The first family with what is now called X-linked SA was reported by T. Cooley in 1945. Subsequently, more families with apparent X-linked sideroblastic anemia were described. The observation that some affected males responded to pharmacological doses of pyridoxine focussed attention on the erythroid-specific ALAS: indeed, the in vitro activity of this enzyme was invariably reduced. The erythroid-specific ALAS (ALAS2) gene was subsequently cloned (Figure 12.2) and mapped to Xp11.12. This locus is subject to X-chromosome inactivation and as a result female relatives of affected males may demonstrate red cell mosaicism; that is, two populations of red cells, one hypochromic and one normochromic. In general, these women do not have anemia and therefore the disease is regarded as recessive. However, full-blown SA may develop in women, especially late in life, as a result of a skewed X-inactivation pattern: in such cases we would have to regard the same disease as dominant.
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