Congenital afibrinogenemia: mutations leading to premature termination codons in fibrinogen AO-chain gene are not associated with the decay of the mutant mRNAs

ABSTRACT

Congenital afibrinogenemia is a rare coagulation disorder with autosomal recessive inheritance, characterized by the complete absence or extremely reduced levels of fibrinogen in patients, plasma and platlets. Eight afibrinogemic probands, with very low plasma levels of immunoreactive fibriogen were studied. Sequencing of the fibrinogen gene cluster of each proband disclosed 4 novel point mutations (1914C>G, 1193G> T, 1215delT, and 3075C> T) and 1 already reported (3192C>T). All mutations, localized within the first 4 exons of the AO-chain gene, were null mutations predicted to produce severely truncated AO-chains because of the presence of premature termination codons. Since premature termination codons are frequently known to affect the metabolism of the corresponding messenger RNAs (mRNAs), the degree of stability of each mutant mRNA was investigated. Contransfection experiments with plasmids expressing the wild type and each of the mutant AO-chains, followed by RNA extraction and semiquantative reversetranscriptase-polymerase chain reaction analysis, demonstrated that all the identified null mutations escaped nonsense-mediated mRNA decay. Moreover, ex vivo analysis at the protein level demonstrated that the presence of each mutation was sufficient to abolish fibrinogen sectretion. (AU)