Mlh1 mutasyonu için homozigot olan bir çocukta nf1 mutasyon analizi

Hereditary nonpolyposis colorectal cancer (HNPCC) is a common autosomal dominant disease characterized by an inherited predisposition to early onset colorectal cancer and an increased risk of certain other cancers. Cancer susceptibility is due to a heterozygous germ line mutation in one of five mismatch repair (MMR) genes, with the majority of mutations found in MLH1 and MSH2. This study focused on a child with a homozygous germ line mutation in MLH1 [C676T→Arg226Stop] inherited from consanguineous parents (Ricciardone et al., 1999). This child presented with neurofibromatosis type 1 and hematological malignancy, instead of the colorectal cancer phenotype usually seen in HNPCC individuals. This severe tumorigenic syndrome most likely resulted from a downstream mutation in the NF1 gene that could not be repaired due to complete absence of DNA MMR activity. To confirm this hypothesis and possibly clarify the genetic mechanism involved in this phenotype, exons in the functional domain of NF1 that contained mononucleotide or dinucleotide repeat sequences were analyzed by single strand conformation polymorphism (SSCP) and DNA sequence analysis for the presence of mutations. SSCP analysis of the child's DNA identified altered mobility of NF1 exon 22. Subsequent DNA sequence analysis revealed a heterozygous C3721T transition mutation. TaqI restriction digestion of the exon fragment confirmed loss of the restriction site, thus, verifying the sequencing results. The C3721T mutation results in substitution of a stop codon for an arginine codon at position 1241. The resulting NF1 gene product is truncated at the beginning of the functional GTPase activation domain. A putative somatic mutation in the wild-type allele caused complete loss of neurofibromin function. The resulting impaired regulation of Ras•GTP signaling probably contributed to tumor development -- neurofibromatosis and hematological malignancy. This NF1 mutation was not present in the mother, father or sibling, indicating that this was a de novo mutation that occurred in early embryogenesis, most probably as a downstream consequence of constitutional MMR deficiency.