GGN Repeat Length Research Articles

Comparison of androgen receptor CAG and GGN repeat length polymorphism in humans and apes

Two polymorphic trinucleotide repeats of human androgen receptor gene (hAR), CAG and GGN which encode glutamine and glycine, have been shown to be associated with human diseases. The number of repeats ranges from 8 to 35 for the CAG and from 10 to 30 for the GGN in human populations. Longer CAG repeats are associated with reduced hAR transcriptional activity, spinal bulbar muscular atrophy and lower cognitive function in older men, whereas shorter CAG repeats are associated with increased risk of prostate cancer and infertility in men. The functional roles of the CAG and GGN repeats have not been clarified. In order to compare the sequence of the CAG and GGN regions in apes, we analyzed 57 chimpanzees, 18 gorillas, 20 orangutans, 16 agile gibbons, and 17 siamangs by PCR and electrophoresis. Two bonobos and one long-tailed macaque were also sequenced and the sequences of all species were aligned, respectively, with one human registered sequence. Seventeen different alleles (4, 7, 8, 9, 12, 14, 15, and 17-26 repeats) and 11 alleles (11-14 and 16-22 repeats) were detected at the CAG and the GGN loci, respectively. Although the repeat tract was conserved among apes, chimpanzees had alleles with a wide range of repeat lengths: (CAG)(14-26) and (GGN)(14-22). Gorillas were less polymorphic with the (CAG)(8) and (GGN)(19) alleles being most common, and orangutans exhibited monomorphic (CAG)(11) and (GGN)(22) alleles. On the other hand, agile gibbons and siamangs had the shortest (CAG)(4) allele, but showed variable length of GGN repeats (11-13 in agile gibbons and 16-21 in siamangs). In chimpanzees, frequent haplotypes consisting of short CAG repeats and long GGN repeats or vice versa was observed as in humans.

How Strong Is the Association Between CAG and GGN Repeat Length Polymorphisms in the Androgen Receptor Gene and Prostate Cancer Risk?

Abstract Objective: Although narrative reviews have suggested an association between (CAG)n and (GGN)n polymorphisms in the AR gene and prostate cancer, it has never been quantified systematically. The purpose of this meta-analysis was to provide relative and absolute quantitative summary estimates with sufficient power. Method: Publications were identified through database searches for epidemiologic studies published until February 2004. For each study, mean differences in repeat length between cases and controls were calculated as well as continuous odds ratios (OR) per one CAG or GGN repeat decrement and discrete ORs to compare prostate cancer risk in men with short CAG repeats (≤21 repeats) versus long CAG repeats (>21 repeats) and short GGN repeats (≤16 repeats) versus long GGN repeats (>16 repeats). The study-specific estimates were combined by random effects metaregression analyses. Results: Nineteen case-control studies were included in this review comprising a total of 4,274 cases and 5,275 controls. Prostate cancer cases had on average 0.26 fewer CAG repeats and 0.09 fewer GGN repeats than controls. The continuous ORs of prostate cancer per one repeat decrement were 1.02 and 1.01 for CAG and GGN repeats, respectively. The summary discrete OR (95% confidence interval) were 1.19 (1.07-1.31) and 1.31 (1.06-1.61) for CAG and GGN repeat polymorphisms, respectively. Conclusion: Although the presence of shorter repeats seemed to be modestly associated with prostate cancer risk, the absolute difference in number of repeats between cases and controls is <1 repeat. We question whether such a small difference is enough to yield measurable biological impact in prostate carcinogenesis.

Linkage between cryptorchidism, hypospadias, and GGN repeat length in the androgen receptor gene.

Although sufficient androgen receptor (AR) function is crucial for normal male sexual differentiation, single-point mutations in the AR gene are infrequent in the two most common male congenital malformations, hypospadias and cryptorchidism. Because polymorphic CAG and GGN segments regulate AR function, we investigated whether there was any association between these polymorphisms and mentioned malformations. Genotyping was performed by direct sequencing of DNA from patients diagnosed with hypospadias (n = 51) and cryptorchidism (n = 23) and controls (n = 210). The subjects with hypospadias were divided into subgroups of glanular, penile, and penoscrotal hypospadias. Median GGN lengths were significantly higher (24 vs. 23) among both subjects with cryptorchidism, compared with controls (P = 0.001), and those with penile hypospadias, compared with either controls (P = 0.003) or glanular and penoscrotal hypospadias combined (P = 0.018). The frequency of cases with GGN 24 or more vs. GGN = 23, differed significantly among those with cryptorchidism (65/35%), compared with controls (31/54%) (P = 0.012), and among subjects with penile hypospadias (69/31%), compared with either controls (P = 0.035) or glanular or penoscrotal hypospadias combined (32/55%) (P = 0.056). There were no significant differences in CAG lengths between the cases and controls. Our findings indicate an association between GGN length and the risk of cryptorchidism and penile hypospadias, both conditions considered consequences of low androgenicity.

Shortening of the GGN-repeat in the androgen receptor gene leads to a reduced transcriptional activity in vitro

The N-terminal region of the human androgen receptor (AR) harbours two variable trinucleotide repeats encoding 8–35 glutamine (CAG) and 16–26 glycine (GGN) residues in the normal population. While expansion of the CAG-repeat length >40 repeats is related to spinal and bulbar muscular atrophy (Kennedy syndrome), a rare neuromuscular disorder, no clear linkage of GGN repeat length to any human disease has been described up to now. We analysed three 46,XY patients with severe virilization deficit with suspected androgen insensitivity but without relevant mutation in the AR and found a shortened GGN repeat length of GGN=10. To investigate whether this short GGN-repeat might be responsible for a reduced androgen response due to a reduced transcriptional activation, we constructed four AR expression plasmids with 0, 10, 16 and 25 GGN repeats. Transactivation capacity of the AR constructs was estimated in transient co-transfection experiments using a luciferase reporter gene with a minimal promoter and two androgen responsive elements (ARE2-TATA-Luc) and increasing concentrations (0–100 nM) of either testosterone (T) or dihydrotestosterone (DHT). Transfection analysis demonstrated a GGN-length dependent activation profile with both androgens. The GGN deletion construct as well as the GGN10 construct showed a significantly reduced activation of the AR-dependent reporter gene compared to AR-constructs with GGN-repeats of the normal range. This study gives evidence that the transactivation profile of the human AR is dependent on the length of the GGN-repeat. The impact of the GGN-repeat length on the direct development of androgen insensitivity and its function as a potential modulator of the variable genotype-phenotype correlation in AIS is discussed.

Androgen receptor gene polymorphisms and increased risk of urologic measures of benign prostatic hyperplasia.

The association between androgen receptor gene polymorphisms and benign prostatic hyperplasia was investigated among 510 men randomly selected from Olmsted County, Minnesota. From 1990 through 2000, lower urinary tract symptom severity was assessed by the American Urological Association Symptom Index (AUASI), and peak urinary flow rate, prostate volume, and serum prostate-specific antigen level were measured. Androgen receptor CAG and GGN genotyping was performed. A CAG repeat length of <21 was associated with an enlarged prostate (hazard ratio (HR) = 1.4, 95% confidence interval (CI): 1.0, 1.9) and a serum prostate-specific antigen level >1.4 ng/ml (HR = 1.5, 95% CI: 1.1, 2.0). A GGN repeat length of <16 was associated with an AUASI >7 (HR = 1.6, 95% CI: 1.1, 2.3) and a serum prostate-specific antigen level >1.4 ng/ml (HR = 1.5, 95% CI: 1.0, 2.3). Having <21 CAG repeats and <16 GGN repeats compared with having neither was associated with an enlarged prostate (HR = 2.5, 95% CI: 1.5, 4.2), a serum prostate-specific antigen level >1.4 ng/ml (HR = 2.8, 95% CI: 1.6, 4.7), a peak flow rate <12 ml/second (HR = 1.9, 95% CI: 1.1, 3.4), and an AUASI >7 (HR = 1.6, 95% CI: 1.0, 2.7). Androgen receptor gene polymorphisms may have a potential role in the pathogenesis of benign prostatic hyperplasia.