Mismatch Repair-deficient Human Cell Research Articles

Abstract 3368: Insertion and deletion frameshift rates and mutational spectra of tetranucleotide microsatellites in DNA mismatch repair-deficient human cells

Abstract Background. Oxidative DNA damage and inflammatory cytokines cause loss of function of the MMR protein MSH3. MSH3-MSH2 (MutSβ) function is required for correction of postreplicative polymerase errors in the form of frameshifts at tetranucleotide microsatellite sequences to prevent elevated microsatellite alterations at selected tetranucleotides (EMAST). EMAST is increasingly being used as a biomarker for MSH3 deficiency in subtyping colorectal cancers due to its association with metastasis and poor patient outcome. Here, we sought to understand frameshift mutational behavior of tetranucleotide sequences in the absence of MMR. Methods. We generated EGFP-based mutation reporter model systems containing the D9S242 tetranucleotide microsatellite at its native length of (AAAG)18 and at modified lengths of (AAAG)15 and (AAAG)12 to represent other endogenous microsatellite lengths. Plasmid vectors were constructed for each length that placed EGFP +1 bp or -1 bp out-of-frame for protein translation, enabling us to measure deletion and insertion frameshifts. These constructs were stably-integrated into MMR-deficient HCT116 cells, and monoclonal cell lines containing the constructs were isolated to measure mutation rates and spectra. Deletion and insertion frameshift mutations restored the EGFP reading frame to become detectable by flow cytometry. Genomic DNA from mutated cells were isolated and sequenced for insertion/deletion frameshift mutational biases. Results. Combined insertion/deletion frameshift mutation rates at D9S242 locus ranged from 31.6 X 10-4 to 71.1 X 10-4 mutations/cell/generation and was strongly correlated with longer length of tetranucleotide microsatellites (r2 = 0.986, P = 0.0375). We observed slightly higher deletion mutation rates over insertion rates at longer microsatellites, but equivalent insertion and deletion rates at shorter microsatellites. However, accumulation of more deletion frameshifts over time contributed to a distinct mutational bias for deletions for each length, likely due to continual frameshift mutation at insertions. About 79% of all observed frameshifts (insertion or deletion) were one- repeat (e.g., four nucleotides), 16% two-repeat, and 5% three or more repeat mutations that were consistent with a slipped strand mispairing mutation model. Conclusions. Our results suggest that intact MMR recognizes and repairs shorter length tetranucleotide frameshifts more efficiently than larger ones, and preferentially corrects loops on the template strand (preventing deletions) over loops on the newly-synthesized strand (preventing insertions) in human cells. MMR deficiency thus manifests in human tissue with a deletion mutation bias at tetranucleotide sequences (EMAST). Longer tetranucleotide microsatellites have increased propensities for larger DNA slippage errors. Citation Format: Maide O. Raeker, Jovan Pierre-Charles, John M. Carethers. Insertion and deletion frameshift rates and mutational spectra of tetranucleotide microsatellites in DNA mismatch repair-deficient human cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3368.

Relative rates of insertion and deletion mutations in dinucleotide repeats of various lengths in mismatch repair proficient mouse and mismatch repair deficient human cells

Microsatellites are DNA elements composed of short tandem repeats of 1–5 bp. These sequences are particularly prone to frameshift mutation by insertion–deletion loop formation during replication. The mismatch repair system is responsible for correcting these replication errors, and microsatellite mutation rates are significantly elevated in the absence of mismatch repair. We have investigated the effect of varying the number of repeats in a (CA) n microsatellite on mutation rates in cultured mammalian cells proficient or deficient in mismatch repair. We have also compared the relative rates of single-repeat insertions and deletions in these cells. Two plasmid vectors were constructed for each repeat unit number ( n=8, 17, and 30), such that the microsatellites, placed upstream of a bacterial neomycin resistance gene ( neo), disrupted the reading frame of the gene in the (−1) or (+1) direction. Plasmids were introduced separately into the cells, where they integrated into the cellular genome. Mutation rates were determined by selection of clones with frameshift mutations in the microsatellite that restored the reading frame of the neo gene. We found that mutation rates were significantly higher for (CA) 17 and (CA) 30 tracts than for (CA) 8 tracts in both mismatch repair proficient (mouse) and deficient (human) cells. A mutational bias favoring insertions was generally observed. In both (CA) 17 and (CA) 30 tracts, single-repeat insertion rates were higher than single-repeat deletion rates with or without mismatch repair; deletions of multiple repeat units (≥8 bp) were observed in these tracts, where as deletions this large were not found in the (CA) 8 tract. Single-repeat mutations of both types were made at similar rates in (CA) 8 tracts in human mismatch repair deficient (MMR −) cells, but single-repeat insertion rates were higher than single-repeat deletion rates in mouse mismatch repair proficient (MMR +) cells. Results of these direct studies on microsatellite mutations in cultured cells should be useful for refinement of mathematical models for microsatellite evolution.

Mismatch repair deficient human cells: spontaneous and MNNG-induced mutational spectra in the HPRT gene

We have determined both the spontaneous and N-methyl- N′-nitro- N-nitrosoguanidine (MNNG)-induced mutational spectra in the HPRT gene of human cells (MT1) defective in the mismatch repair gene hMSH6 (GTBP). Eight of nine exons and nine of sixteen intronic flanking sequences were scanned, encompassing >900 bp of the HPRT gene. Mutant hotspots were detected and separated by differences in their melting temperatures using constant denaturant capillary electrophoresis (CDCE) or denaturing gradient gel electrophoresis (DGGE). A key finding of this work is that a high proportion of all HPRT inactivating mutations is represented by a small number of hotspots distributed over the exons and mRNA splice sites. Thirteen spontaneous hotspots and sixteen MNNG-induced hotspots accounted for 55% and 48% of all 6TG R point mutations, respectively. MNNG-induced hotspots were predominantly G:C→A:T transitions. The spontaneous spectrum of cells deficient in hMSH6 contained transversions (A:T→T:A, G:C→T:A, A:T→C:G), transitions (A:T→G:C), a plus-one insertion, and a minus-one deletion. Curiously, G:C→A:T transitions, which dominate human germinal and somatic point mutations were absent from the spontaneous hMSH6 spectra.

Sequence-dependent mutations in a shuttle vector plasmid replicated in a mismatch repair deficient human cell line.

We utilized a shuttle vector plasmid (pLSC) to assess the role of DNA sequence and mismatch repair on mutagenesis in human cells. pLSC contains an interrupted 29 bp mononucleotide poly(G) run within a bacterial suppressor tRNA gene, which acts as a highly sensitive mutagenic target for detection of base substitution and frameshift mutations. The frequency of spontaneous mutations in pLSC was found to be similar after replication in either the hMSH6 (GT binding protein) mismatch repair-deficient MT1 line or its parental, mismatch repair-proficient line, TK6. However, the classes of plasmid mutations showed distinct differences in the two cell lines. Single base deletions comprised 48% of the mutations in the 56 independent pLSC plasmids sequenced from MT1 cells while these represented only 18% of the 40 independent pLSC mutants sequenced from the wild-type TK6 cells (P = 0.001). Virtually all the deletions included the mononucleotide run. In contrast, in pSP189, which contains the unmodified supF tRNA without the mononucleotide sequence, no single base deletions were observed for either cell line (P < 0.001). UV treatment of pLSC and pSP189 resulted in a 12-140-fold increase in mutations in TK6 and MT1 cells. These were predominately single base substitution mutations without a large increase in deletion mutations in the mononucleotide run in pLSC. These data indicate that a mononucleotide poly(G) run promotes single base deletion mutations. This effect is enhanced in a hMSH6 mismatch repair-deficient cell line and is independent of UV-induced mutagenesis.