmental controls of total homocysteine (tHcy) levels. We assessed the inheritance of tHcy levels in two inbred strains, A/J and C57BL/6J (B6), under controlled physiological conditions and assessed the relative importance of genetic, diet, gender, and parental effects. Diet affected mean tHcy levels, whereas gender affected both the mean and variance of tHcy levels. Moreover, gender of the parents influenced mean tHcy levels in reciprocal F1 hybrids, suggesting maternal effects. Finally, gene-diet interactions affected heritability of mean tHcy levels. These studies showed that each of these factors contributes to tHcy levels and provided important clues to understanding homocysteine homeostasis in humans.genetics; folate; gene environment HYPERHOMOCYSTEINEMIA, or elevated levels of total homocysteine (tHcy) in the blood, is an independent risk factor for neural tube defects (NTDs) (27a, 37), vascular disease (4,43,46), and Alzheimer's disease (36) and is often associated with diseases such as colon cancer (20), osteoporosis (25, 41), and Down syndrome (3, 16). Although it is unclear whether elevated tHcy levels are a cause or consequence of disease, at least one meta-analysis suggests that hyperhomocysteinemia causes cardiovascular disease (43).Normal tHcy blood levels range from 5 to 12 mol/l (31), and hyperhomocysteinemia is classified into three categories: mild (12-30 mol/l), moderate (30-100 mol/l), and severe (Ͼ100 mol/l) (17, 31). Several environmental and genetic factors affect tHcy levels in the blood. Environmental factors include age (tHcy increases with age), gender (women tend to have lower levels than men), and diet (intake of vitamin cofactors such as folate and vitamin B 6 and B 12 usually reduces tHcy levels) (26). Deficiencies in genes involved in homocysteine and folate metabolism result in hyperhomocysteinemia: these genes include cystathionine ␤-synthase (CBS; Refs. Ref. 9,11,19,21).Homocysteine and folate pathways are tightly linked, and their regulation is complex. tHcy and folate levels are usually inversely related (34). Homocysteine, a sulfur-containing amino acid, is remethylated through the trans-methylation pathway or irreversibly catabolized through the trans-sulfuration pathway (29) and contributes to methylation of DNA, proteins, and lipids and indirectly to glucose metabolism and citric acid cycle. Folate metabolism manages the major pool of single carbon units and participates in purine and pyrimidine biosynthesis and amino acid metabolism (34).Because the levels of h...