Congenital afibrinogenaemia caused by uniparental isodisomy of chromosome 4 containing a novel 15-kb deletion involving fibrinogen Aα-chain gene

Abstract

Among rare inherited deficiencies of coagulation factors, congenital afibrinogenaemia is characterised by the lack of fibrinogen in plasma. In the last few years, several genetic defects underlying afibrinogenaemia (mostly point mutations) have been described in the fibrinogen gene cluster. In this study, the molecular basis responsible for afibrinogenaemia in a Thai proband was defined. Point mutation screening was accomplished by directly sequencing the three fibrinogen genes. The impossibility to amplify fibrinogen Aa-chain gene (FGA) exons 5 and 6 suggested the presence of a homozygous deletion. A specific longrange PCR assay enabled the identification of a novel 15-kb deletion, representing the largest afibrinogenaemia-causing deletion described so far. Direct sequencing of the deletion junction allowed mapping of the breakpoints in FGA intron 4 and in the intergenic region between Aa-and Bb-chain genes. Since the mutation was inherited only from the mother and nonpaternity was ruled out, a maternal uniparental disomy (UPD) was hypothesised. UPD test, carried out with markers covering the whole chromosome 4, revealed that maternal isodisomy was responsible for homozygosity of the 15-kb deletion in the proband. The apparently normal phenotype of the proband, except for afibrinogenaemia, suggests that UPD for chromosome 4 is clinically silent. This represents the first case of a documented complete isodisomy of chromosome 4 causing the phenotypic expression of a recessive disorder. In silico analyses of the regions surrounding the breakpoints suggested that the 15-kb deletion might have originated from an inappropriate repair of a double-strand break by the nonhomologous end joining mechanism.

Recessively inherited coagulation disorders

“…26,27 The first case of complete isodisomy of chromosome 4 was recently identified as a cause of afibrinogenemia. 28 Dysfibrinogenemia is an exception to the general paradigm of RICDs as recessive disorders, because it is usually transmitted as an autosomal dominant trait. Most patients are clinically asymptomatic, 17 some present with a bleeding diathesis, others with 29 and occasionally with both.…”

Molecular Basis of Fibrinogen Deficiency

“…The first, a deletion of 1.2 kb eliminating the entire FGA exon 4, with breakpoints situated in FGA introns 3 and 4, was identified in homozygosity in a Japanese patient (30). The second, a deletion of 15 kb, with breakpoints situated in FGA intron 4 and in the FGA-FGB intergenic region, thus eliminating FGA exon 5 and most of the intergenic region was identified in a Thai patient (31). Interestingly, although the patient was apparently homozygous for the deletion, this was found to be transmitted only by the heterozygous mother.…”

Molecular Genetics of Quantitative Fibrinogen Disorders

“…Of a total of 28 mutations reported in FGA, only three are large deletions (1238 bp, 11 kb, and 15 kb), all identified in the homozygous state in afibrinogenemic probands [37,39,40]. In particular, the 1238-bp deletion described in a Japanese proband is an intragenic deletion involving FGA Table 1) and are characterized by the presence of flanking short direct repeats, it has been proposed that these genomic regions could be particularly prone to breakage events [40].…”

“…In particular, the 1238-bp deletion described in a Japanese proband is an intragenic deletion involving FGA Table 1) and are characterized by the presence of flanking short direct repeats, it has been proposed that these genomic regions could be particularly prone to breakage events [40].…”