Cancer in twins: genetic and nongenetic familial risk factors

The search for cancer risk factors: when can we stop looking?

In recent decades, countless cohort, case-control, and ecologic studies have been conducted in the search for cancer risk factors. On the basis of knowledge gained from these studies, various influential commentaries have endeavored to classify the extent to which the total cancer burden is attributable to general categories of risk, such as diet, tobacco, sun exposure, and others. These commentaries have led to the conventional wisdom that most of the cancer burden is caused by environmental factors and relatively little is directly attributable to genetic susceptibility. In the face of the apparent knowledge that the cancer burden is essentially fully "explainable" on the basis of known environmental risks, this article addresses the conceptual and empirical basis of the continued search for new risk factors. It proposes that the extent of the aggregation of cancer within individuals in the population--that is, the occurrence of second primary cancers--is a crucial statistic in this context. A study of the incidence of second primary melanoma suggests that the bulk of the risk variation in this disease cannot be explained by known risk factors. The implications of these ideas for research strategy and for public health policy are discussed.

Molecular genetics of prostate cancer

The molecular mechanisms underlying the development and progression of prostate cancer are poorly understood. Epidemiological studies have suggested that 5-10% of all prostate cancers are familial, and numerous chromosomal loci have been associated with prostate cancer in multicentre linkage studies. However, no putative susceptibility genes harboured in these chromosomal regions have thus far been identified. Several recurrent chromosomal alterations in prostate cancer have been detected in comparative genomic hybridization (CGH) and loss of heterozygosity (LOH) analysis. The target genes for many of these aberrations are still not known. It seems that the androgen receptor (AR) signalling pathway plays a crucial role in both early development as well as in late progression of the disease. Both germ-line and somatic genetic alterations in the AR gene have been demonstrated in prostate cancer patients. The intention of this review is to summarize the current knowledge of molecular mechanisms in the development of prostate cancer.

Sibling risks in cancer: clues to recessive or X-linked genes?

A systematic analysis of cancer risks to offspring and to siblings of cancer cases was carried out based on the nation-wide Swedish Family-Cancer Database. For all 13 cancer sites examined, risks to both offspring and siblings of cases of cancer at the same site were significantly elevated. The relative risk to siblings was approximately 2 fold more than the offspring risk for cancers of the prostate, testis, kidney and bladder, suggesting that recessive or X-linked susceptibility genes may be important for these cancers. Risks to siblings of cases where a parent was also affected were increased >20 fold over population rates for colorectal, ovarian, prostate and renal cancer, and for leukaemia, consistent with the effects of rare high-risk susceptibility alleles.

Cancer risks to spouses and offspring in the Family-Cancer Database

It is generally accepted that cancer is caused by environmental and inherited factors but these are only partially identified. Family studies can be informative but they do not separate shared lifestyles and genes. We estimate familial risks for concordant cancers between spouses in common cancers of both sexes in order to quantify cancer risks from the shared environment. The risks are compared to those seen between parents and offspring in order to estimate the inherited component. The nation-wide Family-Cancer Database was used as the source of family and cancer data. Standardized incidence ratios (SIRs) were calculated for concordant cancer in offspring by parental cancer and in spouses. Among the 23 cancer sites considered, all but two showed an increased SIR for offspring by father or mother. Only two sites, stomach and lung, showed an increase in SIR of concordant cancer among spouses. Additionally, pancreatic cancer and melanoma were increased in couples where at least one spouse was diagnosed before age 50. If both spouses presented melanoma before age 40, SIR was 3.82 for husbands. SIRs of colon, renal, and skin (squamous cell) cancers were unchanged by spouses' concordant cancer. Shared lifestyle among spouses seems to explain only a small proportion of cancer susceptibility. Because lifestyles are likely to differ more between parents and offspring than between spouses, familial cancer risks between parents and offspring are likely to be more due to heritable rather than environmental effects.

Familial risks in invasive and in situ cervical cancer by histological type

The Swedish Family-Cancer Database was used to analyse familial relationships in mothers and daughters with invasive and in situ cervical cancers by histological type during the years 1958-1996, including a total of 21,727 and 191,081 cases, respectively. Familial standardized incidence ratios (SIRs) were calculated separately for mothers and daughters and for invasive and in situ squamous cell carcinoma (SCC) and adenocarcinoma. Familial risks were about 2.0 in invasive SCC and less in in situ SCC. Limited analyses could be carried out on adenocarcinoma because the number of cases was small. However, familial risks were not much smaller in families where only SCC was diagnosed compared with those where both SCC and adenocarcinoma were present. A comparison of cancers between mothers and daughters showed an association between cervical cancer and SCC of skin, and between cervical cancer and smoking-related cancers. The familial risks were unaffected in Poisson regression analysis on many possible intervening variables. The data suggest that host factors modulate an individual's response to human papillomavirus infections.

Genetic epidemiology of multistage carcinogenesis

It is commonly believed that cancer is a multistage, polygenic disease. Even though conceptually appealing, the evidence supporting the multistage theory remains limited. Most known tumor suppresser genes are associated with monogenic dominant cancers following a two-hit pathway. We review results from a recent twin study on 90000 individuals that give support to the multistage theory. Statistically significant heritability estimates were shown for cancers of the colorectum (35%), breast (27%), and prostate (42%). These estimates are much higher than those obtained from family studies in which parents and offspring, or sibs are compared. The difference can be accounted for by the involvement of many genes. A polygenic cancer would show small effects in family studies but large effects in twin studies. We present calculations on the decrease in familial risks when the number of genes involved increases or when the penetrance decreases. We test the apparent number of stages involved in the main cancers from the Swedish Family-Cancer Database. The logarithms of the slopes suggest large differences in the apparent numbers of mutations involved in different cancers. The number of mutations required appears to be less in familial breast cancer compared to sporadic breast cancer. Study designs for gene identification should be revised to accommodate polygenic cancers.

Estimation of genetic and environmental components in colorectal and lung cancer and melanoma

Cancer has predominant environmental and somatic causes but the assessment of hereditary (genetic) causes is difficult, except for highly penetrant single-gene causes. Family studies are only partially informative in this regard because family members share diet and life-styles. Twin studies have been classically used to disentangle the effects of heredity and environment on disease etiology. We estimate the genetic and environmental components in colorectal and lung cancer and melanoma by comparing cancer risks in family members. The Swedish Family-Cancer Database, comprising more than 6 million individuals, was used as the source of family and cancer data. Tetrachoric correlations were used to describe similarity in cancer liability among family members. Structural equation modelling was used to derive estimates of the importance of genetic and environmental effects. The estimated genetic component ranged from 10% in colon and colorectal cancer to 18% in melanoma. For lung cancer, the share was 14%. If assortative mating were important for liability to cancer, these heritability estimates may be an underestimation of the true genetic effects. Non-shared environmental effect was 67-68% in colorectal cancer and melanoma, and 71% in lung cancer. Shared and childhood environments were equally important in colorectal cancer and melanoma, whereas no childhood effect was observed for lung cancer.

Cancer incidence for Swedish twins studied by means of bivariate frailty models

Cancer incidence rates for Swedish twins born between 1928 and 1965 and who both were alive at age 30 are studied by means of bivariate frailty models. Altogether, 7,280 fraternal (DZ) and 4,699 identical (MZ) twin pairs were followed up through December 31, 1995, for cancer status. The association between cancer incidence rates was statistically greater among the MZ than among the DZ pairs and stronger between women than between men; however, the magnitude of this association is relatively small and decreases over time. The relative decrease in dependency (association) is most easily detected using shared frailty models but may also be demonstrated, at least for women, using correlated frailty models. We also demonstrate that estimates of the correlation coefficient are similar when using any correlated frailty models derived from the power variance family but that these estimates disagree regarding the age at which the dependence is most important. The relative importance of dependence across age may sometimes be more interesting than the correlation coefficient itself. The latter may usually be estimated using alternative methods. Furthermore, when estimating correlation coefficients close to the boundary of the parameter space, simulation studies indicate that the correlated inverse Gaussian frailty model is more robust than the gamma frailty model.

High constant incidence in twins and other relatives of women with breast cancer

The incidence of breast cancer rises steeply between ages 25 and 50, and more slowly thereafter. In contrast, the incidence in the unaffected (contralateral) breast of women who have had breast cancer remains constant at about 0.7% per year for at least the next 20 years after diagnosis, irrespective of age at first diagnosis. The incidence in relatives of the patients seems to show a similar pattern. The incidence in a prospective study of monozygotic twins of patients was approximately constant at 1.3% per year (77 cases), again about 0.7% per breast. At ages older than a patient's age at diagnosis, her mother and sisters have an incidence of 0.3-0.4% per year. Above the index patient's age at diagnosis, the rate in relatives shows no temporal trend and is independent of the patient's age at diagnosis. A statistically simple explanation is that incidence in susceptible women increases to a high constant level by a predetermined age that varies between families, but this seems inconsistent with conventional models of carcinogenesis and susceptibility. The very high incidence in monozygotic twins of patients indicates that a high proportion, and perhaps the majority, of breast cancers arise in a susceptible minority of women.

Heritability and risk factors of uterine fibroids--the Finnish Twin Cohort study

OBJECTIVES

Our aim was to study heritability, risk factors and hospitalization for uterine fibroids.

METHODS

A random sample of 80 MZ and 80 DZ twins from the Finnish Twin Cohort were invited and 51% of the eligible women (n=82, 17 MZ and 16 DZ pairs, 40-47 years, mean age 43.0), underwent a transvaginal ultrasound. The entire cohort of 13872 women was linked to the national hospital discharge registry 1972-1990.

RESULTS

Prevalence of fibroids was 66% and the average number of fibroids 1.7. The casewise concordance for being hospitalized for uterine fibroids was higher in MZ (0.31, 95% CI 0.24-0.37) than in DZ pairs (0.18, 95% CI 0.14-0.22). The proportion of variance in liability to fibroid hospitalization accounted for by genetic factors was 54.8% (95% CI 46.2-62.7%). Women with fibroids had higher body mass index (23.7 vs 21.7, P=0.0086), lower age at first birth (25.7 vs 29.3, P=0.012) and higher parity (3+ children 48.2 vs 29.6%, P=0.009) than women without fibroids. Risk ratio (RR) for fibroids in a MZ twin whose sister had been diagnosed with fibroids was 1.1 (95% CI 0.08;15), for a DZ twin 1.1 (95% CI 0.16;8.8) and for all twins 1.3 (95% CI 0.3; 6.1). Intraclass correlation for the number of fibroids was 0.24 for MZ and 0.11 for DZ twins, yielding an heritability estimate of 0.26.

CONCLUSION

Reproductive and anthropometric factors may have at least as large role in pathogenesis of fibroids than genetic factors.

Improving the validity and generalizability of studies with underserved U.S. populations expanding the research paradigm

The gap in cancer incidence and mortality between ethnic minority groups and European Americans in the U.S. is significant and growing. Compared with a drop in incidence and mortality rates in the U.S. since 1992, the rates for ethnic minority groups have increased or stayed the same. A major transformation in the predominantly deductive social and behavioral research paradigm in cancer is required to reduce these disparities in cancer outcomes and improve the quality of life for ethnic minority populations with cancer. This article highlights the fundamental changes that are required to transform the prevalent paradigm for social and behavioral research in cancer care from a monocultural, Eurocentric framework to a multicultural one. Use of the expanded framework would enhance the scientific rigor, validity, generalizability, applicability, and acceptability of multicultural behavioral research. The strategies developed from such research would have a greater likelihood of success in providing optimal cancer care for underserved populations and reduce the disparities in cancer outcomes between U.S. ethnic minority populations and the white population. Examples from the cancer literature are used to illustrate each of the seven steps of the expanded paradigm.

The inherited nature of lung cancer: a pilot study

The role of both primary and secondary cigarette smoke exposure in the causation of lung cancer appears certain. An estimated 90% of lung cancer is attributed to cigarette smoke. Remarkably, however, less then 20% of cigarette smokers develop lung cancer. Investigators have suggested that a genetic predisposition to lung cancer may contribute to familial aggregation of this cancer. To understand the contribution of familial aggregation to this type of cancer and potentially identify individuals and families, which may be important in identifying gene(s) responsible for lung cancer, we developed criteria for identification of high-risk families. We have tested the feasibility and utility of these criteria at three Denver, CO hospitals with very different patient populations. Four hundred eighteen individuals were diagnosed with lung cancer at these three hospitals between 1/1/95 and 8/31/95. Twenty-nine percent of individuals expired prior to the time of initial contact. Family history data were obtained on 182 individuals. To be considered positive (suggesting possible autosomal dominant inheritance of lung cancer), families must have at least two first-degree relatives with lung cancer, one of which must be diagnosed before the age of 55. Seventeen of 182 (9.3%) families in the study population met these criteria. We reviewed the remaining family histories that did not meet the established criteria and identified another 2.3% (5/182) of families had evidence for autosomal dominant transmission of lung cancer. An additional 15% (23/182) of families had histories which could not be classified without further information. This study suggests that at least 11.6% of individuals diagnosed with lung cancer will have a positive family history of lung cancer. Use of the criteria developed for this study may lead to an underestimation of the inherited etiology of lung cancer.

Two identical triplet sisters carrying a germline BRCA1 gene mutation acquire very similar breast cancer somatic mutations at multiple other sites throughout the genome

Monozygotic twins, each of whom has breast cancer, offer a natural study population for gene-environmental interactions as causation of cancer, because they are genetically identical. If heritable factors play a large role in the origin of a neoplasm, disease concordance should be significant in monozygotic twins. Two monozygotic triplet sisters carrying a germline BRCA1 gene mutation (5382insC) who both developed breast cancer at early ages were studied for loss of heterozygosity (LOH) in their microdissected, paraffin-embedded tumors along with control blood and stromal breast tissue at 19 chromosomal arms using 161 microsatellite markers. Microdissected areas of normal lobular and ductal epithelium and ductal in situ carcinoma were also studied for LOH using a subset of microsatellite markers. The mother's DNA (extracted from peripheral blood lymphocytes) was analyzed to determine the parental allele under LOH in each case. Both tumors demonstrated similar histologic features suggestive of a secretory variant of ductal carcinoma. The tumors from both sisters had similar overall LOH frequency expressed by the fractional allelic loss (FAL) indices (0.56 vs. 0.60) and demonstrated concordance for loss or retention at 82 of 97 informative markers (85% correlation). In addition, detailed mapping analysis of several chromosomal arms revealed that identical breakpoints were detected in both tumors at several chromosome regions. Finally, in both sisters' tumors, when a chromosome exhibited allelic loss, all of the markers exhibited LOH of the same parental allele even when there were intervening regions of retention of heterozygosity. In contrast, 17 archival sporadic breast carcinomas demonstrated a wide range of FAL indexes and highly individual patterns of LOH. Our findings support the hypothesis that inherited factors play a role in the development of the multiple somatic deletions occurring in breast carcinomas. Whether one of these factors is the mutant BRCA1 allele or some other gene(s) remains to be determined. Genes Chromosomes Cancer 28:359-369, 2000.

The p53 gene in patients under the age of 40 with gastric cancer: mutation rates are low but are associated with a cardiac location

BACKGROUND

Determining both the frequency and the spectrum of p53 gene mutation in young patients with gastric cancer might provide clues to the host related genetic mechanism(s) in gastric carcinogenesis.

PATIENTS AND METHODS

p53 mutations were assessed (by means of polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP), followed by DNA sequencing) in a cohort of 105 consecutive Italian patients in whom gastric cancer was ascertained before the age of 41.

RESULTS

A low prevalence of p53 mutations (eight of 105) was observed, with no significant difference between intestinal (three of 31; 10%) and diffuse (five of 74; 7%) phenotypes. A significantly higher prevalence of p53 mutations was associated with the cardiac location (odds ratio, 7.09; confidence interval, 1.56 to 32.11). In all but one case, p53 mutations were associated with a stage higher than I. All eight mutations were located at CpG sites, where G : C to A : T transitions have been associated with frequent methylation at the C5 position of cytosine.

CONCLUSIONS

These findings show that, unlike what has been consistently demonstrated in the general population, p53 mutations are uncommon in gastric cancer occurring in young patients, and in such patients, p53 alterations are significantly associated with the cardiac location.

Confirmation of prostate cancer susceptibility genes using high-risk families

Data from many types of studies support the hypothesis that strong familial components are involved in the etiology of prostate cancer. One way to access such genes is through the study of families with multiple affected family members and, in particular, families with individuals affected comparatively early in life. Several prostate cancer susceptibility loci have been described to date. Confirmation of the linkage and estimation of the proportion of families who are linked in large independent datasets is essential to understanding the significance of susceptibility genes. We explore the methodology used to perform such studies and the factors that can limit the ability to confirm linkage results. We report specifically the example of the HPC1 gene on 1q24-25.

Environmental and heritable factors in the causation of cancer--analyses of cohorts of twins from Sweden, Denmark, and Finland

BACKGROUND

The contribution of hereditary factors to the causation of sporadic cancer is unclear. Studies of twins make it possible to estimate the overall contribution of inherited genes to the development of malignant diseases.

METHODS

We combined data on 44,788 pairs of twins listed in the Swedish, Danish, and Finnish twin registries in order to assess the risks of cancer at 28 anatomical sites for the twins of persons with cancer. Statistical modeling was used to estimate the relative importance of heritable and environmental factors in causing cancer at 11 of those sites.

RESULTS

At least one cancer occurred in 10,803 persons among 9512 pairs of twins. An increased risk was found among the twins of affected persons for stomach, colorectal, lung, breast, and prostate cancer. Statistically significant effects of heritable factors were observed for prostate cancer (42 percent; 95 percent confidence interval, 29 to 50 percent), colorectal cancer (35 percent; 95 percent confidence interval, 10 to 48 percent), and breast cancer (27 percent; 95 percent confidence interval, 4 to 41 percent).

CONCLUSIONS

Inherited genetic factors make a minor contribution to susceptibility to most types of neoplasms. This finding indicates that the environment has the principal role in causing sporadic cancer. The relatively large effect of heritability in cancer at a few sites suggests major gaps in our knowledge of the genetics of cancer.

National database of familial cancer in Sweden

A family cancer database was constructed from the nationwide Swedish registries and includes approximately 6 million persons and >30,000 cancers in offspring diagnosed at ages 15-51 years and their parents. A particular advantage of the database is that the contribution of both parental lineages on cancer risk can be examined. Cancer risk in the offspring was increased approximately 1.1 times when the father had cancer, and no increase was noted when the mother had cancer. If both parents had cancer, the risk for sons was 1.4 and for daughters 1.3. The sites of increased cancer risk in the offspring were colorectum, breast, cervix, corpus uteri, ovary, testis, melanoma, eye, other endocrine glands, and multiple myeloma. The results among young and middle-age adults suggest that cancer in both parents increases the cancer risk in the offspring at many sites. The molecular genetic explanation may be that rare dominant single genes increase susceptibility at many sites, or that overlapping sets of genes control susceptibility at multiple sites.

A genome screen of multiplex sibships with prostate cancer

Analysis of a genome screen of 504 brothers with prostate cancer (CaP) who were from 230 multiplex sibships identified five regions with nominally positive linkage signals, on chromosomes 2q, 12p, 15q, 16p, and 16q. The strongest signal in these data is found on chromosome 16q, between markers D16S515 and D16S3040, a region suspected to contain a tumor-suppressor gene. On the basis of findings from previous genome screens of families with CaP, three preplanned subanalyses were carried out, in the hope of increasing the subgroup homogeneity. Subgroups were formed by dividing the sibships into a group with a positive family history (FH+) that met criteria for "hereditary" CaP (n=111) versus those which did not meet the criteria (n=119) and by dividing the families into those with a mean onset age below the median (n=115) versus those with a mean onset age above the median (n=115). A separate subanalysis was carried out for families with a history of breast cancer (CaB+ [n=53]). Analyses of these subgroups revealed a number of potentially important differences in regions that were nonsignificant when all the families were analyzed together. In particular, the subgroup without a positive family history (FH-) had a signal in a region that is proximal to the putative site of the HPC1 locus on chromosome 1, whereas the late-age-at-onset group had a signal on 4q. The CaB+ subgroup revealed a strong linkage signal at 1p35.1.

Quadruple primary neoplasms in a 27-year-old woman

A27-year-old woman was referred to our department on 3 February 1998 with a diagnosis of malignant melanoma associated with cystic teratoma of ovary. She had had a laparotomy and bilateral salphingo-oophorectomy on 5 January 1998. Pathology revealed tumour involvement of left ovary, left parametrium and malignant ascites, the right ovary showed polycystic disease, the uterus showed simple hyperplasia of endothelium, the omentum had no tumour and subdiaphragmatic scrapping was negative. On histological examination of the tumour, the cystic portion was basically a cystic teratoma with presence of skin, glial and bony tissue, the solid portion consisted of sheets of undifferentiated tumour cells, which on immunohistochemical staining shown strong positivity to HMB 45 and S-100. Ultrastructual examination confirmed melanocytic differentiation.

Her chest X-ray was clear. Computed tomography of abdomen and pelvis suggested bilobar liver metastasis and significant para-aortic lymphadenopathy. The alpha-fetoprotein (αFP) and beta-human chorionic gonadotrophin (β-hCG) were within normal limits. The diagnosis was a malignant melanoma arising in cystic teratoma of ovary, stage IV disease.

Her past medical history included multiple primary neoplasms. She had right thigh cutaneous histiocytoma at the age of 20 years, with complete excision performed. She developed poorly differentiated invasive ductal carcinoma of left breast (stage pT2N0M0) at the age of 21 years and had a left mastectomy and left axillary clearance; no postoperative adjuvant treatment was given. She also recently had multiple thyroid nodules which shown atypical cells, suggestive of a follicular neoplasm, and total thyroidectomy was contemplated.

Evaluation of her family revealed a strong family history of malignancy. Her twin sister died of a primary brain tumour at the age of 8 years. Her father died of lung cancer at the age of 45 years. Her paternal grandfather died of lung cancer in his 60s. Her paternal grandmother died of primary liver cancer in her 60s. One of her paternal half-uncles (mutual grandfather, different grandmothers) died of a primary brain tumour in his 50s while another died of lung cancer in his 70s. There was no positive family history of malignancy on her maternal side (Figure 1).

She was treated with combination chemotherapy using cisplatin (75 mg/m2) and cyclophosphamide (750 mg/m2). However, she deteriorated rapidly after the first cycle of chemotherapy and died on 16 March 1998.

Familial prostate cancer from the family-cancer database

The aim of this study was to calculate the familial risk for prostate cancer (PC) for different family relationships. PC was studied in the Swedish Family-Cancer Database, updated in 1999 to cover individuals born after 1934 with their biological parents, totalling 9.6 million persons. Cancer data were obtained from the Swedish Cancer Registry from 1958 to 1996 and included 1035 PC cases amongst offspring. 188 families were identified where a father and a son had PC, giving a familial standardised incidence ratio (SIR) of 2.44 (2.10-2.80). The proportion of familial cancers was 18.2% amongst all PC amongst all PC amongst sons. There were only 5 pairs of affected brothers, of which 3 had an affected father. Age of onset modified familial risks modestly; the highest SIR of 4.43 (1.40-9.17) was for sons diagnosed before 50 years of age when the father was diagnosed before 65 years of age. When analysed across sites, an association of PC in one generation and stomach, liver and skin cancer and myeloma in another generation was observed. The link was most consistent for skin cancer. No maternal site was associated with a son's PC, although the SIR of breast cancer was 1.22 (0.95-1.53). No increased risk of malignancy was observed in wives of affected men excluding any shared environmental effect for PC and female cancers.

Genetic predisposition, environment and cancer incidence: a nationwide twin study in Finland, 1976-1995

Twin studies integrate genetic and environmental (including physical environment and life-style) information by comparing monozygotic and dizygotic twins for the occurrence of disease. Our objectives were to compare cancer incidence in twins with national rates and to estimate both the probability that co-twins of affected twins may develop cancer and the importance of genetic predisposition and environment in cancer development. The nationwide record linkage of the Finnish Twin Cohort Study, the Finnish Cancer Registry and the Central Population Register allowed the follow-up of 12,941 same-sexed twin pairs for incident primary cancers from 1976 to 1995. Zygosity was determined by use of a validated questionnaire in 1975. Methods included calculation of standardized incidence ratios and concordances and fitting of structural equation models. A total of 1,613 malignant neoplasms occurred in the cohort. The overall cancer incidence among twins resembled that among the general population. Monozygotic co-twins of affected twins were at 50% higher risk than were dizygotic co-twins. Based on genetic modeling, inherited genetic factors accounted for 18% (95% confidence interval 4-32%) of the liability in inter-individual variation in the risk of overall cancer, while non-genetic factors shared by twins accounted for 7% (0-16%) and unique environmental factors for 75% (65-85%). Our results appear to exclude a contribution greater than one-third for genetic predisposition in the development of cancer in the general population, thus pointing to the earlier confirmed substantial role of environment.

Infection, immunity, and cancer

A significant percentage of human cancers worldwide are associated with infections due to known viruses, including human papillomaviruses (cervical cancer and other skin cancers), human T-lymphotropic viruses (adult T-cell leukemias and lymphomas in endemic areas), hepatitis B virus (liver cancer), and Epstein-Barr virus (Burkitt lymphoma and nasopharyngeal carcinoma). The fraction of human cancers attributable to infection may now need to be revised in light of the fact that new viral associations have been discovered and other nonviral associations have been identified. This article addresses the increasingly recognized role of infectious agents as precipitants of human neoplasia and the possibility that novel diagnostic, therapeutic, and chemopreventive strategies may emanate directly from research directed at identifying and understanding these agents.

Familial risks in cervical cancer: is there a hereditary component?

The Swedish Family-Cancer Database was used to analyze familial relationships in mothers and daughters in invasive and in situ cervical cancers from years 1958-1994, including a total of 125,000 in situ and 14,000 invasive cancers. In situ cancers were diagnosed on average 10 years earlier than invasive cancers. Familial relative risks (FRRs) were calculated separately for mothers and daughters, and were between 1.8 and 2.3 for the 2 forms. The risks were only slightly modified by age of onset, except at higher ages where there was no familial risk; in mothers, the risks increased if more than one daughter was affected. Aggregation of in situ cases among sisters was observed in families. Heritability estimates were between 0.11 and 0.15 for in situ and 0.22 and 0.34 for invasive cervical cancer. A comparison of cancers in mothers and daughters showed an association between cervical cancer and many cancer types observed in immunosuppressed patients, suggesting a role for a mild form of immunosuppression, in addition to sexual behavior leading to human-papilloma-virus infections, in familial cervical cancer.

DNA copy number losses at 1p32-pter in monozygotic twins concordant for breast cancer

To find similarities that may possibly indicate novel mutations, we performed comparative genomic hybridization (CGH) analysis following degenerate oligonucleotide primed polymerase chain reaction (PCR) for DNA obtained from unique material of breast cancer that developed in monozygotic twin-pairs. Polymerase chain reaction amplification was successful in 12 samples for 11 patients, including 3 pairs. Six samples exhibited DNA copy number changes. Gains (76%) were more frequent than losses (24%). Gains or high-level amplifications in 8q were present in all but 1 of the abnormal cases. Frequent gains were detected with a minimal common overlapping region at 5p (4 cases), at 1q25-qter (3 cases), and at 20q12-qter (2 cases). The most frequent loss, detected in half of the abnormal cases, was at 1p32-pter. One twin-pair showed similar changes in 4 chromosomal locations involving loss of 1p32-pter and gains in 1q25-qter, 5, and 8q.

Familial cancers in a nationwide family cancer database: age distribution and prevalence

We calculated sex- and age-specific familial relative risks (FRRs) of cancer in offspring of cancer probands at 19 male and 20 female cancer sites, based on the nationwide Family Cancer Database from Sweden. The proportion of familial cancers among all cancers was also determined. The database contained 550,000 primary cancers. The familial risk at known sites: colon, breast, ovary, testis, skin (melanoma), nervous system, thyroid and other endocrine glands were confirmed. The FRR of thyroid cancer exceeded any other cancer and was over twice as high for male as for female offspring, and appeared to constitute an early- and late-onset component. Novel register-based findings were familial risks in cervical and uterine cancer, and in male offspring of male probands kidney and skin (mainly squamous cell) cancer. Familial risks were noted also for lung cancer, lymphoma and leukaemia but they may have largely environmental causes. The proportion of familial cancers depended on the site, ranging from 11% in prostate to 8.7% in female breast and to well below 1% at many sites.

Association between smoking and tumor progression in Japanese women with adenocarcinoma of the lung

We studied the effect of smoking on tumor progression in 3312 patients with lung cancer registered at the National Matsudo Hospital and National Cancer Center Hospital East between 1977 and 1996. The odds ratios of the following variables for tumor extent (localized versus advanced disease) and hazard ratios for survival were calculated in both sexes separately using the logistic regression and Cox proportional hazard models, respectively: smoking history, number of cigarettes smoked per day, pack-years smoked, age, histological type, and the year of admission. Of the 943 women, 367 (38.9%) were smokers and 694 (73.6%) had adenocarcinoma, whereas of the 2369 men, 2255 (95.2%) were smokers and 1010 (42.6%) had adenocarcinoma. In female adenocarcinoma patients, the odds ratio (95% confidence interval) for advanced disease and the hazard ratio (95% confidence interval) for survival with an increase of 30 cigarettes smoked per day were 2.86 (1.49-5.49) and 1.52 (1.13-2.04), respectively, but in those with non-adenocarcinoma, the odds ratio and hazard ratio were 0.96 (0.41-2.23) and 1.13 (0.75-1.70), respectively. In male patients, smoking history influenced tumor progression regardless of histological type, but the odds ratios and hazard ratios were lower than those for women with adenocarcinoma. In conclusion, smoking habit was closely correlated with progression of adenocarcinoma in women. This association was not observed in women with non-adenocarcinoma and was weaker in men, suggesting various effects of smoking on lung cancer development depending on gender and the histological type of the tumor.

Hereditary factors in basal cell carcinoma of the skin: a population-based cohort study in twins

The contribution of hereditary factors in basal cell carcinoma of the skin has not been well defined at the population level. We aimed to assess the hereditary component in basal cell carcinoma by comparing its occurrence in monozygotic and dizygotic twin pairs. The Finnish Twin Cohort, comprising 12,941 adult, like-sex twin pairs with established zygosity and resident in Finland in 1975, was linked with the Finnish Cancer Registry. We identified 335 twin pairs in which at least one twin had basal cell carcinoma diagnosed between 1953 and 1996. Standardized incidence ratios, concordances, tetrachoric correlations and pairwise relative risks were computed by standard methods. Components of variance in liability were estimated by structural equation modelling. There was an elevated risk of basal cell carcinoma for the co-twin of a diseased twin, but no difference in risk by zygosity. During the prospective follow-up in 1976-96, the probandwise concordance was 7.7% in monozygotic and 7.0% in dizygotic pairs. Model fitting indicated that genetic factors were not needed to account for the distribution of basal cell carcinoma in twin pairs. These results confirm the major role of environmental factors in the aetiology of basal cell carcinoma.

Evidence for autosomal dominant inheritance of prostate cancer

A family-history cancer survey was conducted on 5,486 men who underwent a radical prostatectomy, for clinically localized prostate cancer, in the Department of Urology at the Mayo Clinic during 1966-95; 4,288 men responded to the survey. Complex segregation analysis was performed to assess the genetic basis of age at diagnosis and the familial clustering of prostate cancer. For the total group, no single-gene model of inheritance clearly explained familial clustering of disease, which could be partly explained by lack of Hardy-Weinberg equilibrium, with an excess of homozygotes. After accounting for deviations from Hardy-Weinberg equilibrium, the best-fitting model that explained the familial aggregation and age at diagnosis was a rare autosomal dominant susceptibility gene, and this model fitted best when probands were diagnosed at <60 years of age. The model predicts that the frequency of the susceptibility gene in the population is .006 and that the risk of prostate cancer by age 85 years is 89% among carriers of the gene and 3% among noncarriers. A strength of our study is its large size, such that genetic models could be fitted within strata defined by the age of the proband. Although the autosomal dominant model was consistently the best model, the parameter estimates differed somewhat (P=.03) across the different age groups, suggesting genetic heterogeneity. Additional evidence that the hereditary basis of prostate cancer is likely to be genetically complex was provided by the following: (1) there was a significantly elevated age-adjusted risk of prostate cancer among brothers of probands, compared with their fathers (relative risk 1.5 [95% confidence interval 1.4-1.7]); (2) the autosomal dominant model predicted an excess of homozygotes, over that predicted by Hardy-Weinberg equilibrium; and (3) the model-predicted risk of prostate cancer among relatives was inadequate when probands were diagnosed at age >=70 years.

Use of twins in search for tumor suppressor genes

A new approach is applied in the mapping of tumor suppressor genes: analysis of loss of heterozygosity (LOH) in concordant tumors of monozygotic and dizygotic twins. The method relies on recognition of genome locations undergoing loss in both twins in a high proportion of the set of all twin pairs examined. The method effectively pinpoints, and excludes, the loci of potential tumor suppressor genes. With the help of a high density linkage map any such candidates can be placed within a narrow region of a chromosome arm and perhaps matched with known genes. The analysis of the Swedish Twin Registry has shown a clear genetic component for breast cancer. We have identified mono- and dizygotic twins concordant for breast cancer and collected the pathology specimens. Tumor and normal tissue was microdissected and microsatellite analysis carried out to test for allelic loss (LOH) in entirely new and putative chromosomal loci in this cancer. It can be calculated that using only six pairs of informative monozygotic twins, a locus can be incriminated with a high probability. Using increasingly dense markers and search for homozygous deletions, it should be possible to map one or more candidates for breast cancer.