Fareena Bilwani

Molecular analysis of six factor xiii-a-deficient families in southern pakistan

Factor XIII deficiency is a severe autosomal recessive bleeding disorder which is commonly due to absence of the A subunit protein. Molecular studies have shown that factor XIII-A deficiency is genetically heterogeneous and over 40 mutations have been described in the A subunit gene. We have recently performed genetic analysis on six apparently unrelated families with inherited factor XIII-A deficiency from the Southern region of Pakistan. In five families, affected individuals were shown to have an identical homozygous splice site mutation ± a single nucleotide substitution (G!A) of the last base of exon 14. This mutation, which has been previously described in two Caucasian patients in the UK, results in exon skipping of exon 14 and a truncated mRNA transcript (Anwar et al, 1995). Further analysis using the HUMF13A01 short tandem repeat (STR) marker closely linked to the A subunit gene (Polymeropoulos et al, 1991) showed that 9 of the 10 alleles with the exon 14 G!A mutation were associated with an identical (AAAG)5 repeat. The level of consanguinity in the Pakistan population is high and this observation would suggest a founder effect (i.e. inheritance of a common ancestral gene mutation). One patient, however, was heterozygous for the (AAAG)5 allele and an (AAAG)7 repeat. As microsatellite instability is apparently rare in STR sequences with repeat numbers of , 10 and almost invariably involves loss or gain of a single repeat (Brinkmann et al, 1998), it seems likely that the two identical exon 14 splice site mutations in this patient have arisen independently. Direct sequence analysis of the factor XIII-A gene in the sixth patient identified a homozygous missense mutation (G!A) in exon 8 which predicts an Arg326Gln substitution in the catalytic core domain of the A subunit protein. This defect has been previously reported in a German family and leads to abnormal folding and destabilization of the A subunit protein (Mikkola et al, 1996). Again, the different ethnic backgrounds suggests that the two mutations have arisen independently in these families. Further information on the frequency of independent mutations at specific loci in the factor XIII-A gene will help clarify whether certain sequences are hotspots for recurrent mutational events.