abstract
Background & Aims
Patients with bi-allelic germline mutations in mismatch repair (MMR) genes (MLH1, MSH2, MSH6, or PMS2)
develop a rare but severe variant of Lynch syndrome called
constitutional MMR deficiency (CMMRD). This syndrome is characterized by
early-onset colorectal cancers, lymphomas or leukemias, and brain
tumors. There is no satisfactory method for diagnosis of CMMRD because
screens for mutations in MMR genes are non-informative for 30% of
patients. MMR-deficient cancer cells are resistant to genotoxic agents
and have microsatellite instability (MSI), due to accumulation of errors
in repetitive DNA sequences. We investigated whether these features
could be used to identify patients with CMMRD.
Methods
We
examined MSI by PCR analysis and tolerance to methylating or thiopurine
agents (functional characteristics of MMR-deficient tumor cells) in
lymphoblastoid cells (LCs) from 3 patients with CMMRD and 5 individuals
with MMR-proficient LCs (controls). Using these assays, we defined
experimental parameters that allowed discrimination of a series of 14
patients with CMMRD from 52 controls (training set). We then used the
same parameters to assess 23 patients with clinical but not genetic
features of CMMRD.
Results
In the
training set, we identified parameters, based on MSI and LC tolerance to
methylation, that detected patients with CMMRD vs controls with 100%
sensitivity and 100%. Among 23 patients suspected of having CMMRD, 6 had
MSI and LC tolerance to methylation (CMMRD highly probable), 15 had
neither MSI nor LC tolerance to methylation (unlikely to have CMMRD),
and 2 were considered doubtful for CMMRD based on having only 1 of the 2
features.
Conclusion
The presence of
MSI and tolerance to methylation in LCs identified patients with CMMRD
with 100% sensitivity and specificity. These features could be used in
diagnosis of patients.
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