Complex MSH2 and MSH6 mutations in hypermutated microsatellite unstable advanced prostate cancer

Abstract

A hypermutated subtype of advanced prostate cancer was recently described, but prevalence and mechanisms have not been well-characterized. Here we find that 12% (7 of 60) of advanced prostate cancers are hypermutated, and that all hypermutated cancers have mismatch repair gene mutations and microsatellite instability (MSI). Mutations are frequently complex MSH2 or MSH6 structural rearrangements rather than MLH1 epigenetic silencing. Our findings identify parallels and differences in the mechanisms of hypermutation in prostate cancer compared with other MSI-associated cancers.

Figure 1. MSH2 and MSH6 rearrangements are associated with loss of protein expression and MSI.

(a) Four of seven hypermutated cases had complex rearrangements in MSH2 and MSH6 or both genes. Shown is a representative complex MSH2 rearrangement present in hypermutated cases LuCaP 147 and 05–165 (LuCaP 147 was derived from autopsy patient 05–165). Breakpoints were confirmed by Sanger sequencing. Genomic coordinates are hg19. Detail on additional structural rearrangements and other mismatch repair gene mutations is provided in Tables 1 and 2 and Supplementary Figs 2–9. (b) Hypermutated tumors exhibited microsatellite instability by PCR. Shown is representative data for LuCaP 58, which is positive for MSI in 3/5 mononucleotide marker systems (MONO-27, BAT-25 and NR-24, arrows). All hypermutated tumors tested were MSI-PCR positive in at least 2/5 loci (Supplementary Data 1). (c) Hypermutated tumors LuCaP 58, 73 and 147 have loss of MSH2 and MSH6 proteins by IHC. Similar results were observed in hypermutated tumors from rapid autopsy patients (Supplementary Fig. 11). A representative non-hypermutated tumour (LuCaP 23.1) has intact expression. LuCaP 145 had mono-allelic mutations in MSH2 and MSH6 but was not hypermutated. IHC shows loss of MSH6 protein expression in some tumour cells. Scale bars, 0.1 mm.