Lynch syndrome (LS) is a common cause of hereditary colorectal cancer (CRC). Some CRC patients develop mismatch repair deficiency without germline pathogenic mutation, known as Lynch-like syndrome (LLS). We compared the risk of CRC in first-degree relatives (FDRs) in LLS and LS patients. LLS was diagnosed when tumors showed immunohistochemical loss of MSH2, MSH6, and PMS2; or loss of MLH1 with BRAF wild type; and/or no MLH1 methylation and absence of pathogenic mutation in these genes. CRC and other LS-related neoplasms were followed in patients diagnosed with LS and LLS and among their FDRs. Standardized incidence ratios (SIRs) were calculated for CRC and other neoplasms associated with LS among FDRs of LS and LLS patients. In total, 205 LS (1205 FDRs) and 131 LLS families (698 FDRs) had complete pedigrees. FDRs of patients with LLS had a high incidence of CRC (SIR, 2.08; 95% confidence interval (CI), 1.56–2.71), which was significantly lower than that in FDRs of patients with LS (SIR, 4.25; 95% CI, 3.67–4.90; p < 0.001). The risk of developing other neoplasms associated with LS also increased among FDR of LLS patients (SIR, 2.04; 95% CI, 1.44–2.80) but was lower than that among FDR of patients with LS (SIR, 5.01, 95% CI, 4.26–5.84; p < 0.001). FDRs with LLS have an increased risk of developing CRC as well as LS-related neoplasms, although this risk is lower than that of families with LS. Thus, their management should take into account this increased risk.

BACKGROUND: Lynch syndrome (LS) is characterized by mismatch repair (MMR) deficiency. However, there is a group of patients where LS is suspected because of MMR deficiency but there is no germinal mutation in MMR genes.These patients are known as Lynch-like syndrome (LLS) and there is no consensus about their management. The aim of this study is to describe a large series of LLS patients and to analyze if there are clinical, pathology or molecular differences in patients with suspected hereditary or sporadic origin.METHODS: Patients with colorectal cancer (CRC) were included in a national registry when their tumors show immunochemical loss of MSH2, MSH6, PMS2 or loss of MLH1 with BRAF-wild type and/or no MLH1 methylation and absence of pathogenic mutation in these genes. Demographic, clinical and pathological variables, as well as family history of neoplasms were registered. RESULTS: We included 160 patients with LLS. Mean age at diagnosis of CRC was 55 years. A total of 66 patients were female (41%). Amsterdam I and II criteria were fulfilled by 11%, revised Bethesda guidelines by 65% of cases and 24% were diagnosed because of universal screening. There were no differences in sex, indication for colonoscopy, immunochemistry, pathology findings or personal history of CRC or other LS related tumors between patients fulfilling Amsterdam or Bethesda guidelines and patients diagnosed because of universal screening of LS without family history.

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