Male infertility is becoming an increasingly serious medical concern; 15% of adult couples experience infertility, with male infertility occurring in nearly half of the cases worldwide (1-3). The etiology of infertility is complex and involves various factors, including genetics, endocrine diseases, immune dysfunction, reproductive tract infections or abnormalities, and sexual dysfunction, among other factors. Patient genetics is a particularly important factor in male infertility, contributing to ~20% of all cases (3). However, the roles of genetic factors in male infertility are unclear. Chromosomal abnormalities are detected in 2.2-33% of men suffering from infertility (4). Azoospermia factor (AZF) is a gene associated with spermatogenesis that is located in region 1 band 1 of the long arm of the Y chromosome (Yq11). AZF microdeletion occurs in 6.6% of patients with azoospermia and severe oligozoospermia (5-7). Chromosomal abnormalities and AZF microdeletion are important genetic factors associated with male infertility, and are among the most common genetic abnormalities observed in clinical cases. Further etiological examinations are needed to improve clinical treatment and guide the application of assisted reproductive technologies to ultimately reduce the emotional and economic burden of patients. The present study conducted karyotype and molecular genetic analyses in 1,980 infertile men to elucidate the occurrence of chromosomal abnormalities and AZF microdeletions.

A chromosome karyotype abnormality is one of the most common genetic indicators of male infertility (7). In the present study, abnormal karyotypes accounted for 9.0% of cases and the 47, XXY karyotype accounted for 44.9% of all abnormal karyotypes, which is consistent with the findings of previous reports (4,9,10). 47, XXY, also known as Klinefelter syndrome, is characterized by congenital testicular convoluted seminiferous tubule hypoplasia, spermatogenic tubule hyalinosis and fibrosis without spermatogenesis. The SRY gene on the Yq controls testicular development and the male phenotype; however, the function of the Yq is inhibited due to the additional X chromosome. This influences the development of the convoluted canal, hyaline degeneration and fibrosis in the spermatogenic tubule, and hyaline degeneration in the convoluted ducts, resulting in infertility owing to reduced sperm production (11). This syndrome is also associated with the failure of chromosome segregation during meiosis. 47, XYY accounted for 2.2% of abnormal karyotypes. Most patients with 47, XYY are tall, have a normal phenotype and are fertile; however, they exhibit personality and behavioral disorders (12). A few exhibit dysplasia of external genitalia, cryptorchidism and reduced fertility owing to inhibited Yq segregation during meiosis II (13). This results in the formation of 24, YY sperm, which forms a 47, XYY ovum on fertilization with a normal 23, X oocyte. In the present study, 48, XXYY accounted for 2.2% of the abnormal karyotypes. The XXYY genotype could have resulted from reduced chromosomal segregation during meiosis in the patients' parents. Severe fibrosis and hyperplasia of the testicular tissue in these patients could have caused thickening of the nonspecific barrier and severe destruction of the blood-testicular barrier. These changes could lead to serious obstacles and pathological changes in the formation of spermatogenic cells, resulting in male infertility (14). The karyotype 46, X, inv(Y)(q11) accounted for 5.6% of abnormal karyotypes. Yq inversion can result in the loss of local genes associated with the development of male gonads, leading to gonadal dysplasia and infertility (15). In addition, Robertsonian translocation accounted for 5.6% of abnormal karyotypes. This phenotype has been associated with normal intelligence; however, semen examination has indicated oligospermia, asthenospermia and azoospermia. The oligospermia could be attributed to abnormal conjunctions during meiosis and the resulting inability to form gametes (16). Sex reversal syndrome accounted for 6.7% of abnormal karyotypes. The sex of these patients is inconsistent with their chromosome set owing to abnormal sex determination and differentiation. They do not have a normal Yq, but have an extra X chromosome, resulting in testicular tissue dysplasia, which influences sperm production (17).

AZF exists in the q11 region of the long arm of Yq, which consists of three non-overlapping regions, AZFa, AZFb and AZFc. The gene locus regulates spermatogenesis and the deletion of one or more regions can cause spermatogenic dysfunction, leading to asthenia and infertility. AZF microdeletions are common genetic causes of male infertility due to their associations with spermatogenic disorders (18). In the present study, AZF microdeletions were detected in 10.7% of the cases, which is consistent with results from several countries in Europe (8-18%) (19) and Southwest Asia (10-16%) (20,21). In the present study, the most common abnormality was the deletion of sY1192 in the AZFb/c region, accounting for 66.4% of all deletions. The second most common was the AZFb/c+c deletion (18.0%). sY1192 is located between AZFb and AZFc, and the deletion of sY1192 in the AZFb/c region occurs within the extended site near the AZFc region. This deletion has similar physiological relevance to that of the AZFc deletion, which is associated with various clinical manifestations; patients with this abnormality may exhibit normal spermatogenesis or oligospermia/azoospermia. In patients with oligospermia exhibiting the AZFc deficiency, sperm count decreases progressively, thus requiring the cryopreservation of semen to enable future utilization of intracytoplasmic sperm injection-assisted reproductive technologies (22). The present study found that 12 patients had AZF deletions (5.7% of all deletions), all of whom exhibited SRY-positive sex reversal syndrome (46, XX males). In these patients, testicular development was poor, testosterone level was reduced, and follicle-stimulating hormone and luteinizing hormone feedback was increased, suggesting hypogonadism and severe inhibition of spermatogenesis. AZFa deletion was relatively rare, accounting for 3.7% of all deletions. Patients with the AZFa deletion exhibited sterility owing to spermatogenesis disorders caused by Sertoli-cell-only and Klinefelter syndromes. In such cases, the only viable option for the couple is to use sperm from a donor. AZFb deletion accounted for 1.9% of all deletions and resulted in the retarded development of spermatogonia; the corresponding patients presented with aspermia, which is often accompanied by simultaneous deletion of other regions and results in diverse clinical manifestations (23). Among the 211 patients with the AZF deletion, 18% harbored abnormal karyotypes. This result may be underestimated as AZF microdeletion is difficult to detect using chromosomal karyotype detection. Therefore, future studies should employ alternative detection techniques to detect AZF in patients with the XY chromosomal karyotype.

Yq microdeletions were detected in seven of the 12 patients with 46, X, Yqh-(small Y), including four with AZFb/c deletions, two with AZFb/c+c deletions, and one with the AZFa deletion. The AZFb/c deletion was detected in two of the 13 patients with 46, X, Yqh+ (large Y). Therefore, the rate of AZF microdeletion was higher in men with the Yqh-chromosome, which is consistent with previous results (24). Indeed, infertility in patients with a large Yq may not be related to AZF microdeletion (25-27). The present study observed only few Yq length changes. The effects of this abnormality on male infertility should be analyzed with larger sample sizes, which will prove to be useful in guiding personalized treatment strategies. Among the 12 patients with 46, X, del(Y)(q11), seven had the AZFb+c+b/c deletion and fivehead the AZFc+b/c deletion, suggesting that these patients may be AZF deficient. Further molecular examinations should be performed to confirm these findings. Among the 80 cases of 47, XXY, the AZF gene was normal in 78 cases and the AZFb/c sY1192 deletion was detected only in two. There were no associations between azoospermia and AZF microdeletion on the Yq in patients with the 47, XXY karyotype. Therefore, AZF microdeletion analysis may not be useful in determining the etiology of infertility in patients with this karyotype. The key gene that determines the development of spermatogenic cells in the testicular curved spermatogenic cells is located in the q11 position of Yq. The deletion of this part may lead to the deletion or alteration of a series of genes related to spermatogenic function on Yq.

There are some limitations to the present study. First, the sampling principal employed may not guarantee a good reflection of the whole male infertile population. Secondly, the present study only reported the clinical results, but did not investigate the molecular mechanisms. There is scope for improving the molecular biological techniques used to further elucidate the molecular genetic mechanisms underlying male infertility.

In conclusion, chromosomal karyotype abnormalities and AZF microdeletions in the Yq are two major genetic factors related to male infertility. Regular cellular and molecular genetic examinations in men with infertility could help physicians determine the patient-specific etiology of infertility and promote the development of appropriate clinical treatments and effective assisted reproductive technologies to ultimately improve fertility and prevent birth defects.