Progress in prostate cancer

The clinical genetics of prostate cancer

Prostate cancer is the most common cancer in men and the second highest cause of cancer-related mortality in the U.K. A genetic component in predisposition to prostate cancer has been recognized for decades. One of the strongest epidemiological risk factors for prostate cancer is a positive family history. The hunt for the genes that predispose to prostate cancer in families has been the focus of many research groups worldwide for the past 10 years. Both epidemiological and twin studies support a role for genetic predisposition to prostate cancer. Familial cancer loci have been found, but the genes that cause familial prostate cancer remain largely elusive. Unravelling the genetics of prostate cancer is challenging and is likely to involve the analysis of numerous predisposition genes. Current evidence supports the hypothesis that excess familial risk of prostate cancer could be due to the inheritance of multiple moderate-risk genetic variants. Although research on hereditary prostate cancer has improved our knowledge of the genetic aetiology of the disease, a lot of questions still remain unanswered.

This article explores the current evidence that there is a genetic component to the aetiology of prostate cancer and attempts to put into context the diverse findings that have been shown to be possibly associated with the development of hereditary prostate cancer. Linkage searches over the last decade are summarised. It explores issues as to why understanding the genetics of prostate cancer has been so difficult and why despite this, it is still a major focus of research. Finally, current and future management strategies of men with Hereditary Prostate Cancer (HPC) are discussed.

Short tandem repeat polymorphism linkage to the androgen receptor gene in prostate carcinoma

BACKGROUND

Due to high polymorphism, common sequences, and ubiquitous presence, short tandem repeats (STRs) may enhance genomic typing to determine prostate carcinoma (CaP) predisposition. The human phosphoglycerate kinase (PGK1) gene is located within Xq11-Xq13, a region implicated in familial prostate carcinoma, androgen insensitivity, perineal hypospadias, and other genitourinary abnormalities. The PGK1 STR is the most polymorphic site described in the Xq11-Xq13 interval and was investigated for its ability to detect differences comparing a heterogeneous CaP population versus controls.

METHODS

We compared PGK1 STR allele sizes in 103 localized CaP patients with 299 control subjects to evaluate the STR's ability to detect potential CaP predisposing genetic factors. Allele sizes were measured with an automated DNA sequencer after polymerase chain reaction (PCR) based copying of the PGK1 STR region. Allele sizes were compared using chi square and Mann-Whitney U tests.

RESULTS

Among 402 subjects, there were 10 distinct allele sizes consisting of five common and five relatively rare alleles. The PGK1 STR, 12 allele (12 tetrameric repeats) was more common among patients with CaP (p=0.03). Allele 13 was more common in CaP patients > 60 years old than among younger patients (p< 0.005).

CONCLUSIONS

Our findings suggest that STRs in the Xq11-Xq13 region and other regions may provide a means to rapidly scan genetic loci in large populations of CaP patients and controls. Within limitations, STRs offer the advantage of relatively uniform protocols that could potentially provide a means to comprehensively scan genomes at known predisposing loci.

Mxi1 is a potential cellular target of carcinogens and frequently mutated in experimental rat tumors and tumor cell lines

Mxi1, a member of the Myc family of transcription factors, negatively regulates Myc oncoprotein activity and thus may be a tumor suppressor gene. It is mutated in a few human prostate cancers. Rat Mxi1 was isolated as a selective overexpressive message in rat esophageal cancer induced by N-nitrososarcosine ethyl ester using differential display and polymerase chain reaction cloning. Reverse transcription, single-strand conformation polymorphism analysis and subsequent DNA sequencing were used to screen mutations for the rat Mxi1 coding region including the functional domains, Sin3-interacting, helix-loop-helix and leucine zipper in samples from 24 rat tumor tissues and various cell lines. Seven mutations were revealed to exist in six rat tumors (including two esophageal tumors and a breast cancer), and three rat tumor cell lines: Leydig cell tumor, osteogenic sarcoma, and pituitary tumor. No coding changes were detected in 34 samples of human sporadic gastric adenocarcinoma. A silent base substitution (GAG to GAA) at codon 131 was also identified in six rat tumors as well as in one human gastric cancer. Our results indicate that Mxi1 is often mutated in experimental rat tumors but mutations are rare in human sporadic cancers. The Mxi1 tumor suppressor gene may be a cellular target of strong carcinogens. Considering the frequency of mutations in chemical carcinogen-induced tumors, searches for Mxi1 mutation in human tumors should be directed toward patients with a specific epidemiological background.

High frequency of deletion on chromosome 9p21 may harbor several tumor-suppressor genes in human prostate cancer

Chromosome 9p has been reported to be a critical region of loss in various cancers. Our present study was designed to determine the frequency of deletions at different loci of chromosome 9p in microdissected samples of normal prostatic epithelium and carcinoma from the same patients. For this purpose, DNA was extracted from the microdissected sections of normal and tumor cells of 40 prostate specimens, amplified by PCR and analyzed for loss of heterozygosity (LOH) on chromosome 9p using 15 microsatellite markers. Only 6 of 15 microsatellite markers exhibited LOH in prostate cancer specimens (D9S162, D9S1748, D9S171, D9S270, D9S273 and D9S153). LOH on chromosome 9p was identified in 29 of 40 cases (72.5%) with at least 1 marker. The main deletion was found on 9p21, at loci D9S1748 (50%), D9S171 (51.4%) and D9S270 (21.8%). There was also a deletion on 9p22 at locus D9S162 (8.3%), on 9p13 at locus D9S273 (13.8%) and on 9p11 at locus D9S153 (7.7%). LOH data were correlated with stage of prostate cancer and revealed a high frequency of LOH at 3 or more loci in samples with stage T(3)N(0)M(0) (46%) compared with stage T(2)N(0)M(0) (15%), which suggests a higher incidence of LOH in the advanced stage of prostate cancer. One of the candidate target tumor-suppressor genes, p16 (MTS-1/CDKN2), has been identified within the 9p21 deleted region in tumor cell lines. Expression of P16 protein was either absent or very low in prostate cancer samples, suggesting that loss of the p16 gene may be involved in prostatic carcinogenesis.

Chromosomal basis of adenocarcinoma of the prostate

Prostate cancer is the most frequent malignancy and the second leading cause of cancer deaths among males in the Western world. The clinical course of the disease is highly complex, and genetic factors underlying tumorigenesis are poorly understood. The challenge that lies ahead is to identify the important gene(s) that causes adenocarcinoma of the prostate. Chromosomal findings by cytogenetic and molecular methods, including Southern blotting, microsatellite analysis, fluorescence in situ hybridization, and comparative genomic hybridization, revealed a high frequency of chromosomal aberrations of heterogeneous nature, including: -1, +1, -1q, +4, -6q, -7, +7, -8, -8p, -8q, +i(8q), -9, -9p, -10, +10, +11, -12, -13q, -16, -16q, +16, -17, +17, +17q, -18, +18, -18q, +19p, +20q, +X, -Xq, -Y, and +Y. Specific chromosomal regions of alterations were 1q24-25, 2cen-q31, 5cen-q23.3, 6q14-23.2, 7q22-q31, 8p12-21, 8p22, 8q24-qter, 10q22.1, 10q23-25, 11p11.2, 16q24, 17p13.1, 18q12.2, and Xq11-12. Recently, a predisposing gene for early onset has been localized on 1q42.2-43. The losses of heterozygosity at specific chromosomal loci from chromosomes 5q, 6q, 7q, 8p, 8q, 10q, 13q, 16q, 17p, 17q, and 18q are generally correlated with poor prognosis in advanced tumor stage. In addition, an abnormal function of known tumor suppressor genes from these regions have been observed in prostate cancer. Although, the amplification of the androgen receptor gene at Xq11-13 and HER-2/neu gene at 17q11.2-q12 are novel findings, no single gene has been implicated in harboring prostate cancer. Frequent inactivation of PTEN/MMAC1 tumor suppressor gene at 10q23, MXI-1 at 10q25, KAI-1 at 11p11.2, Rb at 13q14.2, and p53 at 17p13.1 and deregulation of c-myc oncogene at 8q24 have recently been the subject of intense scrutiny and debate.

Evidence for a rare prostate cancer-susceptibility locus at chromosome 1p36

Combining data from a genomic screen in 70 families with a high risk for prostate cancer (PC) with data from candidate-region mapping in these families and an additional 71 families, we have localized a potential hereditary PC-susceptibility locus to chromosome 1p36. Because an excess of cases of primary brain cancer (BC) have been observed in some studies of families with a high risk for PC, and because loss of heterozygosity at 1p36 is frequently observed in BC, we further evaluated 12 families with both a history of PC and a blood relative with primary BC. The overall LOD score in these 12 families was 3.22 at a recombination fraction (theta) of .06, with marker D1S507. On the basis of an a priori hypothesis, this group was stratified by age at diagnosis of PC. In the younger age group (mean age at diagnosis <66 years), a maximum two-point LOD score of 3.65 at straight theta = .0 was observed, with D1S407. This linkage was rejected in both early- and late-onset families without a history of BC (LOD scores -7.12 and -6.03, respectively, at straight theta = .0). After exclusion of 3 of the 12 families that had better evidence of linkage to previously described PC-susceptibility loci, linkage to the 1p36 region was suggested by a two-point LOD score of 4.74 at straight theta = .0, with marker D1S407. We conclude that a significant proportion of these families with both a high risk for PC and a family member with BC show linkage to the 1p36 region.

Loss of heterozygosity of the TP53 tumor suppressor gene and detection of point mutations by the non-isotopic RNAse cleavage assay in prostate cancer

Mutation within the TP53 tumor suppressor gene is a frequent occurrence in human cancers, resulting in a poor prognosis, response to therapy, and overall survival time. Mutations have been primarily detected in advanced prostate cancer; however, the involvement of the gene through loss of heterozygosity (LOH) in primary prostate cancers has not been investigated due to lack of identifiable polymorphisms within this gene. Using the nonisotopic RNAse cleavage assay (NIRCA), we screened for point mutations and identified an ApaI restriction site polymorphism located in intron 7 within the TP53 gene. This polymorphism allowed us to detect LOH in informative samples in a population of patients that underwent prostate biopsies and a population that underwent radical prostatectomies. Within the combined study population, 31 of 80 patients (38.75%) were informative for the polymorphism. Loss of heterozygosity was detected in 10 of the 31 samples (32.3%). Point mutations were identified in two samples. The identification of LOH in these patients suggests that the TP53 tumor suppressor gene may play a more active role in prostate cancer than was previously believed.

Linkage analysis of chromosome 1q markers in 136 prostate cancer families. The Cancer Research Campaign/British Prostate Group U.K. Familial Prostate Cancer Study Collaborators

Prostate cancer shows evidence of familial aggregation, particularly at young ages at diagnosis, but the inherited basis of familial prostate cancer is poorly understood. Smith et al. recently found evidence of linkage to markers on 1q, at a locus designated "HPC1," in 91 families with multiple cases of early-onset prostate cancer. Using both parametric and nonparametric methods, we attempted to confirm this finding, in 60 affected related pairs and in 76 families with three or more cases of prostate cancer, but we found no significant evidence of linkage. The estimated proportion of linked families, under a standard autosomal dominant model, was 4%, with an upper 95% confidence limit of 31%. We conclude that the HPC1 locus is responsible for only a minority of familial prostate cancer cases and that it is likely to be most important in families with at least four cases of the disease.

Genetic variation of 3 beta-hydroxysteroid dehydrogenase type II in three racial/ethnic groups: implications for prostate cancer risk

BACKGROUND

Elevated prostatic dihydrotestosterone (DHT) has been suggested to increase the risk of prostate cancer. The HSD3B2 gene encodes the type II 3 beta-hydroxysteroid dehydrogenase: one of two enzymes that initiate the inactivation of DHT. Thus, the HSD3B2 gene is a candidate gene for predisposition to prostate cancer.

METHODS

We have determine the distribution of a complex dinucleotide repeat in the HSD3B2 gene in high-risk African-Americans, intermediate-risk Euro-Americans, and low-risk Asians. Genomic DNA from 312 individuals was amplified by polymerase chain reaction (PCR) and analyzed by electrophoresis on denaturing polyacrylamide gels.

RESULTS

We have found that certain alleles are either unique to or much more common in either African-Americans, Asians, or Euro-Americans. Our data also substantially expand the number of alleles reported for the complex dinucleotide repeat polymorphism in the HSD3B2 gene.

CONCLUSIONS

Our report demonstrates substantial genetic variation in the HSD3B2 gene. We hypothesize that allelic variants of the HSD3B2 gene may play a role in predisposition to prostate cancer, and in explaining the substantial racial/ethnic variation in risk.

Deletion of chromosome 11p15, p12, q22, q23-24 loci in human prostate cancer

Loss of heterozygosity (LOH) on chromosome 11 is frequently altered in various epithelial cancers. The present study was designed to investigate LOH on chromosome 11 in microdissected samples of normal prostatic epithelium and invasive carcinoma from the same patients. For this purpose, DNA was extracted from the microdissected normal and tumor cells of 38 prostate cancers, amplified by polymerase chain reaction PCR and analyzed for LOH on chromosome 11 using 9 different polymorphic DNA markers (D11S1307, D11S989, D11S1313, D11S898, D11S940, D11S1818, D11S924, D11S1336 and D11S912). LOH on chromosome 11 was identified in 30 of 38 cases (78%) with at least one marker. Four distinct regions of loss detected were: 1) at 11p15, at loci between D11S1307 and D11S989; 2) at 11p12, on locus D11S131 (11p12); 3) at 11q22, on loci D11S898, D11S940 and D11S1818; and 4) at 11q23-24, on loci between D11S1336 and D11S912. We found 25% of the tumors with LOH at 11p15; 39% had LOH at 11p12; 66% had LOH at 11q22; and 47% had LOH at 11q23-24. These deletions at 11p15, 11p12, 11q22 and 11q23-24 loci were not related to the stage or grade of the tumor.

Chromosome 3p24-26 and 3p22-12 loss in human prostatic adenocarcinoma

Identification of loss of heterozygosity on specific genetic loci is crucial for understanding the pathogenesis of prostate cancer at the molecular level. This is especially important because the deleted regions may contain putative tumor suppressor genes. Chromosome 3p loss appears to be frequently associated with various epithelial cancers. To our knowledge, there is no report on loss of heterozygosity (LOH) of chromosome 3 in human prostate cancer. The present study was designed to investigate the LOH on chromosome 3p in microdissected samples of delineated regions of normal and invasive carcinoma areas of prostatic epithelium from the same tumor sections. For this purpose, DNA was extracted from microdissected normal and tumor cells of 38 prostate cancers, amplified by PCR and analyzed for LOH on chromosome 3p using 6 different polymorphic DNA markers (D3S1560, THRB, D3S647, D3S1298, D3S1228 and D3S1296). Our results suggest that LOH was identified in 34 of 38 cases (89%) with at least one marker. Twelve of 30 informative cases showed LOH at D3S1560; 18 of 22 informative cases showed loss at THRB; 20 of 38 informative cases showed deletion at D3S647; 16 of 38 informative cases showed loss at D3S1298; 12 of 34 informative cases showed LOH at D3S1228; and 6 of 34 informative cases showed LOH at D3S1296 regions. Our results suggest that the LOH is on the 3p24-26 and 3p22-12 regions of the short arm of chromosome 3, indicating 2 discrete areas of deletion on chromosome 3p. The deletion at 3p24-26 and 3p22-12 was not related to the stage or grade of the tumor.

How is individual risk for prostate cancer assessed?

A man's risk of developing prostate cancer is influenced by both genetic and nongenetic factors. Genetic factors are particularly important at younger ages, and the attributable risk of strong genetic factors could be as high as 43% among men less than 55 years of age; however, only about 9% of all cases may be directly attributable to a family history of prostate cancer. Race appears to be an important determinant of risk; African-American men are at high risk, whereas men of oriental ancestry are at lower risk. The bases of these racial differences remain obscure but may be related to hormonal differences. Modifiable risk factors are most important from a public health perspective. Diet or closely related factors appear to hold the most promise for prevention, although the precise factors are unknown. The strongest evidence indicates that some component of animal fat intake appears to act as a promoter of prostate cancer. Other dietary factors, including vitamin D, vitamin E, and beta-carotene and lycopene, may confer protection, but these require more study. Many but not all studies that have examined long-term effects of vasectomy suggest that this procedure may increase risk of prostate cancer, but whether this association is causal is not established. Occupational factors, smoking, and physical activity level do not appear to be major determinants of prostate cancer risk.

Age-dependent expression of the androgen receptor gene in the prostate and its implication in glandular differentiation and hyperplasia

The senescence phenotype is the product of both cumulative physical damages during the life span and a species-specific genetic program. The genetic program of aging appears to have co-evolved with the sexual mode of reproduction. The same developmental processes that prepare the animal for maximum vitality and reproductive competence during young adulthood, if allowed to continue, can be detrimental during old age. Androgen receptor-mediated development and growth of the prostate gland is an example of such "antagonistic pleiotropy." The prostate gland is composed of two major groups of cells: the epithelial and stromal. Among the epithelial type, the columnar cells on the luminal surface produce the prostatic secretions, and the basal cells are presumed to serve as progenitors of the columnar cells. Within the stromal cell population, fibroblastic and smooth muscle cells are thought to produce growth factors that support the development and function of the epithelial cells. Both epithelial and stromal cells are dependent on androgens. In this study, we have examined age-dependent expression of the androgen receptor gene in the prostatic tissues of rats and dogs. Unlike the rat, in which the prostatic growth ceases after sexual maturation, the dog prostate continues to grow during aging. Similar to the dog, the antagonistic pleiotropy of the prostatic growth in the human causes the pathological condition of benign prostatic hyperplasia (BPH), the major health problem in old men. Quantitation of the androgen receptor (AR) mRNA in the total prostate extracts from young and old animals by the reverse transcriptase-polymerase chain reaction (RT-PCR) method showed about a 30% decline in AR mRNA in the 24-month-old rat prostate, as compared to the prostate of 3-month-old young adult animals. However, no significant difference in AR mRNA contents between 1-year-old and 10-year-old dog prostates was observed. In situ immunostaining for the androgen receptor protein revealed that in the case of rat, developmental maturation during the first month of life is associated with an increase in AR immunoreactivity in the luminal columnar epithelium, with a concomitant loss of immunoreactivity in the basal cells. Furthermore, with aging, there was a marked increase in the proportion of AR-negative basal cells in comparison to luminal columnar cells. Surprisingly, in both young adult (approximately 1-year-old) and old (approximately 10-year-old) dogs, most of the AR immunoreactivity was localized in the fibroblastic stromal cells rather than in the epithelial cells. Based on these observations and the existing literature, we propose that normally, in most mammalian species, an age-dependent decline in the conversion of basal to columnar epithelial cells after sexual maturation serves as a stop signal for the prostate growth. However, in certain species, such as the dog, robust AR expression in the stromal cells overrides this regulatory blockage and leads to prostatic hyperplasia in old age.