PYRIDOXINE-REFRACTORY CONGENITAL SIDEROBLASTIC ANEMIA WITH EVIDENCE FOR AUTOSOMAL INHERITANCE - EXCLUSION OF LINKAGE TO ALAS2 AT XP11.21 BYPOLYMORPHISM ANALYSIS

A son and daughter of unaffected parents had transfusion dependent, py ridoxine-refractory sideroblastic anaemia from birth. Their haemoglobi n levels were 4.3 and 6.4 g/dl, respectively. delta-Aminolaevulinate s ynthase activity in erythroblasts from fractionated marrow of the sist er was 135 pmol delta-aminolaevulinate formed/10(6) erythroblasts/hour (normal range = 110-650 pmol). While mutations of the erythroid-speci fic 6-aminolaevulinate synthase gene (ALAS2) at Xp11.21 have been repo rted in patients with X linked sideroblastic anaemia, sequence analysi s of the ALAS2 gene in the son did not identify any mutations in the c oding region, the intron/exon boundaries, or the 1 kb 5' promoter regi on. A useful polymorphism was found in the 3' region of the ALAS2 gene , a G to A transition, 220 nt 3' of the AATAAA polyadenylation signal. Mismatch PCR at this site and subsequent discrimination by XmnI restr iction analysis of 148 alleles identified the gene frequency of this p olymorphism to be 25%. Analysis of the inheritance of this intragenic polymorphism showed that the affected sibs received different maternal alleles at the ALAS2 locus, excluding mutations in this gene as the c ause of their sideroblastic anaemia. Furthermore, the absence of a dim orphic erythrocyte population in the mother, coupled with the demonstr ation of random X inactivation in her peripheral leucocytes, showed th at the mother was not the carrier of any X linked sideroblastic anaemi a mutation. These results strongly suggest that the sideroblastic anae mia in this family is an autosomal recessive trait.