21067 Background: Hereditary non-polyposis colorectal cancer (HNPCC) is a genetic disorder that results in an increased risk of early onset colorectal cancer (CRC). HNPCC is caused by germline mutations in DNA mismatch repair (MMR) genes, including MLH1 and MSH2, and is transmitted in an autosomal dominant manner. De novo germline mutations in MMR genes are exceedingly rare. We now describe a case of a de novo germline mutation in MLH1 associated with early onset CRC in a young woman with a negative family history. Methods: We present a case of a 31-year-old Caucasian female who presented with abdominal pain. Colonoscopy revealed a moderately differentiated adenocarcinoma with focal mucin production involving the sigmoid colon, with a final staging of T4N0M0. Genetic testing was offered because of her young age at diagnosis, having fulfilled the Bethesda guidelines. Microsatellite instability (MSI) testing using a panel of six microsatellite loci was performed on the tumor sample from the affected patient. Immunohistochemistry (IHC) testing for MLH1 and MSH2 was performed. Following counseling and with informed consent, DNA was isolated and sequenced using bi-directional PCR of the MLH1 gene. All exons of MLH1 and MSH2 were analyzed by standard Southern blot methods. Paternity was established using eight genetic loci. Results: The patient's tumor revealed high MSI and complete absence of MLH1 immunoreactivity. MSH2 IHC staining was normal. A large deletional mutation involving exons 5–12 of MLH1 was identified by Southern blot analysis. The patient's parents and siblings were tested and found to have wild type MLH1. Paternity was confirmed with greater than 99.9% certainty. Conclusions: De novo mutations in MMR genes are a rare cause of HNPCC. We report the first case of a large de novo deletion in the MLH1 gene accounting for early onset CRC. Such de novo mutations, albeit rare, must be considered in patients who present with early onset CRC and a negative family history. These results support the use of the Bethesda guidelines to identify individuals who may carry mutations in the MMR genes. No significant financial relationships to disclose.
Germline mutations in the spindle assembly checkpoint genes BUB1 and BUB3 are infrequent in familial colorectal cancer and polyposis
