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- MATERIALS AND METHODS
- FUNDING SUPPORT
- CONFLICT OF INTEREST DISCLOSURES
Oxaliplatin (OXA)-based combination regimens, either in the form of leucovorin, 5-fluorouracil, and oxaliplatin (FOLFOX) or oxaliplatin plus capecitabine (XELOX), have demonstrated prolonged disease progression-free and overall survival in patients with colorectal cancer (CRC) in both the adjuvant and advanced or metastatic setting.[1, 2] Peripheral neuropathy (PN) is currently recognized to be among the major and dose-limiting nonhematological adverse events of OXA treatment.
OXA-induced peripheral neuropathy (OXAIPN) presents as 2 clinically distinct syndromes. The acute, neuromyotonia-like, transient syndrome occurs in the majority of patients with cold-related paresthesias, pharyngolaryngeal dysesthesias, jaw spasms, and cramps. Conversely, the dose-dependent, chronic form occurs at a threshold dose of 600 to 700 mg/m2 and is a pure sensory, axonal neuropathy with a stocking-and-glove distribution that affects between 50% and 70% of patients.
Although clinical predictors for both acute and chronic OXAIPN have been suggested, to the best of our knowledge no reliable genetic or molecular biomarkers have been identified to date with which to detect those patients at high risk of developing OXAIPN.[5-7] The voltage-gated sodium channels (SCNAs) are fundamental to facilitate the initiation and propagation of action potentials in neurons and other electrically excitable cells. These membrane proteins are encoded by > 10 genes in mammals, whereas mutations in SCNAs are associated with diseases of both the central and peripheral nervous system (PNS).
Briefly, mutations in the SCN1A and SCN2A genes have been associated with several seizure and migraine disorders. SCN3A is found in a cluster of 5 α subunit genes on chromosome 2. The SCN4A gene is expressed in skeletal muscle, and its mutations have been linked to several myotonias and periodic paralysis disorders. The integral membrane protein encoding the SCN5A gene is found primarily in cardiac muscle and defects in this gene cause autosomal-dominant cardiac disease. Mutations in the SCN8A gene are associated with mental retardation, pancerebellar atrophy, and ataxia; mutations in the SCN9A gene play a significant role in nociception signaling and have been associated with channelopathy-associated insensitivity to pain and paroxysmal extreme pain disorder, whereas the protein encoded by the SCN10A gene is a tetrodotoxin-resistant SCNA α subunit that may be involved in painful PN. Finally, SCN11A mediates voltage-dependent gating and conductance.[9-11]
Previous pharmacogenetic studies focused on OXA presented results that were controversial and inconclusive. Lack of a prestudy hypothesis based on the known role of the investigated targets in the PNS and the inappropriate outcome measure for neurological impairment are major drawbacks. Moreover, the majority of these studies were retrospective and based on a post hoc analysis of oncology-based databases of different, not preplanned, size. The current study was designed to investigate in an adequately powered, prospective cohort of well-characterized patients a series of single nucleotide polymorphisms (SNPs) in genes coding for neurologically relevant targets.
Considering that differences in nerve excitability measures bolster the critical involvement of SCNAs in the pathogenesis of acute OXAIPN, the objective of the current hypothesis-driven study was to prospectively investigate whether SNPs of SCNA genes, which can account for the clinical features of acute but also of cumulative/chronic OXAIPN (ie, SCN4A, SCN9A, and SCN10A) might confer liability to the increased incidence and severity of OXAIPN in a large and homogenous cohort of patients with CRC treated with either FOLFOX or XELOX.
- Top of page
- MATERIALS AND METHODS
- FUNDING SUPPORT
- CONFLICT OF INTEREST DISCLOSURES
Despite the progress in research, the mechanism of OXAIPN remains elusive. Existing knowledge had demonstrated that the acute neuromyotonia-like syndrome after OXA infusion could be considered as a channelopathy because of the interaction between OXA-induced oxalate and ion channels located in the cellular membrane. OXA appears for the most part to impair the calcium-dependent lymph node axonal SCNAs rather than potassium channels, thereby resulting in hyperexcited motor sensory nerves and muscles because of the reduction in the overall sodium current.[6, 20] To our knowledge to date, there is no evidence that the other cytotoxic drugs contained in the FOLFOX or XELOX regimens (5-fluorouracil or capecitabine) may affect the ion channels.
The pathogenic hallmark of cumulative OXAIPN is the decreased cellular metabolism and axoplasmic transport resulting from the accumulation of OXA in dorsal root ganglia (DRG) cells, together with mitochondrial dysfunction and oxidative stress, thus producing DRG neuron apoptosis.
There is a lack of a reliable and sensitive molecular or genetic predictive surrogate marker to demonstrate a causal relationship with the development of OXAIPN.[5, 21] The objective of the current study was to provide mechanistic insight into the significant clinical problem of OXAIPN, and to possibly elucidate new therapeutic targets for improved treatments.
The main finding of the current study was that the overdominant model (CT vs CC + TT) of the skeletal muscle SCN4A-rs2302237 and the tetrodotoxin-resistant SCN10A-rs1263292 polymorphisms have emerged as being significantly associated with an increased incidence of acute OXAIPN. The overdominant model of SCN4A-rs2302237 was also able to predict the severity of acute OXAIPN. A weaker association was found between the overdominant model of SCN4A-rs2302237 and the development of cumulative OXAIPN.
The skeletal muscle channelopathies are caused by sodium channel, chloride channel, or calcium channel defects. It has been previously demonstrated that mutations in the SCN4A gene confer liability to various combinations of both typical and atypical hyperexcitability syndromes of skeletal muscles, including cold-induced myotonia, periodic paralysis, and paramyotonia congenita. The finding that another tetrodotoxin-resistant SCNA SNP (ie, SCN10A-rs1263292), which is known to be linked with neuropathic pain, was associated with the incidence of acute OXAIPN points to a complex phenomenon and reinforces the findings of the current study. These results, together with recent findings suggesting that SCN8A plays a central role in mediating acute cooling-exacerbated symptoms following OXA, put sodium channels center stage in the pathogenesis of OXAIPN.
The latter genetic susceptibility supports the results of the current study because acute OXAIPN is considered to be a cold-related channelopathy, mostly related to sodium channel dysfunction. In addition, it clinically resembles a neuromyotonia-like syndrome evoking clinical symptoms such as cold-induced paresthesias, jaw spasm, fasciculations, and muscle cramps due to its effect on both neurons and muscle cells.
Nonetheless, the findings of the current study may seem at odds for 2 reasons. First, heterozygous (CT) patients have a higher possibility of developing OXAIPN, thereby suggesting that the risk follows an overdominant trait. This heterozygote advantage is not common and as such one could argue that the rare inheritance mode reported herein might be peculiar. However, there are instances present in the literature describing an overdominant behavior in other diseases or conditions, such as sickle cell anemia, cystic fibrosis, and resistance to hepatitis C virus infection.[24, 25] Based on the results of the current study, it appears that the heterozygote genotype of the skeletal muscle sodium channel (rs2302237) has a higher relative fitness than either the homozygote-dominant or homozygote-recessive genotype in the context of OXAIPN. Therefore, the current study data either suggest that there is a disadvantage to expressing both forms of sodium channel in a cell, possibly due to a different activation mode, or that the 2 homozygous traits are protective for different reasons, similar to the case for sickle cell anemia.
Second, we found that the CT allele of 2 SCNA SNPs might be able to identify patients at high risk of developing first the acute and eventually the cumulative/chronic form of OXAIPN. Taking into account the different putative mechanisms conferring acute and cumulative/chronic OXA-induced neurotoxicity, this finding might appear to be strange. However, there is evidence at least in the clinical setting that acute OXAIPN may predispose patients to the cumulative/chronic neurotoxicity.[15, 26, 27]
From the theoretical pathogenic point of view, the interrelation between acute and cumulative OXAIPN might be because of the cellular stress affecting the sensory nerve cells as a result of the prolonged activation of SCNAs superimposed with the decreased cellular metabolism and axoplasmic transport in the DRG cells. The latter pathogenetic hypothesis is supported by preclinical results that are in keeping with alterations in sodium channel inactivation kinetics of the sural nerve after OXA application and prolonged opening of sodium channels resulting in an increase in sodium currents. This pathogenic hypothesis, although speculative, could also partly explain the lack of correlation with the severity of cumulative OXAIPN because other mechanisms, such as platinum detoxification and DNA repair enzymes, could modulate the severity of the cumulative neurotoxicity, and the cellular stress induced by the hyperexcitability might only be a trigger of this process.
We acknowledge that the causal relationship between acute and chronic OXAIPN is still unproven, although the results of the current study suggest that this relationship might occur. In any case, the possibility that acute and cumulative neurotoxicity share, at least in part, the same genetic susceptibility requires further investigation.
The lack of a validation population represents a limitation of the current study. This limitation aside, the current study has several advantages. It was a multicenter, international study that tested the genetic susceptibility of selected SCNAs in a homogenous cohort of patients with CRC. Our series is larger than most of existing relevant publications. Contrary to previous studies focusing on the pharmacogenetics of OXAIPN, which applied an oncology-oriented approach on the basis of mechanistic hypotheses relevant mainly to cancer cells and not to cells of the PNS,[5, 29, 30] the current study was hypothesis-driven based on a neurologically rational assumption, at least as far as the acute form of OXAIPN is concerned. In addition, it focused exclusively on OXAIPN by applying prospective and detailed neurological examinations with validated grading tools such as TNSc, in addition to the National Cancer Institute Common Toxicity Criteria.
Finally, we tried to ensure the best-quality results by applying duplicate analysis of samples by real-time PCR and sending randomly selected samples for retesting and validation of results using DNA sequencing at an independent institution. To the best of our knowledge, the external control of pharmacogenetic results has hardly ever been performed.
The results of the current study demonstrated that the overdominant model of the skeletal muscle sodium channel SCN4A-rs2302237 and the tetrodotoxin-resistant SCN10A-rs1263292 polymorphisms appear to be related to the development of acute OXAIPN. The association between the overdominant model of SCN4A-rs2302237 and the development of cumulative OXAIPN, which we also found, requires further data to be fully accredited. The rs2302237 polymorphism was also able to detect patients at high risk of developing clinically significant acute OXAIPN. The results of the current study provide evidence to support a causal relationship between SCNA SNPs and OXAIPN. Further studies from independent groups are warranted to test these results and, if confirmed, to be the basis for further research toward the elucidation of new therapeutic targets for improved treatment against OXAIPN.