Determinação das alterações moleculares em pacientes com deficiencia de proteina C

AUTOR(ES)
DATA DE PUBLICAÇÃO

1999

RESUMO

Protein C is the central component of an important natural system of antithrombotic regulation. 1t is a pIasmatic vitamin-K dependent glicoprotein that acts degrading proteolitycal1y the procoagulant active factors V and vrn. In addition, protein C stimulates fibrinolysis through the fonnation of a complex with plasminogen activator inhibitor (PAI). The hereditary protein C deficiency is an important cause of venous thromboembolic disease. The first reports about protein C deficiency suggested that it had a dominant autossomic hereditary pattem. Nevertheless, subsequentlyan hypothesis of a recessive autossomic pattem in which only homozygotes and double heterozygotes would develop thrombosis was proposed. For a long time, the molecular basis for these conflicting observations were not elucidated, in spite ofthe hypothesis that in thrombofilic families other genetic defects which interfere with protein C gene or the association with other defects which predispose to thrombofilia might be presents. The gene which controIs the synthesis of protein C is locatedin chromosome 2, at position q13-q14. 1t spans 9 exons and 8 introns comprising a.region over llkb, which originates a rnRNA of 1.6 to 1.8kb. According to the protein C gene mutation database published by Reitsma et al.,1996; 160 mutations were described. Among these, 135 were different point mutations, 79% were miss~nse mutations, 8% were splice site mutations, 6.6% were silent mutations and 6% were nonsense mutations. Thirty two percent occurred in CpG dinucleotides, which are considered mutation hotspots. Eight different insertions, twelve short and one large deletion were descnDed. In addition, four polymorphisms were also described. In this work, 6 patients with protein C deficiency who developed venous thrombosis were studied. Other defiCÍenCÍes which predispose to thrombosis were also evaluated. All the nine exons and exon-intron boundaries of the proteinC gene were studied using a strategy which combines polymerase chain reaction (PCR), nonradioactive single-strand confonnational polymorphism (SSCP) and onfonnation-sensitive-gel electrophoresis (CSGE) ana1ysis for mutation screening. The PCR amplified fragments which revealed an altered pattem related to normal controls on SSCP or CSGE ana1ysis were sequenced to precise determination ofthe molecular alterations. The strategy which combines PCR-SCGE resulted more effective to detect alterations than the PCR SSCP strategy. This metodology allowed to detect mutations in 83% of studied patients: 5 point mutations, inc1uding a silent mutation, apart ftom a polymorphism in some patients. Only one of these mutations was not previously descnlJed. T.ae first one was detected in two unrelated patients, a C~T substitution (position 6182), that generated a premature stop codon at Arg 157, in exon 7. This exon code for the activation peptide. This mutation results in exclusion ofthe mutant allele or in a truncated protein. The second one, a G~A substitution (position6246), converted Arg178 to Gln, in exon 7. This alteration affects aminoacids interactions leading to a confonnational change withincreased degradation, that impair protein secretion. The third is a A~G substitution (position -1533), in the promoter region. This alteration affects transcriptional efficiency, since it occurs within HNF-3 (hepatocite nuclear factor) binding site consensus sequence. Finally, a G~T substitution (position 3190), converted Gly 83 to Cys, in exon 5. Tbis exon code for the first EGF domain. This mutation was not described, and could atfects aminoacids interactions, leading to terciary structure alterations and subsequent decrease of protein secretion. These alterations must be responsibles for the hereditary protein C deficiency, because they segregate with deficiency in families, and 3 of them were previously described as the cause of the disease in other patients. Our results indicate that in protein C deficiency the incidence of recurrent mutations is high1y ftequent, since 4 mutations were previously described in families with protein C deficiency in other countries and 2 of them involved the hypermutable region of CpG dimmers, in exon 7. The determination of the molecular alterations in the protein C gene is an important tool to elucidate doubtfu1 diagnostics of protein C deficiency and to investigate the relation between structure and function of protein C

ASSUNTO(S)

genetica molecular trombose mutação (biologia)

ACESSO AO ARTIGO

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