Xlinked sideroblastic anemia with ataxia

This pyridoxine-resistant form of SA with congenital cerebel-lar ataxia has been described in two families. It is due to missense mutations of the human adenosine triphosphate binding cassette 7 (hABC7) gene, which maps to Xq1.3-q13.3. ABC7 belongs to a wider family of proteins involved in the transportation of substrates across cell and organelle membranes. As studies of its yeast ortholog indicate, hABC7 is involved in the transportation of clusters of Fe-Su (iron-sulfur) across the mitochondrial membrane to the cytosol. These clusters are required for the functional maturation of a number of

4.9 kb deletion

ATPase8 » CO3 ATPase6

4.9 kb deletion

ATPase8 » CO3 ATPase6

OH, OL origin and direction of synthesis of heavy and light strands

^ PH, PL origin and direction of transcription of heavy and light strands

I I Genes encoding proteins

tRNA genes

Fig. 12.3 The human mitochondrial genome

Both replication and transcription of the heavy (H) and light (L) strands of mtDNA run in opposite directions, as indicated in the figure. The products of transcription are large, and are cleaved to generate RNAs for individual genes. Note the absence of introns and close apposition of genes. ATPase 8 and ATPase 6 overlap. The approximate location of the common 4.9 kb deletion (see text) is indicated by the broken line. nND1-ND6 encode NADH dehydrogenase subunits; CO1-CO3 encode cytochrome c oxidase subunits and CYB encodes cytochrome b. Modified from Strachan T, Read AP. (1996) Human Molecular Genetics, 1st edn. BIOS.

proteins. Mutations of hABC7 interrupt this process and lead to iron accumulation in the mitochondrion.

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