Currently, MMR mosaicism appears to be rare with only a handful of cases reported to date [2–5] (Table 1). The MSH6:c.1135_1139del p.Arg379* variant is the first report of a mosaic pathogenic variant in the MSH6 gene and was identified as a somatic mutation in both the EC and CRC following panel testing in a woman presenting with MSH6-deficiency in both her tumors. ddPCR of the MSH6 variant enabled confirmation of mosaicism demonstrating the variant at low-levels in multiple tissue samples encompassing the endoderm (colon), ectoderm (saliva) and mesoderm (blood). This suggests the variant occurred early in embryogenic development and is potentially present in the primordial germ cells. The MSH6 pathogenic variant may therefore be heritable. Since the patient did not have and can no longer have biological offspring, we did not test for gonadal mosaicism but preimplantation genetic testing may be recommended where a patient is planning to have children, since detection of a mosaic pathogenic MMR variant in gonadal cells would increase the risk of cancer for all carrier children. The patient (ID_151-1), now diagnosed with mosaic Lynch syndrome, can undergo risk-appropriate clinical management while the father and sister, who had no evidence of the MSH6 variant in their DNA samples, can now be confirmed as non-carriers and are released from intensive screening surveillance.

This case of an MSH6 mosaic variant was identified in a person with a diagnosis of suspected Lynch syndrome/Lynch-like syndrome. Tumor testing of suspected Lynch syndrome cases has shown that the predominant etiology is two somatic MMR mutations causing biallelic MMR gene inactivation [5]. When considering who to screen for MMR mosaic variants, cases with somatic MMR mutations in the absence of a germline MMR pathogenic variant are the ideal candidates. Of the MMR mosaic cases identified to date, 3/5 developed multiple Lynch syndrome spectrum cancers (Table 1) raising the suspicion of an undetected germline pathogenic variant. It is likely that the presentation of these MMR mosaicism cases with multiple cancers is a bias of cases selected for mosaicism testing. The ability to test multiple tumors for somatic MMR mutations, both with loss of MSH6 expression, enabled us to target the ddPCR screening to a single variant shared between the tumors. A common MMR mutation in multiple tumors from the same patient may also be indicative of a primary and metastatic lesion, although more than one somatic mutation in common would support this rather than mosaicism. Testing of multiple adenomas to identify a common somatic mutation via the “adenoma first” approach, has been successfully used to identify APC mosaic variants in adenomatous polyposis [8]. This approach has shown APC mosaicism to be a more common mechanism than previously thought in unexplained adenomatous polyposis [1]. Guillerm et al. [5] and Lucia Jansen et al. [1] have both proposed decision tree models for triaging individuals diagnosed with suspected Lynch syndrome for identifying MMR gene mosaicism in patients and first-degree relatives, albeit for the research setting.

This study highlights the importance of screening for mosaicism in patients with a diagnosis of suspected Lynch syndrome and somatic MMR mutations in their tumors. Consistent with literature, this study has shown that Sanger sequencing is not sensitive enough to reliably detect low level variants [9] and alternate, more sensitive methods are required to screen for mosaic variants. The stepwise approach of MMR gene sequencing using NGS methodology in MMR-deficient tumors followed by sensitive ddPCR testing of a specific variant in DNA from multiple tissue sources from different germ layers is a recommended approach moving forward. As tumor screening and sensitive methods such as ddPCR become more widely adopted, the prevalence of MMR mosaicism may also be shown to be higher. As the true prevalence of MMR mosaicism becomes known, improvements to the diagnostic workflow can enable efficient and cost-effective screening approaches to detect all cases of Lynch syndrome, including those with mosaicism.