HETEROPLASMIC POINT MUTATIONS OF MITOCHONDRIAL-DNA AFFECTING SUBUNIT-I OF CYTOCHROME-C-OXIDASE IN 2 PATIENTS WITH ACQUIRED IDIOPATHIC SIDEROBLASTIC ANEMIA

Mitochondrial iron overload in acquired idiopathic sideroblastic anemi a (AISA) may be attributable to mutations of mitochondrial DNA (mtDNA) , because these can cause respiratory chain dysfunction, thereby impai ring reduction of ferric iron (Fe3+) to ferrous iron (Fe2+). The reduc ed form of iron is essential to the last step of mitochondrial heme bi osynthesis. It is not yet understood to which part of the respiratory chain the reduction of ferric iron is linked. In two patients with AIS A we identified point mutations of mtDNA affecting the same transmembr ane helix within subunit I of cytochrome c oxidase (COX I; ie, complex IV of the respiratory chain). The mutations were detected by restrict ion fragment length polymorphism analysis and temperature gradient gel electrophoresis. One of the mutations involves a T --> C transition i n nucleotide position 6742, causing an amino acid change from methioni ne to threonine. The other mutation is a T --> C transition at nt 6721 , changing isoleucine to threonine. Both amino acids are highly conser ved in a wide range of species. Both mutations are heteroplasmic, ie, they establish a mixture of normal and mutated mitochondrial genomes, which is typical of disorders of mtDNA. The mutations were present in bone marrow and whole blood samples, in isolated platelets, and in gra nulocytes, but appeared to be absent from T and B lymphocytes purified by immunomagnetic bead separation. They were not detected in buccal m ucosa cells obtained by mouthwashes and in cultured skin fibroblasts e xamined in one of the patients. In both patients, this pattern of invo lvement suggests that the mtDNA mutation occurred in a self-renewing b one marrow stem cell with myeloid determination. Identification of two point mutations with very similar location suggests that cytochrome c oxidase plays an important role in the pathogenesis of AISA. COX may be the physiologic site of iron reduction and transport through the in ner mitochondrial membrane. (C) 1997 by The American Society of Hemato logy.