Diagnóstico genético na síndroma de Lynch: implicações da localização de mutações germinais em genes de reparação do ADN

S. Ferreira¹, I. Claro¹, I. Francisco², R. Sousa¹, P. Lage¹, C. Albuquerque², B. Filipe², A. Suspiro¹, P. Rodrigues¹, M. Cravo^1,2, P. Fidalgo¹, C. Nobre Leitão^1,2

 

 

Resumo

Introdução: O diagnóstico clínico da Síndroma de Lynch (SL) baseia-se nos critérios de Amesterdão (CA); adicionalmente, algumas famílias são identificadas com base nos critérios de Bethesda (CB). A SL resulta de mutações germinais em genes de reparação do ADN, sobretudo no MLH1 e MSH2, mas também no MSH6, PMS1 e PMS2. Não foram ainda identificadas localizações preferenciais das mutações nestes genes que permitam orientar o diagnóstico genético.

Objectivos: Em doentes de famílias com SL com mutações identificadas nos genes MLH1, MSH2 ou MSH6, correlacionar as características clínicas com a localização das mutações.

Doentes e Métodos: Incluíram-se 58 doentes (21 H/37 M) pertencentes a 33 famílias com CA e 7 famílias com CB, todos com mutação germinal identificada num dos genes de reparação do ADN. Registou-se o tipo de tumor desenvolvido, a idade de diagnóstico e as características patológicas dos carcinomas do cólon e recto (CCR). A análise mutacional nos genes MLH1, MSH2 e MSH6 foi efectuada por DGGE, seguida de sequenciação directa a partir do produto de PCR. Nas famílias cujo diagnóstico genético foi inconclusivo por DGGE, procedeu-se a MLPApara identificação de grandes delecções.

Resultados: Desenvolveram CCR 48/58 (83%) doentes, com uma média de idades de 45 anos (25-74). Os restantes 10 doentes apresentaram outros tumores do espectro da SL (6 endométrio, 2 ovário, 1 urotélio e 1 estômago). Foram identificadas 22 famílias com mutações no gene MLH1, 17 no gene MSH2 e uma no gene MSH6. A maioria (76%) das mutações patogénicas no gene MLH1 encontrava-se entre os exões 10 e 19, sendo neste grupo a média de idades de desenvolvimento do CCR mais tardia, 49,8 versus 32,5 anos (p=0,01) e mais frequente a presença de tumores extra-cólicos. No gene MSH2, 71% das mutações patogénicas encontravam-se entre os exões 1 e 8, tendo também estas predominado em famílias com tumores extra-cólicos.

Conclusões: Os resultados observados sugerem que, de acordo com as características das famílias, se deva iniciar o diagnóstico genético pelos exões mais frequentemente mutados em cada gene.

 

Summary

Background: HNPCC diagnosis is based on the Amsterdam criteria (AC), although some families are also identified using the Bethesda guidelines (BG). HNPCC is associated with germline mutations in the DNA mismatch repair genes, particularly MLH1 and MSH2, but also MSH6, PMS1 and PMS2. At present, no "hot-spots" have been identified that could direct genetic diagnosis.

Aims: In patients belonging to HNPCC families with identified mutations in MLH1, MSH2 or MSH6 genes, to correlate tumor characteristics with the location of the mutation.

Patients and Methods: We studied 58 patients (21M/37F) belonging to 33 families with AC and 7 families with BG, all with an identified germline mutation. Age of diagnosis and pathological characteristics of the colorectal cancer (CRC) were recorded, as well as the presence of HNPCC extracolonic cancers. Mutational analysis in MLH1, MSH2 and MSH6 genes was performed by DGGE and direct sequencing. In families with no identified point mutations, we also performed MLPA for detection of large deletions.

Results: A total of 48/58 (83%) of the patients had CRC, with a mean age at diagnosis of 45 years (25-74). The remaining 10 patients had an HNPCC-associated cancer other than CRC (6 endometrial, 2 ovarian, 1 urinary tract and 1 stomach). We identified 22 families with MLH1 mutations, 17 with MSH2 mutations and one with a MSH6 mutation. Most (76%) of the pathogenic mutations in MLH1 gene were located between exons 10 and 19. In this location mean age of CRC diagnosis was higher, 49.8 versus 32.5 years (p=0.01) and more associated HNPCC extracolonic tumors were found. In MSH2, 71% of the mutations were located between exons 1 and 8 and, in this group, more extra-colonic tumors belonging to HNPCC spectrum were identified.

Conclusions: Our results suggest that based on family characteristics, genetic diagnosis should be started by the more frequently mutated exons in each gene.

 

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(1) Serviço de Gastrenterologia

(2) Centro de Investigação de Patobiologia Molecular.

Instituto Português de Oncologia de Francisco Gentil, Centro de Lisboa, Portugal.

 

 

Recebido para publicação: 12/10/2005

Aceite para publicação: 08/02/2006