The Implication of X-Linked Genetic Polymorphisms in Susceptibility and Sexual Dimorphism of Cancer

Methodology

PubMed and Web of Sciences databases were searched using the key words “X-chromosome”, “polymorphism” and “cancer”. Then the CancerGenetics web database (http://www.cancergenetics.org) was used and genes associated with cancer in the articles retrieved by the database search were categorized by chromosome. Thereafter, only the genes located on the X-chromosome were selected and a new search for each gene and its genetic association with cancer susceptibility was then performed in PubMed and Web of Science. Thirty-nine articles corresponding to the study’s scope of X-chromosome variants association with cancer susceptibility were published during the period 2005-2020.

Germline Genetic Polymorphisms on the X-Chromosome Involved in Cancer

Genetic variants of genes lying on the X-chromosome have been proposed as candidate factors for increasing cancer susceptibility in a growing number of studies. Table I summarizes the genetic polymorphisms on the X-chromosome that have been associated with cancer risk in case–control studies. Genetic variants which have been detected in patients with cancer and have not been studied in a control group were not included in this review, as their involvement in cancer susceptibility is not clear.

The X-chromosome can be divided into six different strata according to the probability of genes escaping inactivation. Strata 1 to 5 contain regions that have very low, low, moderate, high and highest probability of escaping X-chromosome inactivation, respectively. On the other hand, genes located on the pseudoautosomal regions 1 and 2 (PAR1 and PAR2) always escape X-chromosome inactivation (4). The genetic polymorphisms on the X-chromosome that are implicated in cancer susceptibility reviewed here are observed in genes located in PAR1 locus and in strata with very low, low, and high probability of escaping X-chromosome inactivation.

Genetic Polymorphisms of Genes Located in the PAR1

Cluster of differentiation 99 (CD99). CD99 gene is a protein-coding gene which is located in PAR1 of X- and Y-chromosomes and escapes X-chromosome inactivation. This gene encodes a cell-surface glycoprotein involved in leukocyte migration, transport of surface molecules, T-cell adhesion, differentiation and apoptosis (7-9). High CD99 expression is a key clinical feature of patients with Ewing sarcoma (EWS) and is routinely used as a prognostic marker (10). Martinelli et al. evaluated the single nucleotide polymorphism (SNP) rs311059 C>T in CD99 gene as a predisposing factor for EWS in children in an Italian population. Rs311059 is a non-coding SNP, but it may increase CD99 expression through affecting CD99 epigenetic regulation mechanisms. Another CD99 variant (rs312257) identified in the same study was associated with better event-free survival in EWS patients, but more studies are needed to validate this finding (11).

Genetic Polymorphisms in Genes With Very Low Probability of Escaping X-Chromosome Inactivation

Angiotensin II receptor type 2 gene (AGTR2). An increasing number of studies have shown that the components of the renin–angiotensin system play a role in tumorigenesis. Angiotensin II is the main effector of the renin–angiotensin system acting through G-protein-coupled receptors, type 1 (AGTR1) and type 2 (AGTR2). AGTR2 protein promotes tumor development by enhancing both malignant cell proliferation and tumor angiogenesis in renal cell carcinoma, gastric cancer and breast cancer (12-14). On the other hand, AGTR2 expression is inversely correlated with cell proliferation and migration in colorectal cancer (15). A genetic variant in the AGTR2 gene, rs5194 G>A, has been associated with aldosterone-producing adenoma. This SNP is located in the 3’-untranslated region (UTR) of the AGTR2 gene. The risky A-allele may enhance the binding of AGTR2 mRNA to micro-RNAs and lead to down-regulation of AGTR2 mRNA, which may stimulate hyperplasia and overgrowth of adrenal cortical cells in patients with aldosterone-producing adenoma (16).

Androgen receptor (AR). Extensive evidence has highlighted the role of this gene in cancer susceptibility. This gene codes for AR, a steroid hormone receptor that acts as a transcriptional factor in androgen signaling. Up-regulation of the AR gene promotes tumorigenesis in many cancer types, including prostate, bladder, kidney cancer, lung, breast and liver cancer but its role in cancer metastasis is contradictory, acting either as a suppressor or stimulator (17). A trinucleotide cytosine-adenine-guanine (CAG) repeat expansion in exon 1 of the AR gene has been associated with cancer risk in different populations. Specifically, the CAG length is inversely correlated with transcriptional activity of the AR gene, meaning that short CAG repeats might promote tumorigenesis. These CAG repeats might serve as a biomarker of prostate cancer predisposition, especially in Asians and Caucasians, as carriers of short CAG repeat expansions have increased susceptibility to prostate cancer (18, 19).

Furthermore, a meta-analysis revealed short CAG repeat length to be associated with ovarian cancer risk in African Americans and Chinese, whereas the reverse association was observed in Caucasians and Italians, again, more studies are needed to validate these results (20). Moreover, a meta-analysis implicated CAG repeats in predisposition to breast cancer, with long CAG repeats increasing the risk of breast cancer in Caucasian women (21). Regarding CAG repeats in colorectal cancer, the findings are inconsistent, as long CAG repeats were associated with higher colorectal cancer risk in one study (22) and with increased colon cancer risk only in males (23), whereas no association between CAG repeats and colorectal cancer was revealed in another study (24).

In addition, exon 1 of the AR gene contains also a GGN microsatellite repeat polymorphism, the length of which varies among different populations. Short GGN repeat length has been reported to enhance AR expression (25). The association of this polymorphism with prostate cancer risk is unclear, as the results of two meta-analyses are contradictive. One study demonstrated that short GGN repeat length was associated with increased risk of prostate cancer, especially in Caucasians (19), while another meta-analysis revealed no association between GGN repeat length and prostate cancer susceptibility (26).

Glypican 3 (GPC3) and GPC4. Genetic polymorphisms in GPC3 and GPC4 have also been related to cancer susceptibility. These genes encode the GPC3 and GPC4 heparan sulfate proteoglycans involved in proliferation, migration, and cell survival modulation in several tissues. GPC3 gene has controversial roles in cancer development, acting either as a tumor-suppressor gene in breast cancer, mesothelioma and ovarian cancer, or as an oncogene in other cancer types such as hepatocellular carcinoma (HCC), colorectal cancer, melanoma, Wilms tumor, neuroblastoma and salivary gland tumors (27, 28). A genetic polymorphism (rs2267531 G>C) in the promoter region of GPC3 was associated with HCC in Egyptians. The risky allele C was found at a significantly higher frequency in patients with HCC compared to the control group and the C allele was correlated with higher GPC3 gene expression (28).

Another member of the glypican gene family, the GPC4 gene, is also involved in cancer risk. The SNP rs1048369 C>T of GPC4 leads to the coding amino acid Ala-Val (p.Ala442Val) mutation and its relationship with cancer susceptibility was studied in Chinese populations. The T-allele of rs1048369 was proposed as risk factor for Epstein– Barr virus-associated gastric cancer by influencing WNT signaling (29), while the C-allele was significantly more frequent in patients with Epstein–Barr virus-positive nasopharyngeal carcinoma (NPC) (30). This difference may be attributed to the different interaction of Epstein–Barr virus with GPC4 in different cancer types; the exact mechanism needs further investigation (30).

Interleukin-1 receptor-associated kinase 1 (IRAK1). IRAKs constitute a family of serine-threonine kinases involved in the signaling cascades of toll-like receptors (TLRs) and interleukin-1 receptors. The first member of the family, IRAK1, has a crucial role in cancer, as its expression is elevated in many solid tumor types and hematological malignancies and is often correlated with metastasis or poor prognosis (31, 32). A recent study in a Greek population correlated the genetic polymorphism rs3027898 C>A, in the 3΄-UTR of IRAK1 with papillary thyroid cancer (PTC) risk. The frequency of the minor allele A was higher in patients with PTC compared to controls, and after adjusting for sex, the A-allele was found to be the risky allele only in men, suggesting a possible role of rs3027898 in PTC development (32). The same polymorphism was also associated with autoimmune diseases in a large number of studies (33-36).

L1 cell-adhesion molecule (L1CAM). A key factor in tumorigenesis in many cancer types, this gene encodes L1CAM transmembrane protein, which is a neuronal cell adhesion molecule involved in cell migration, adhesion and differentiation (37). L1CAM gene has a significant role in cancer initiation and progression, as alterations in its expression can affect cancer cell migration, invasion, growth and metastasis (38, 39). An L1CAM intron SNP, rs4646263 C>A, was associated with susceptibility for epithelial ovarian cancer. Carriers of the AA genotype were at higher risk of developing epithelial ovarian cancer but the mechanism involved remains unclear (40).

Melanoma antigen gene A1 (MAGEA1) and MAGEA11. The MAGE gene family is a known for its role in cancer as its members are tumor biomarkers and targets of immunotherapies. Type I MAGEs (MAGEA, -B, and -C subfamily members) are cancer testis antigens located on the X-chromosome and involved in cancer immunity (41). Genetic association between MAGEs and cancer susceptibility was revealed in two case–control studies. In the first study, the minor T-allele of the rs3788749 SNP on MAGEA1 was more frequent in patients with colorectal cancer and when tumor location was taken into account, this correlation was observed only in patients with rectal cancer. Thus, the rs3788749 C>T polymorphism was proposed as a rectal cancer biomarker, since the T-allele seems to increase MAGEA1 gene expression through its independent functioning as a binding site for transcription factors (42).

The second study identified an association between two intronic SNPs (rs6641352 T>C and rs6540341 C>T) on MAGEA11 gene and renal cell carcinoma (RCC) risk. Carriers of the minor alleles of these SNPs, C and T, respectively, showed increased susceptibility to RCC compared to those homozygous for the major alleles, indicating a possible contribution of these two SNPs to RCC development (43).

Proteasome 26S subunit non-ATPase 10 (PSMD10). PSMD10, encoding the enzyme also known as gankyrin, is a component of the 19S regulatory cap of the 26S proteasome which is involved in diverse biological processes, including cellular growth, proliferation, and invasion. Several studies have reported that PSMD10 gene is up-regulated in a variety of cancer types and have established its role as a candidate oncogene and a tumor biomarker (44-47). Liu et al. investigated the involvement of a SNP located in the 3΄UTR of PSMD10 gene (rs111638916 G>A) in gastric cancer development. They concluded that the minor A-allele of rs111638916 may act as a tumor-promoting factor and increase gastric cancer risk through affecting post-transcriptional regulation of PSMD10 mRNA by miR-505. Carriers of the GA and AA genotypes had also larger tumor size and higher risk of metastasis, suggesting an association between the rs111638916 SNP and clinical features of gastric cancer (48).

Transcription elongation factor A-like 7 (TCEAL7). This gene codes for the cell death-regulatory protein TCEAL7, which acts as a transcriptional repressor of nuclear factor-kappa-B signaling. It has been reported that the TCEAL7 gene is down-regulated in ovarian, breast, brain, prostate, non-small-cell lung and gastric cancer, as well as glioblastoma, and may function as a tumor-suppressor gene (49). In a case–control study, Peedicayil et al. reported three SNPs (rs5987515, rs5987724, rs5945971) upstream of the TCEAL7 gene that may affect ovarian cancer susceptibility. They found that the minor alleles of these SNPs were significantly associated with reduced risk of invasive serous ovarian cancer, and therefore they may have a role in the development of this cancer type (50).

X-linked inhibitor of apoptosis protein (XIAP). Genes of the XIAP apoptotic signaling pathways are also implicated in tumorigenesis. XIAP inhibits cell death mainly through blocking apoptosis. Overexpression of XIAP has been validated in many cancer types and has been linked to cancer development and poor prognosis (51-55). Furthermore, SNP rs8371 C>T in the 3’-UTR of the XIAP gene was associated with esophageal squamous cell carcinoma susceptibility in a Chinese case–control study. The T-allele was observed at lower frequencies in patients compared to the control group, suggesting its protective role in susceptibility (56).

Genetic Polymorphisms in Genes With a Low Probability of Escaping X-Chromosome Inactivation

Factor 9 gene (F9). An X chromosome-wide association study for SNPs in a Chinese population (57) revealed genetic association of F9 with NPC risk, which was validated in Taiwanese (58) and Malaysian replication cohorts (59, 60). This gene encodes factor IX protein, a plasma serine protease which circulates as a zymogen and is involved in blood coagulation (61). Combined analysis of the three population groups revealed that an intron F9 polymorphism, rs371000 C>T, was associated with NPC risk only in males (57).

Fibroblast growth factor 13 (FGF13). The FGF gene family has a crucial role in regulating cellular proliferation, migration, and differentiation. FGF13 protein has an oncogenic activity, as its overexpression is involved in tumor development and progression in many cancer types, including pancreatic endocrine carcinoma, melanoma, multiple myeloma and lung cancer (62). Furthermore, in a genome-wide association study, the intron FGF13 rs619373 G>A variant was associated with increased breast cancer risk in BRCA2 DNA repair-associated (BRCA2) mutation carriers, with the A-allele being the risky variant (63).

Forkhead box P3 (FOXP3). FOXP3 encodes a transcription factor involved in regulation, activation, and differentiation of T-cells. Alterations in FOXP3 expression were found in autoimmune diseases, benign tumors, and carcinomas. FOXP3 is up-regulated in colorectal cancer, non-small lung cancer, thyroid cancer, melanoma and cervical cancer. On the other hand, high levels of FOXP3 are associated with good prognosis in breast, prostate and gastric cancer (64). The association of two FOXP3 promoter polymorphisms, rs3761549 C>T and rs3761548 C>A, with cancer susceptibility has been investigated in many studies in Asian populations but the results were conflicting (65-69). However, a meta-analysis of these studies revealed the rs3761549 (C>T) and rs3761548 (C>A) polymorphisms not to be associated with the risk of breast cancer but with the risk of HCC and non-small-cell lung cancer, respectively (70). Additional studies indicated that the minor A-allele of rs3761548 is a risk factor for colorectal cancer and differentiated thyroid cancer (71, 72), while it is related to lower risk of endometrial cancer (73). Moreover, the same allele was associated with autoimmune diseases susceptibility (74, 75). Regarding the rs3761549 polymorphism, although the major C-allele has been associated with HCC risk, a study in an Iranian population revealed a possible involvement of the T-allele in susceptibility to lung cancer (76). These results might be explained by the dual role of FOXP3 gene in carcinogenesis, acting either as a transcriptional activator or repressor (73, 76). Another FOXP3 promoter polymorphism, rs5902434 (del/ATT), and especially the ATT/ATT genotype, may act as a protective factor reducing endometrial cancer risk in Chinese women (73). The same polymorphism has also been associated with low risk of recurrent respiratory papillomatosis, a benign neoplasm of the larynx and trachea (77). Additionally, the intron FOXP3 rs220883 T>C polymorphism appeared to reduce the risk of differentiated thyroid cancer (C-allele) (72) and to increase the susceptibility to hepatitis B-related HCC and small-cell lung cancer (T-allele) (66, 76). Finally, the heterozygous genotype of the intronic rs2294021 C>T polymorphism was reported to interact with skewed X-chromosome inactivation and elevate breast cancer predisposition by breaking the balance between FOXP3-controlled immune tolerance and tumor suppression (68).

Monoamine oxidase-A (MAOA). This gene encodes the mitochondrial enzyme which degrades monoamine neurotransmitters, such as serotonin and norepinephrine, by the production of hydrogen peroxide. Previous studies have documented that up-regulation of the MAOA gene promotes cancer development and progression in prostate cancer, renal cell carcinoma, classical Hodgkin lymphomas, glioma and non-small cell lung cancer, while reduced MAOA levels may serve as a biomarker for cholangioma and HCC prognosis (78). A recent study proposed the rs144551722 G>A polymorphism, which lies in the promoter region of the MAOA gene, as a predictive biomarker for glioblastoma in males (79).

Nudix hydrolase 10 (NUDT10) and NUDT11. NUDT10 and NUDT11 genes encode two diphosphoinositol polyphosphate phosphohydrolases involved in a variety of biological processes, including vesicle trafficking, maintenance of cell wall integrity, and mediation of cellular stress responses (80). A meta-analysis assessed that a NUDT10 -NUDT11 intergenic polymorphism, rs5945572, is associated with prostate cancer susceptibility. More specifically, the minor G-allele of rs5945572 increases prostate cancer susceptibility and may promote prostate carcinogenesis via multiple signaling pathways (81). This association was confirmed not only in patients with sporadic, but also in those with hereditary prostate cancer (82). Another SNP in the same genetic region, rs5945619, was also reported to be associated with prostate cancer risk in several studies, suggesting a significant role of the NUDT10-NUDT11 intergenic region in prostate cancer susceptibility (83-85).

Tissue inhibitor matrix metalloproteinase 1 (TIMP1). TIMP1 gene inhibits the proteolytic activity of matrix metalloproteinases and is involved in cell proliferation and anti-apoptotic activity. TIMP1 overexpression is associated with cancer progression and poor patient prognosis in papillary thyroid carcinoma, cutaneous melanoma, and gastric, breast, lung and colorectal cancer (86). The rs4898 T>C TIMP1 missense variant has been proposed as a predictive marker for breast and lung cancer, as the C-allelic frequency was increased in patients of both cancer types compared to controls in a Taiwanese population (87, 88). After adjusting for gender, the distribution of rs4898 genotypes did not differ significantly between males and females, so this polymorphism was reported not to contribute to the sex disparities in lung cancer susceptibility (88).

Genetic Polymorphisms in Genes With a High Probability of Escaping X-Chromosome Inactivation

Ras homolog GTPase-activating protein 6 (ARHGAP6). ARHGAP6 is a novel protein involved in regulation of actin polymerization in several cellular processes. It has been reported in in vitro and in vivo studies that ARHGAP6 has an inhibitory effect on the cell growth and metastasis of cervical carcinoma and lung cancer (89, 90) and might serve as a biomarker for colorectal cancer development and progression (91). An intron SNP on ARHGAP6 gene (rs5933886) showed correlation to NPC predisposition, but only in females (57).

Duchenne muscular dystrophy (DMD). This gene is the largest human gene and codes for dystrophin protein, which is part of a protein complex that links the intracellular cytoskeleton network to the extracellular matrix. DMD dysregulation is involved in the pathogenesis of sarcoma, leukemia, lymphoma, melanoma, carcinoma and nervous system cancer (92). In an X-chromosome-wide association study, the intron SNP rs5927056 T>G on the DMD gene was associated with NPC susceptibility, with the minor G allele to be the risky variant. This polymorphism might alter regulatory motifs and affect DMD expression and thus contribute to NPC predisposition (57).

Shroom family member 2 (SHROOM2). A meta-analysis of five genome-wide association studies revealed an association of the rs5934683 polymorphism in the promoter region of the SHROOM2 gene with colorectal cancer susceptibility (93). The SHROOM2 gene is involved in endothelial sprouting, migration, and angiogenesis. The SHROOM2 gene is implicated in carcinogenesis of esophageal squamous cell carcinoma and NPC, acting as a tumor-suppressor gene (94). Moreover, a meta-analysis identified a correlation between the SHROOM intron SNP rs2405942 A>G and prostate cancer risk, which was then confirmed in the replication stage of the same study (95). However, an association study by Cremers et al. did not validate this finding (82).

Toll-like receptor 7 (TLR7). This gene encodes an endosomal receptor that has a key role in innate and adaptive immunity. This protein has been studied due to its immunostimulatory action that can be used in antitumor therapy (96). The rs179008 A>T TLR7 polymorphism has been proposed to have a protective effect on the risk of Hodgkin disease in individuals carrying the minor T-allele (97). Another TLR7 SNP, rs179019 A>C, has been associated with increased susceptibility to urinary bladder cancer (UBC) in males, but more studies are needed to confirm this finding (96). This polymorphism has also been proposed as a predisposing factor for systemic lupus erythematosus (98, 99).

Interaction Between Genes Involved in Cancer Susceptibility

The study of the interactions among the genes described above may provide a better overview of their implication in cancer susceptibility. GeneMANIA (http://genemania.org) is a flexible user-friendly web site for generating hypotheses about gene function, analyzing gene lists and prioritizing genes for functional assays. The 22 corresponding genes from Table I were submitted to GeneMANIA and their interaction network is shown in Figure 1. The interaction network shows direct and indirect interactions among the 22 genes (co-expression, shared protein domains, co-localization, pathway, and predicted interactions). Co-expression means that two genes are linked if their expression levels are similar across conditions in a gene-expression study. Shared protein domains are a protein interaction type in which the genes are linked because their protein products have the same protein domain. Moreover, two genes are considered to be ‘linked’ if they are both expressed in the same tissue or identified in the same cellular location (co-localization) or if their products participate in the same reaction within a pathway (pathway). Finally, the interaction network shows the predicted functional relationships between genes, which may be protein interactions.

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Figure 1.

Interaction network of the 22 X-chromosome-linked genes (left column), the polymorphisms of which are involved in cancer susceptibility. The right column shows other genes which participate in the indirect interactions among the 22 X-linked genes (http://genemania.org).

In detail, CD99 shows similar expression level to IRAK1, TIMP1 and GPC4, and has the same protein domain and predicted interaction with IRAK1. Regarding the heparan sulfate proteoglycans GPC3 and GPC4, GPC3 has a similar expression level to DMD, SHROOM2, MAGEA1 and MAOA, while GPC4 shows co-expression with CD99 and DMD. GPC3 and GPC4 also share the same protein domain. AR is co-expressed with DMD and participates in the same pathway with F9. SHROOM2 has similar expression levels to GPC3 and MAOA and the same protein location as MAGEA1. Furthermore, MAGEA1 and MAGEA11 are co-expressed and share the same protein domain, while MAGEA1 also has the same protein location as SHROOM2 and L1CAM. Co-expression is also observed between XIAP and PMSD10. Moreover, IRAK1 participates in the same pathway as TLR7 and has predicted interaction with XIAP. From the remaining genes, FOXP3, TCEAL7 and NUTD11 show only indirect interactions in the network, while FGF13, AGTR2 and ARHGAP6 have neither direct nor indirect interactions (Figure 1).

The multiple interactions among the 22 genes described in Table I demonstrate the complicated way in which these genes are involved in cancer susceptibility and illustrate that these genes may contribute to tumorigenesis through their interplay. A gene polymorphism may not contribute to cancer susceptibility directly, but it is possible that it can affect cancer development, indirectly affecting expression of other genes. Specifically, alterations in gene expression caused by a polymorphism can affect the expression of other genes participating in the same pathway or alter the balance among genes that are co-expressed, promoting tumorigenesis, Moreover, a combination of genetic variants in genes that interact with each other may promote tumorigenesis. Taking into account that most of these genes are located in regions with low or very low probability of escaping X-chromosome inactivation, random or skewed X-chromosome inactivation events further complicate the understanding of the mechanisms involved in cancer development.

Sexual Dimorphism in Cancer

Sex is a significant factor that affects the incidence, progression and treatment responses of various diseases including cancer. Sex disparities have been observed not only in sex-specific cancer, but also in a variety of other cancer types. Generally, there is a male predominance in cancer susceptibility for many cancer types, such as hematological malignancies, head and neck squamous cell cancer, and esophageal, urinary bladder and liver cancer. On the other hand, females have an increased risk of developing thyroid, gallbladder, biliary tract and anal cancer (100). Sex disparities in cancer susceptibility are believed to be the result of both physiological and genomic differences between the sexes (101).

Physiological differences. The main reason for the physiological differences leading to different cancer susceptibility between males and females is sex hormones. Sex steroid hormones derive from cholesterol and include the androgens (testosterone), estrogens (17β-estradiol, estriol, and estrone) and progestogens. Sex hormones act through four types of receptors: the estrogen receptors ERα, ERβ and G protein-coupled estrogen receptor 1 (GPER1), and the AR. By binding to these receptors, sex hormones are involved in cell-signaling pathways affecting cancer susceptibility through various mechanisms (101). Most of these mechanisms influence immune surveillance, cancer stem cell self-renewal, the tumor microenvironment, and the regulation of systemic metabolism. In many cancer types, such as head and neck squamous cell carcinoma, colonic, glioma, skin, esophageal and non-small-cell lung cancer, significant differences in incidence are observed when comparing males or postmenopausal females with premenopausal females, suggesting a mechanism of sex hormone-mediated tumor development (102). Sex hormone signaling often has controversial roles in regulating cancer development. For example, estrogen signaling has a protective role in HCC, reducing the cancer risk in females, while in many other cancer types, such as of the breast and the female reproductive system, it promotes carcinogenesis (103). Moreover, studies have shown that sex-related hormones, such as prolactin, gonadotropins (Luteinizing hormone and Follicle stimulation hormone) and gonadotropin-releasing hormone have a role in tumor development in prostate, ovarian and breast cancer (102).

Genomic differences. The main reason for the genomic differences in cancer susceptibility between sexes is the sex chromosomes. As described previously, the X-chromosome carries a significant number of genes involved in tumorigenesis. The presence of these genes in a single copy in males and in two copies in females, in combination with random, skewed or escaping X-chromosome inactivation events and LOH in females, is a mechanism that explains the differences in cancer predisposition between sexes. Alterations on the Y-chromosome, sex-specific expression of protein-coding genes and micro-RNAs, and differences in epigenetic profiles of autosomal and sex chromosome genes can also alter the cancer incidence rate between males and females (101). This part of the review focuses on the possible implication of genetic polymorphisms on the X-chromosome, described above, in sex disparities in cancer susceptibility.

Polymorphisms on the X-chromosome and sex disparities in cancer susceptibility. Polymorphisms in eight genes (AR, FGF13, FOXP3, L1CAM, NUDT11, SHROOM, TCEAL7, TIMP1) described in Table I were associated with sex-specific cancer, such as prostate cancer in males, and breast or ovarian cancer in females (referred to as sex-restricted in Table II).

As far as sex-influenced cancer, Jiang et al. reported that the frequency of the AA or AC genotypes of the FOXP3 rs3761548 (C>A) polymorphism was increased in females compared to males. These genotypes were associated with differentiated thyroid cancer risk (72). Taking into account that thyroid cancer incidence has a male to female ratio of 1:4 (100), rs3761548 may explain the increased incidence of thyroid cancer in females. Moreover, the rs3027898 risky A-allele on the IRAK1 gene was correlated with PTC risk in males. In male controls, the major rs3027898 C-allele was more frequent compared to in females, leading to high IRAK1 expression, which may act as a protective factor for PTC in males (32).

Another SNP, rs179019, on the TLR7 gene may increase UBC susceptibility in males. In females, no significant difference in the allelic frequency for rs179019 was observed between patients with UBC and the control group (96). As UBC has higher prevalence in males compared to females, with a male to female ratio of 4:1 (96), it is of great importance to investigate the possible role of the rs179019 polymorphism in increasing UBC risk in males.

Furthermore, two other polymorphisms of the MAGEA11 gene (rs6641352 T>C and rs6540341 C>T) were associated with RCC risk in the overall population and after performing gender-stratified analysis, rs6641352 showed stronger association with RCC in males, in contrast to rs6540341 in females (43). RCC frequency did not differ much between sexes (male to female ratio 1.5:1) (43), therefore more studies are needed to confirm these findings and clarify how these polymorphisms contribute to RCC cancer risk in the two sexes.

Additionally, the study of Zuo et al. revealed two X-linked SNPs, rs371000 C>T on the F9 gene and rs5933886 on the ARHGAP6 gene to be associated with NPC susceptibility. The rs371000 polymorphism was shown to increase risk of NPC only in males, while rs5933886 had a protective effect on NPC risk in females. Given that there is a male predominance in NPC incidence (male to female ratio of 2-3:1), these polymorphisms may contribute to the gender disparities in NPC susceptibility (57).

Finally, the rs144551722 G>A polymorphism on the MAOA gene was associated with glioblastoma risk only in males. As glioma is more common in males compared to females (male to female ratio 1.4:1), this SNP might have a role in the differences observed in glioma risk between sexes (79).

A confirmation of the sex-associated involvement of the above reviewed genetic polymorphisms in cancer susceptibility may be indirectly derived from studying gene expression in normal compared to certain tumor tissues. Taking as examples breast cancer and thyroid cancer, a female-specific cancer and a cancer with high female to male incidence ratio, respectively, the expression levels of the sex-influenced and sex-restricted genes described above were studied in breast and thyroid cancer. Specifically, for this analysis the GENT2 database (http://gent2.appex.kr) was used, which is a platform for exploring gene-expression patterns across normal and tumor tissues. Most of these genes have significantly altered expression in breast or thyroid tumor compared with normal tissues as shown in Table II. Thus, this finding may enforce the association of some of the genes discussed above with sex-limited or sex-influenced cancer types.

Statistical Analysis of X-Chromosome Genetic Association Studies

In case–control genetic association studies, many statistical methods for testing the association between autosomal markers and phenotype have successfully been established. However, due to the different inheritance patterns of X-chromosome-linked traits and the X-chromosome inactivation process, there is a lack of statistical tools for handling X-chromosome genotypic data. As a result, in most studies, the X-chromosomal variants are either incorrectly analyzed or completely excluded from analysis. Recently, many statistical models for testing X-chromosomal associations have been developed. Zheng et al. proposed several approaches for analyzing X-chromosomal variants (104) and Clayton was the first who incorporated the random X-inactivation process in X-chromosome data analysis (105). Other methods have taken into account the different patterns of X-chromosome inactivation and the deviations from Hardy–Weinberg equilibrium (106). Different methods for testing Hardy–Weinberg equilibrium on the X-chromosome have also been developed (107). In addition, Gao et al. introduced the XWAS software toolset, which facilitates the integration of X-chromosome data in genome-wide association studies, combining several previously proposed statistical models (108). However, this software does not incorporate the skewed X-chromosome inactivation pattern.

Even though there are efforts to reduce mistakes in X-chromosome genetic association studies, it is obvious that there is a lack of a standardized, complete, and user-friendly method to analyze X-chromosome genomic data. Therefore, current genetic association studies of X-chromosome-linked variants may have bias or errors in their analyses or may conceal the reluctance of researchers to include X-chromosome variants in their studies.