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

Congenital afibrinogenemia is a rare coagulation disorder with autosomal re cessive inheritance, characterized by the complete absence or extremely red uced levels of fibrinogen in patients' plasma and platelets. Eight afibrino genemic probands, with very low plasma levels of immunoreactive fibrinogen were studied. Sequencing of the fibrinogen gene cluster of each proband dis closed 4 novel point mutations (1914C >G, 1193G >T, 1215delT, and 3075C >T) and 1 already reported (3192C >T). All mutations, localized within the fir st 4 exons of the A alpha -chain gene, were null mutations predicted to pro duce severely truncated A alpha -chains because of the presence of prematur e termination codons. Since premature termination codons are frequently kno wn to affect the metabolism of the corresponding messenger RNAs (mRNAs), th e degree of stability of each mutant mRNA was investigated. Cotransfection experiments with plasmids expressing the wild type and each of the mutant A a-chains, followed by RNA extraction and semiquantitative reverse-transcrip tase-polymerase chain reaction analysis, demonstrated that all the identifi ed null mutations escaped nonsense-mediated mRNA decay. Moreover, ex vivo a nalysis at the protein level demonstrated that the presence of each mutatio n was sufficient to abolish fibrinogen secretion. (C) 2001 by The American Society of Hematology.