Introduction

Early detection and modern treatments for cancer have contributed to improved survival rates for many types and sites of disease, such that there are presently 14.5 million cancer survivors in the United States alone1. As the number of cancer survivors increases, so does the need to understand and moderate the factors that may adversely impact quality of life in survivorhood. One such factor is that of hearing loss, which has been shown in general populations to have adverse consequences for cognition2, and mental health3, if untreated. Specifically, hearing loss is a significant risk factor for dementia2,4. Given the vulnerability of many cancer survivors, the understanding of ototoxicity arising from cancer therapies is of high importance.

Treatment with cisplatin (cis-diamminedichloroplatinum II or CDDP) chemotherapy continues to be a mainstay of curative therapy for many common cancers including breast, testis, and ovarian cancer in adults, and paediatric neuroblastoma. The propensity for cisplatin to instigate cochlear dysfunction, and hence deficits in auditory sensitivity and discrimination abilities has long been known5, although this is still not clearly communicated to the cancer patient in the current medical practice4. The prevalence of hearing loss following cisplatin treatment is dependent upon cumulative dose6, and has been reported as being up to 90%7. Given the recent classification of hearing problems as the 4th leading cause in years lived with disability by the WHO8, interventions that cause hearing problems as a side-effect can have significant adverse consequences for quality of life. Whilst cisplatin administration leads to changes in auditory function that are detectable during and immediately after treatment, specifically bilateral progressive and irreversible high frequency hearing loss, the long-term persistent presence of the platinum compounds in the cochlea9,10 can increase the vulnerability to subsequent insults (age related metabolic change, noise, or viral, for example) and cumulate towards greater social communication impediments and burden.

The anti-cancer actions of cisplatin are mainly due to its interference with tumour cell proliferation11. Through its binding to nuclear DNA, cisplatin blocks transcription and causes double-strand breaks leading to cell cycle arrest12,13. However, since cells from the cochlea do not proliferate, it is thought that platination of mitochondrial DNA is a more likely cause of hearing loss than nuclear DNA damage14,15. It is generally known that cisplatin ototoxicity has 3 major targets, hair cells, spiral ganglion neurons and the stria vascularis (the metabolic hub of the cochlea)16,17. Several molecular mechanisms have been described as mediators of cisplatin-induced ototoxicity. Cisplatin has been shown to target the NOX3 anti-oxidant system by causing the formation of reactive oxidative species (ROS), which in turn triggers inflammatory pathways in the cochlea and promotes apoptotic and necrotic cell death18. Downstream of ROS generation is the JNK pathway, which activates STAT-1 mediated inflammatory pathways, leading to the induction of apoptotic cascades involving caspase 3 and 919. Elegant in vitro and in vivo animal experiments have recently evidenced the involvement of the ATM-Chk2-p53 signaling pathway in cisplatin-mediated hair cell damage20. Interestingly, the stria vascularis retains platinum-based compounds for a long period of time, leading to subsequent alterations in potassium homeostasis and in the generation of the endocochlear potential, both being essential for normal hearing function9,21. Consequently, strial pathology could potentially contribute in disruptions in cochlear metabolic balance, production of ROS, and subsequent apoptosis of cochlear hair cells.

Several risk factors for ototoxicity related to cisplatin chemotherapy have been identified, including poor renal function, very young or old age, gender, nutritional status, melanin content, and pre-existing cochlear hearing loss22. A genetic predisposition has also been proposed based upon observations of substantial inter-individual variability in the prevalence and severity of ototoxicity23. Whilst there are multiple potential pathways for ototoxic hearing loss associated with cisplatin, the possibility of genetic susceptibility to ototoxic side effects is of interest from a number of perspectives24. The identification of polymorphisms that render individuals vulnerable to chemotherapy induced hearing loss is an important precursive step to precision individualized medicine approach that might titrate a chemotherapy regimen such that hearing loss was less likely or severe. Further, such knowledge would support translational genomic approaches in this area25. Additionally, it has been suggested that ototoxicity may act as a valid surrogate marker for other, less well defined health tissue damage associated with platinum-based chemotherapy26.

The aim of the present study was to perform a systematic review and meta-analysis of the literature pertaining to potential genetic predisposition to ototoxicity associated with cisplatin chemotherapy in humans.

Methods

Search strategy

A systematic search of the literature was conducted by two of the authors (E. T. & T. N.) from 6 different databases: Embase, Medline, ASSIA, Pubmed, Scopus and Web of Science. For each database, the search was performed using the key terms: (Gene* OR genotype OR genetic) AND (tinnitus OR ototoxic* OR hearing loss OR hearing impairment OR hearing disorder OR cochleotoxicity OR deaf*) AND (Cisplatin OR cisplatinum OR platamin OR neoplatin OR cismaplat OR cis-diamminedichloridoplatinum* OR carboplatin OR paraplatin OR oxaliplatin OR (platinum AND chemotherapy). Literature searches were conducted in October 2017 and updated in September 2018 (Fig. 1).

Figure 1
figure 1

PRISMA flow diagram displaying the methodology used in the systematic review. The number of records identified by the search and the number of records excluded at each stage of screening against the inclusion/exclusion criteria is shown.

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Criteria for considering studies for this review

All studies written in English were considered eligible for this review. There was no restriction on participant age since studies with both children and adults were included. All different study designs were taken into account. Studies that were not available in English were excluded as we did not have the resources to translate them. Both adults and children were included in the review as many of the studies have been in children and since it is known that cisplatin causes more severe ototoxicity in children than in elders22. In vitro and in vivo studies were excluded because cell lines and animals are not fully representative of the ototoxic effects that platinum-based chemotherapy could have on humans.

Data Extraction and Management

Data extracted included study design, demographic characteristics, intervention and genetic association. Data extraction tables were developed and piloted for this purpose. Where data were missing or unclearly reported, an attempt was made to contact the relevant corresponding author of the study. Three articles were excluded after reading the full text. One paper was excluded because platinum-based chemotherapy was only studied by in vitro methods23. Another was excluded because there was no association between cisplatin ototoxicity and the mitochondrial mutations, which they analysed and there is no report of ototoxicity grade27. A third paper was excluded because the statistical results are based on comparison with craniospinal radiation28. A study by Upadhya et al. reported that 31.4% of patients had sensorineural hearing loss 6 months after radiation of the ear29. Radiation causes ototoxic effects, therefore is a confounding factor when investigating the ototoxic effect of chemotherapy. Studies by Brown et al.30, Drögemöller et al.31, Olgun et al.32 and Wheeler et al.33 are included in the socio-demographic and the cisplatin intervention tables but not in the forest plots since not all values about patients with or without ototoxicity in relationship with the genetic profile were available.

The data from each article was extracted and summarized in an extraction form (Table 1 & Sup. Table S1). The extraction form includes socio-demographic data of the study participants, details of the treatment intervention and audiological assessment and the results of the statistical analysis of the genes examined. The Critical Appraisal Skills Program checklist was used to assess the validity and results of each article included in the systematic review (CASP Critical Appraisal Skills Program Oxford UK, 2017).

Table 1 Description of the socio-demographic data from the collected literature.
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Risk of bias (Quality assessment)

Risk of bias assessment was conducted by four authors (E.T., T.N., N.E. & C. R. C) on those study records included in the meta-analysis. Risk of bias criteria that were taken into consideration in this review were the study population (age, gender, ethnicity), type of cancer (any type of malignancies), type of intervention (other ototoxic drugs, irradiation, prior hearing loss) and measurement of hearing outcome. All these criteria were taken into account in the interpretation of the results.

Meta-analysis

Forest plots were created using the Forest Plot add-in (version 8.0) for JMP 13.2.1 data analysis software to visually summarize the results from each study included in this review. The forest plots display the odds ratios (OR) and 95% confidence intervals that demonstrate the association between ototoxicity and the various genes and single-nucleotide polymorphisms (SNPs) reported in the literature. One forest plot was created to demonstrate the results for the genes tested in a single study. A second forest plot was created to compare the findings of different studies examining the same genes and SNPs and provide an estimate of the combined result of these studies. This is a meta-analysis of the data. The combined odds ratios and 95% confidence intervals were calculated using the values for the number of variant SNPs and controls in the patient groups with normal hearing and with ototoxicity after chemotherapy.

Statistical analysis

The number of cases from the included publications were extracted to four groups: Ototoxicity with SNP variant (OtSNP), Ototoxicity no SNP variant (Ot), Normal hearing with SNP variant (NhSNP), Normal hearing no SNP variant (Nh), and arranged the groups in a contingency table. Since the number of observations in all contingency tables considered is not too large, Fisher’s exact test serves for hypothesis testing. Although Fisher’s exact test is preferable whenever the computational power allows to carry it out, we also report results from the commonly used chi-squared (χ2) test to ensure comparability with the literature.

Furthermore, we report odds ratios (OR) for quantifying the risk of ototoxicity. The OR for being affected by ototoxicity if also having the SNP variant was then calculated as:

$$OR=\frac{OtSNP/NhSNP}{Ot/Nh}$$

Employing the Woolf method34,35, the 95% confidence interval (CI) of the odds ratio is given by:

\(95 \% \,CI=\exp (\mathrm{ln}(OR)\pm 1.96\,\times SE)\) where \(SE=\,\sqrt{\frac{1}{NhSNP}+\frac{1}{Nh}+\frac{1}{OtSNP}+\frac{1}{Ot}}\)

For tables containing empty cells (i.e. if no research subjects populated a group), we applied the Haldane-Anscombe correction36,37, that is we added 0.5 to all cells in the table for that publication. The statistical procedures were carried out using JMP 13.2.1. Values of p < 0.05 were considered significant.

Results

From the 30 included papers, 44% were retrospective with a sample size ranging from 39 to 317 (Table 1). Some of them (33%) were performed on patients younger than 25 years of age30,37,38,39,40,41,42,43,44,45,46,47 and 80% on both genders. The ethnicity was rather broad with Northern America and Norwegians representing together the majority of the articles (14%)31,41,48,49,50. However, 12 of the papers did not specify the ethnicity of the patients32,33,37,38,39,40,47,51,52,53,54,55 and 4 papers did not include age32,36,54,56, which is a known risk factor for ototoxicity22.

Supplementary Table S1 presents medical aspects reported in the studies. Fifty-three percent of the studies were performed on osteosarcoma32,37,38,39,40,42,43,44,46,47,51,56,57,58,59,60, medulloblastoma in 33%30,32,38,39,40,41,44,45,46,56, while testicular cancer was studied in 17% of them31,33,48,49,50,53. How the dose was reported varied between the studies with 17 studies reporting median values of cisplatin administration (from 100 to 525.5 mg/m2)30,31,36,42,43,44,47,48,49,50,52,54,55,56,57,58,59, and reporting mean values (from 328.2 to 425.5 mg/m2)32,38,39,45. Seven studies only reported the range33,37,40,46,51,53,60, and 3 the dose per cycle41,60,61. Only 9 studies reported the number of cycles/courses31,41,44,48,51,52,54,59,61. Regarding auditory measures, there was also a large heterogeneity. Information on the tests and metrics used was missing in 9 studies36,41,42,43,51,52,55,58,61. Sixty-three percent of the studies used pure-tone audiometry (PTA), of which 5 included auditory brainstem responses (ABR)32,37,39,44,54 and 4 included distortion products of otoacoustic emissions (DPOAE)32,37,39,40. However, 33% did not measure hearing at baseline, which makes the changes in hearing difficult to assess30,33,42,43,48,49,51,55,56,58. The average percent of patients with ototoxicity (>grade 2) was 41.8%, ranging from 8 to 75%. There were 6 different grading systems of ototoxicity used across the literature: Brock classification, CTCAE, Boston classification, Chang classification, ASHA, Muenster classification and the Standard National Cancer Institute classification. Nevertheless, in 10 papers there was no clarification of the ototoxicity grading system used33,36,38,39,45,48,49,50,51,55.

Radiotherapy or other ototoxic drugs (such as aminoglycosides and vincristine) were used as part of the treatment of patients in 24 of the included studies. Six papers did not specify whether other ototoxic drugs or radiotherapy were used33,37,38,39,40,52. Radiation to the head and neck causes ototoxic effects and is a confounding factor when investigating the ototoxic effect of chemotherapy29. Only 8 studies adjusted the statistical analysis to relevant clinical variables, such as age at diagnosis, gender, ethnic group, cumulative cisplatin dose, vincristine treatment and craniospinal irradiation doses43,47,48,49,50,55,59,60.

Twenty SNPs of 9 genes were investigated once without having been repeated (Fig. 2). Three of the SNPs were shown to be otoprotective36,38,60. Epoxides are among the many targets of GSTs. Converging this pathway, the Epoxide Hydrolase 1 (EPXH1) rs2234922 was related to otoprotection (OR: 0.05; 95% CI: 0.00–0.94; n = 84; p = 0.004)36. Spracklen et al. identified two SNPs predictors of cisplatin otoprotection60; rs6721961 of NFE2L2 gene, involved in the protection of cells against oxidative stress (OR: 0.34; 95% CI: 0.15–0.81; n = 222; p = 0.019) and rs10950831 of ABCB5 gene, which contributes on the cellular efflux of cisplatin (OR: 0.30; 95% CI: 0.12–0.73; n = 222; p = 0.008).

Figure 2
figure 2

Forest plot describing the genes/SNPs tested in one study in alphabetic order. Black indicates a non-significant association with ototoxicity, red indicates a significant association with otoprotection, and blue indicates a significant association with ototoxicity. The square is centred on the odds ratio and the horizontal line represents the 95% confidence interval. n = sample size. The asterisk (*) identifies studies in which the p value to reach significance differed between Fishers and χ2 tests.

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Nineteen SNPs of 15 genes were investigated at least twice and the meta-analysis is shown in Figs 3 and 4. Seven of these SNPs showed no overall effect, namely the copper transport protein 1 CTR1 rs10981694, GSTM1 and T1 deletions, GSTP1 rs1695, SLC16A5 rs4788863, XPC rs2228001 [a component of nucleotide excision repair (NER)] and XPD rs1799793. Albeit, XPD rs1799793 did not present an overall effect, was significantly ototoxic in one study (OR: 2.621; 95% CI: 1.13–6.10; n = 106; p = 0.034)47. The low-density lipoprotein-related protein 2 (LRP2) encoding the protein megalin was shown positively associated with ototoxicity on 2 SNPs (rs2075252 and rs4668123, Fig. 4). Interestingly, while the latter did not appear significant in hypothesis testing in 2 studies40,56, the accumulated data supports a positive association (OR: 3.532; 95% CI: 1.48–8.45; n = 118; p = 0.0059), likely due to an increase in the statistical power. Three SNPs (rs12201199, rs1142345, rs1800460) on the thiopurine S-methyltransferase gene (TPMT) were found significant in 2 studies42,43 and not in 3 others44,46,57. However, the overall pattern of the 5 studies merged together displayed significant associations with increased ototoxic risk from OR 2.47 to 2.82, with a total sample size of 786 (p < 0.0001). Another variant in COMT rs9332377 showed mixed results with 2 studies showing positive associations42,53, and 4 others not43,44,46,57, while Hagleitner et al. presented an otoprotective effect of this SNP (OR: 0.395; 95% CI; 0.19–0.83; n = 148; p = 0.014)57. The meta-analysis showed an overall positive association but with the smallest risk of all genes (OR: 1.55; 95% CI: 1.18–2.05; n = 847; p = 0.002). The acylphosphatase-2 ACYP2 variant rs1872328, showed in Vos et al.58 and in Xu et al.54 reports a significant association (p = 0.0274 and p < 0.0001, respectively, by Fisher’s test), in spite of an extremely large confidence interval (Fig. 3). High OR of this SNP was also presented in another study showing the strong relation with cisplatin ototoxicity50. The combined data reveals a strong positive association with an overall risk of 4.618 (95% CI: 3.04–7.02; n = 696; p < 0.0001). Another gene playing an important role in oxidative stress, superoxide dismutase 2, mitochondrial (SOD2), with the rs4880 showing a positive association with cisplatin ototoxicity in one of the two studies45, but also in the overall meta-analysis with an OR of 1.917 (significant with χ2, but not with Fisher’s test, Fig. 4).

Figure 3
figure 3

Forest plot describing ABCC3 rs1051640, ACYP2 rs1872328, COMT rs4646316, COMT rs9332377, CTR1 rs10981694, GSTM1 del, GSTM3 rs1799735, GSTP1 rs1695, GSTT1 del tested in multiple studies. Black indicates a non-significant association with ototoxicity, blue indicates a significant association with ototoxicity and red a significant association with otoprotection. The square is centred on the odds ratio and the horizontal line represents the 95% confidence interval of each study. The diamond summarises each SNP average OR and the horizontal shows the 95% confidence interval. n = overall sample size. The asterisk (*) identifies studies in which the p value to reach significance differed between Fishers and χ2 tests.

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Figure 4
figure 4

Forest plot describing LRP2 rs2075252, LRP2 rs4668123, SLC16A5 rs4788863, SLC22A2 rs316019, SOD2 rs4880, TPMT rs1142345, TPMT rs12201199, TPMT rs1800460, XPC rs2228001, XPD/ERCC2 rs1799793 tested in multiple studies. Black indicates a non-significant association with ototoxicity, blue indicates a significant association with ototoxicity and red a significant association with otoprotection. The square is centred on the odds ratio and the horizontal line represents the 95% confidence interval of each study. The diamond summarises each SNP average OR and the horizontal shows the 95% confidence interval. n = overall sample size. The asterisk (*) identifies studies in which the p value to reach significance differed between Fishers and χ2 tests.

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In contrast, some genes presented overall otoprotective associations in this meta-analysis such as the antioxidant polymorphism GSTM3*B (rs1799735) was associated with increased otoprotection (OR: 0.275; 95% CI: 0.13–0.59; n = 145; p = 0.001), a drug clearing transporter, namely the solute carrier SLC22A2 rs316019 (OR: 0.485; 95% CI: 0.27–0.86; n = 286; p = 0.017) and rs1051640 of ABCC3 gene (OR: 0.557; 95% CI: 0.39–0.798; n = 539; p = 0.0017). Although the COMT rs4646316 variant appeared ototoxic in one study42, no significant associations were found in the other studies43,44,46,57 and the overall meta-analysis instead presented otoprotective associations with an OR of 0.620 (p = 0.0008).

Discussion

One of the most prevalent adverse effects of cisplatin treatment is ototoxicity, in which the consequent hearing loss - although is not lethal - has a non-negligible impact on life quality. Hearing deficits are now 4th in the leading causes of years lived with disability8. It is thus an important factor to consider when performing cisplatin interventions in patients, not only to inform patients of the risks, but also to determine whether a given individual has a greater risk and for whom the regimen of administration may be adjusted or alternatives to cisplatin being given. The confirmation that there is a non-negligible risk for ototoxicity when treating patients with cisplatin, and that this risk is influenced by genetics, is in support for greater cautiousness in considering auditory impairments that cancer patients may develop.

A striking finding from this systematic review is that studies with non-significant findings in isolation reached sufficient power when combined to show increased risk of developing cisplatin-induced ototoxicity. This is the case for LRP2 rs4668123, which emphasizes the need of considering larger sample sizes when performing such studies in order to provide more statistically solid evidence. Although our meta-analysis did not use individual data nor included adjustments (for instance for age, sex, the ethnic group, and the cumulative cisplatin dose), the summarized analysis emphasizes the need of large sample sizes to reveal biologically relevant associations that would otherwise been underestimated or missed.

We identified 8 different SNPs from 5 different genes (including rs4668123 from LRP2) from repeated studies showing significant associations with cisplatin ototoxicity (Table 2). These genes are mainly related to anti-oxidant regulation, neurotransmission or to auditory function. ACYP2 encodes the acylphosphatase-2 expressed in the cochlea that hydrolyses phosphoenzyme intermediates of membrane pumps that affect Ca2+ ion homeostasis54. While ATP-dependent Ca2+ signalling has been shown to be involved in hair cell development, the exact role of ACYP2 on the cochlea remains unknown54. Interestingly this ACYP2 polymorphism showed the highest average risk (OR: 4.618), which suggests its major involvement in cisplatin ototoxicity and opens the possibility for more investigations addressing the contribution of this polymorphism in cisplatin-mediated ototoxicity.

Table 2 Summary of all ototoxic associations from repeated studies, listed from the greatest OR to the smallest.
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With an OR ranging from 2.8 to 3.53, the LRP2 rs2075252 and rs4668123 polymorphisms also appear as important risk factors for developing cisplatin-mediated ototoxicity. LRP2 or megalin is currently the only gene associated with Donnai-Barrow syndrome, a condition characterized by craniofacial anomalies, ocular abnormalities, sensorineural deafness and developmental delay62. LRP2 is also connected with diabetic nephropathy, Lowe syndrome, Dent disease, Alzheimer’s disease (AD) and gallstone disease63.

TPMT is a methyltransferase, which enzymatic activity varies depending on polymorphisms of TPMT gene in chromosome 664. A decreased enzymatic activity leads to myelosuppression, gastrointestinal intolerance, pancreatitis and hypersensitivity64. Here, 3 polymorphisms were found with significant OR (Table 2), ranging from 2.47 to 2.82 suggesting a strong involvement of TPMT in cisplatin-mediated ototoxicity. Again, 2 out of 5 studies found positive associations, and the overall outcome was a clear risk in spite of the 3 negative associations. Interestingly, rs12201199 has been linked to the 3A haplotype, leading to a reduced activity of the TPMT enzyme and greater toxicity of the anticancer drugs thiopurine and mercaptopurine65. It was recently demonstrated that HEI-OC1 and UB/OC-1 cells derived from the cochlea are more sensitive to cisplatin when expressing the TPMT*3A variant instead of the wild-type Tpmt65. In contrast, Tpmt knock-out mice do not display an increased sensitivity to cisplatin when administered at comparable levels as found in humans66, however this result might not be surprising given the known resistance of mice to cisplatin ototoxicity when compared to rats or guinea pigs67.

Of the two polymorphisms tested for COMT, only rs9332377 appeared as an important risk factor, although displaying the smallest OR of all validated studies (OR: 1.553). Mutations in COMT genes are implicated in sensorineural deafness. Hearing loss is less severe in subjects with COMT Met allele, possible due to the protective effect of dopamine on the hearing system68,69. While COMT has not been described in the cochlea, a homolog sharing 30% sequence identity, Comt2 was found expressed in hair cells and mice homozygous for a missense mutation in Comt2 showed sensorineural deafness due to degeneration of hair cells70. Overall, there are strong indications that catecholamines play a potential role in the auditory function. Thus, a greater vulnerability to cisplatin ototoxicity may arise when the function of the auditory system is already weakened.

Another important polymorphism related with increased risk of cisplatin ototoxicity is SOD2 rs4880 presented an overall OR of 1.917. SOD2 catalyses the metabolism of the highly toxic superoxide anion to less but still toxic hydrogen peroxide. The SNP rs4880, which results in an exchange of valine against alanine, increases the catalytic activity of SOD2, leading to the accumulation of hydrogen peroxide and secondary ROS generation45. It is thus possible that altered mitochondrial function in the cochlea may increase the vulnerability to cisplatin ototoxicity. Notably, SOD2 polymorphisms (IVS3-23T/G; IVS3-60T/G; and V16A) have also been implicated in noise induced hearing loss (NIHL)71,72.

Three polymorphisms, which have been evaluated twice, were found with a significant oto-protective effect, namely ABCC3 rs1051640, GSTM3 rs1799735 and SLC22A2 rs316019. ABCC3 is an ATP-binding cassette member of the MRP subfamily which is involved in multi-drug resistance. This transporter regulates the efflux of organic anions, glutathione S‐conjugates and xenobiotics73,74. MRP expression in cancer cells correlates with resistance to cisplatin75. The mechanisms by which ABCC3 regulates cisplatin-induced hearing loss are unclear, but some studies suggest it may act upstream of GST73. Indeed, consistent with the otoprotective effects of the ABCC3 variant, GSTM3 rs1799735 shown to be otoprotective by Peters et al.40 but also in the overall analysis. GSTM3 variations are indeed thought to alter the susceptibility to potential carcinogens and toxins76. SLC22A2 is a solute carrier that encodes CTR1, which is a plasma-membrane transport-protein that has an essential role in cisplatin uptake into cochlea hair cells. Since cisplatin accumulates in the stria vascularis from the cochlea9, polymorphism that positively affects monocarboxylate transporter function may improve the strial function affected by cisplatin.

Only two studies have evaluated polygenic effects on the vulnerability to cisplatin ototoxicity. Oldenburg et al. evaluated the cumulated risks of combinations in variants of GSTT1, GSTM1 and GSTP148. Such an approach makes sense when considering that the overall results for GSTM1 and T1 appeared inconclusive (Fig. 3). However, the combination of GSTM1 null, T1 null and P1 Ile105/Ile105 alleles had a major impact on the risk for severe hearing impairment48. These findings are consistent with the known association of GSTM1 null, T1 null and P1 Ile105/Ile105 genotypes with greater vulnerability of developing NIHL77,78 and an 8.88-fold increase in the risk of developing presbyacusis (sensorineural hearing loss caused by natural ageing)79. The incapacity of these individuals to conjugate certain metabolites may ultimately cause oxidative stress and damage to the cochlea, which would be exacerbated in presence of cisplatin. Pussegoda et al. also performed plurigenic analyses but included more complex models incorporating clinical and genetic variables43. Here the combination of TPMT rs12201199, ABCC3 rs1051640, and COMT rs4646316 in a high risk group could reach an OR of 11 (95% CI: 3.2–37.6). Such studies highlight the need of considering multigenic screens when assessing the risk for ototoxicity which may be underestimated when considering a single marker.

There are a number of limitations to be noted in the present study. First, our meta-analysis was performed using group data and not individual data, which pre-empted the possibility of adjusting for e.g. age at diagnosis, gender, ethnic group, cumulative cisplatin dose. Second, in all studies reviewed, hearing loss associated with cisplatin chemotherapy was assessed immediately or soon after treatment. Given that cisplatin persists indefinitely in the human cochlea after such treatment10, the possibility of longer-term cochlear vulnerability (and hence progressive hearing loss) cannot be discounted. Third, ototoxic effects do not only lead to hearing loss, but also tinnitus, and vestibular toxicity80, which were not assessed in the present review, may help determining additional impacts on the auditory system that cannot be revealed with traditional audiometry. There are numerous ototoxicity grading scales used across the different studies. In the clinical trial setting, standardization is vital and the variability between different studies makes analysis more challenging81. Currently, there are 2 main categories of ototoxicity assessment criteria: (1) those measuring a change of hearing from baseline, such as the National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE), and (2) those measuring absolute hearing levels, such as the Brock or Chang classifications81. Interestingly, Spracklen et al. used the CTCAE and Chang grading scales, but also the American Speech-Language-Hearing Association criteria (ASHA), and the resulting associations differed depending on which scale is used60. As a matter of fact NFE2L2 polymorphisms presented as significantly ototoxic in ASHA and CTCAE scales (the latter of which was included in Fig. 2) but not when Chang scale is used60. As a consequence, the selection of the grading scale can have a dramatic impact on the outcome of the study. These findings highlight the needs of determining the most sensitive measures in order to standardise the methodologies into the context of genetic testing in ototoxic vulnerability.

Finally, whilst a genetic predisposition to cisplatin mediated sensorineural hearing loss has been identified and may help identifying cancer patients with greater ototoxic risk, the specific mechanisms remain elusive. This would be an essential precursive step to the development of oto-protective therapy together with cisplatin interventions. It has recently been demonstrated that specifically targeting the p53 pathway protects from cisplatin ototoxicity while still maintaining cancer treatment efficacy20. Knowing the genetic predisposition to cisplatin is an important advancement for improving clinical treatment but now new therapies that target specific pathways are being developed to protect against cisplatin-induced ototoxicity.