Non-small cell lung cancer (NSCLC) is a common cancer and the main cause of cancer-related mortality worldwide [1–5]. Although many new target drugs, such as gefitinib and erlotinib, have been used to treat NSCLC, cytotoxic drugs, such as platinum, are still used as first-line agents in the treatment of NSCLC [6, 7]. However, adverse drug reactions, such as nephrotoxicity, hepatotoxicity, hematologic toxicity, and gastrointestinal toxicity, are major obstacles to successful treatment [8–10]. Thus, it is important to identify biomarkers that can be used to predict platinum-based chemotherapeutic toxicity [11].

DNA mismatch repair (MMR) is a key DNA repair system [12, 13]. It is highly conserved and plays an important role in correcting errors generated during DNA replication [14, 15]. MMR proteins interact with one another to form protein complexes that recognize and digest mismatched DNA segments and ultimately fill mismatch gaps [16]. Briefly, MutL homolog 1 (MLH1) dimerizes with postmeiotic segregation increased 1 (PMS1), PMS2, or MLH3 to form the MLH1/PMS2 (MutLα), MLH1/PMS1 (MutLβ), or MLH1/MLH3 (MutLγ) heterodimer and MutS homolog 2 (MSH2) dimerizes with MSH6 or MSH3 to form the MSH2/MSH6 (MutSα) or MSH2/MSH3 (MutSβ) heterodimer so as to bind the DNA helix and recognize DNA mismatches. Together with the abovementioned MutL complexes, the MutSα complex guides the repair of single-base and small-loop mismatches, whereas the MutSβ complex guides the repair of small- to large-loop mismatches [16–19]. Thus, MutSα and MutLα are key proteins in the MMR system and are responsible for mismatch detection and subsequent repair event coordination [20, 21].

There are several partially overlapping DNA repair pathways, including base excision repair (BER), nucleotide excision repair (NER), double-strand break repair (DSBR), and MMR [22]. A previous study showed that mutations in DNA repair genes affect the effectiveness and toxicity of platinum-based chemotherapy in NSCLC patients [23]. MMR plays a key role in maintaining genomic stability through the highly conserved biological pathway. It is an important determinant of platinum cellular toxicity. The formation of platinum/DNA adducts blocks replication and transcription of DNA, and the MMR system plays an important role in removing these adducts. MMR defects lead to replication and recombination errors and cause 6-thioguanine (6-TG)- and O6-methylguanine-induced toxicity in DNA glycosylase-deficient cells [24]. It has been reported that mutations in MMR pathway genes may be associated with platinum-based chemotherapy toxicity in NSCLC patients [23, 25]. In addition, mutations in MMR genes, particularly those in MSH3 and MSH5, may be associated with the risk of lung cancer and even lead to increased alkylation tolerance [26].

Our previous studies showed that genetic polymorphisms were useful clinical markers for chemotherapy response and toxicity prediction in lung cancer patients [9, 27–29]. To investigate the relationship between MMR pathway genetic polymorphisms and platinum-induced toxicity, we evaluated 6 MMR genes (MLH1, MSH2, MSH3, MSH4, MSH5, and MSH6) in Chinese NSCLC patients.

In this study, we investigated whether polymorphisms of MMR genes (MLH1, MSH2, MSH3, MSH4, MSH5, and MSH6) were associated with platinum-based chemotherapy toxicity in 220 NSCLC patients. We evaluated the associations between these gene polymorphisms and gastrointestinal, hematologic, and overall toxicities. Our results showed that MSH2 rs6544991 was associated with gastrointestinal toxicity, MSH3 rs6151627, rs6151670, and rs7709909 were associated with hematologic toxicity, and MSH5 rs707939 and rs805304 were associated with gastrointestinal toxicity and overall toxicity.

A previous study showed that MSH2 was a key protein that influenced 6-thioguanine (6-TG)- and O⁶-methylguanine-induced toxicity in DNA glycosylase-deficient cells, indicating that MSH2 plays an important role in attenuating oxidative DNA damage [24]. In our study, C allele carriers of rs6544991, which features an A/C single-nucleotide variation located in the intron area of MSH2, exhibited poor gastrointestinal toxicity tolerance after being treated with platinum-based chemotherapy. We speculated that this SNP may affect the ability of MSH2 to remove platinum adducts. In addition, our results showed that MSH3 rs6151627 G allele carriers, rs6151670 G allele carriers, and rs7709909 T allele carriers exhibited poor hematologic toxicity tolerance after being treated with platinum-based chemotherapy. MSH3 rs6151627 is an A/G single-nucleotide variation, rs6151670 is a C/G single-nucleotide variation, and rs7709909 is a C/T single-nucleotide variation. All of these polymorphisms are intron variants of MSH3. Methylation of the MSH3 promoter is involved in esophageal tumorigenesis, suggesting that it plays an important role in modulating cell chemosensitivity. As a DNA MMR gene, MSH3 forms the MutSβ heteroduplex with MSH2. The MSH2/MSH3 heterodimer is an ATPase that plays a critical role in mismatch recognition and repair initiation. It binds to DNA mismatches by recognizing 2- to 13-bp insertion-deletion loops [30]. The three SNPs are speculated to affect the function of MSH3; however, the underlying mechanism is still unclear.

Another important finding of our study was that MSH5 polymorphisms were significantly related to overall toxicity. MSH5 rs707939 T allele carriers exhibited better gastrointestinal and overall toxicity tolerance after being treated with platinum-based chemotherapy. Moreover, among MSH5 rs707939 T allele carriers, stratification analysis showed that male patients, patients ≤55 years old, smokers, and patients diagnosed with squamous cell carcinoma faced a lower risk of overall severe toxicity than their counterparts. All these results indicated that MSH5 rs707939 was associated with reduced cisplatin-induced gastrointestinal and overall toxicities in NSCLC patients. In addition, MSH5 rs805304 was also significantly associated with gastrointestinal and overall toxicities. It is noteworthy that rs707939 is a G/T single-nucleotide variation in the intron of MSH5. Previous studies suggested that mutations in MSH5 result in alkylation tolerance in mammalian cells, which is associated with lung cancer risk [26, 31]. Thus, rs707939 and rs805304 may also affect MSH5 activity.

As far as we know, DNA MMR gene defects lead to MMR function loss, which increases the spontaneous mutation frequencies of cells. Cell mutation phenotypes are thought to result in malignant transformation and cause continuous accumulation of gene mutation events. Many error messages across the entire genome eventually affect the effectiveness and toxicity of chemotherapy. However, the detailed mechanisms underlying the effects of these SNPs on gene function need to be studied further.

Our findings indicate that carriers of the MSH5 rs707939 T allele, the MSH2 rs6544991 C allele, the MSH3 rs6151627 and rs6151670 G alleles, and the MSH3 rs7709909 T allele have poor toxicity tolerance. Therefore, these polymorphisms are potential clinical markers for predicting platinum-based chemotherapy toxicity in Chinese NSCLC patients. However, a study with a larger sample size is needed to validate these findings in the future.