Genetics and etiology of human cancer

Genes and oral cancer

Oral cancers have been one of the leading causes of deaths particularly in the developing countries. Prime reason for this high mortality and morbidity is attributed to the delay in diagnosis and prompt treatment. Relentless research in the field of oncology has led to the advent of novel procedures for the early detection of oral cancers. Molecular biology is highly promising in this regard. It is a procedure that detects alterations at a molecular level much before they are seen under a microscope and much before clinical changes occur. Molecular studies serve as the basis by which we will eventually be able not only to augment clinical assessment and classification of oral lesions but also predict malignant potential of oral lesions, thus reducing the incidence and increasing the scope for early diagnosis and treatment of oral cancers. However, making such sophisticated tools available for the common man in developing countries is one of the most important challenges faced today.

Frequent alterations of LOH11CR2A, PIG8 and CHEK1 genes at chromosomal 11q24.1-24.2 region in breast carcinoma: clinical and prognostic implications

To understand the importance of frequent deletions at chromosome 11q24.1-24.2 region in breast carcinoma, alterations (deletion/methylation) of the candidate genes LOH11CR2A, ROBO3, ROBO4, HEPACAM, PIG8 and CHEK1 located in this region were analyzed in 106 breast carcinoma samples. Among these genes, LOH11CR2A showed highest frequency of deletion (56%), followed by PIG8 (35%), CHEK1 (31%) and ROBO3/ROBO4/HEPACAM loci (28%). Comparable frequency of promoter methylation (26-35%) was observed for LOH11CR2A, CHEK1 and PIG8. Overall alterations (deletion/methylation) of these genes were in the following order: LOH11CR2A (60%) > PIG8 (46%) > CHEK1 (41%) and showed significant association with each other. Breast carcinoma samples that were estrogen/progesterone receptor negative showed significantly high deletion and overall alterations than estrogen/progesterone receptor positive samples for LOH11CR2A, CHEK1 and PIG8. The methylation and overall alteration of LOH11CR2A were significantly associated with tumor stages in breast carcinoma. However, in early/late onset and estrogen/progesterone receptor positive/negative breast carcinoma, the overall alterations of LOH11CR2A, PIG8 and CHEK1 were differentially associated with advanced stages, tumor grade and lymph node metastasis. Alterations of PIG8 and CHEK1 were significantly associated with poor prognosis in patients with early age of onset of the disease indicating significant prognostic importance. Quantitative mRNA expression analysis detected reduced expression of the genes in the order LOH11CR2A > CHEK1 > PIG8. Immunohistochemical analysis showed reduced protein expression of PIG8 and CHEK1 that was concordant with their molecular alterations. Thus, our study suggests that LOH11CR2A, PIG8 and CHEK1 are candidate tumor suppressor genes associated with breast carcinoma and have significant clinical as well as prognostic importance.

Molecular pathogenesis of oral squamous cell carcinoma: implications for therapy

The development of oral squamous cell carcinoma (OSCC) is a multistep process requiring the accumulation of multiple genetic alterations, influenced by a patient's genetic predisposition as well as by environmental influences, including tobacco, alcohol, chronic inflammation, and viral infection. Tumorigenic genetic alterations consist of two major types: tumor suppressor genes, which promote tumor development when inactivated; and oncogenes, which promote tumor development when activated. Tumor suppressor genes can be inactivated through genetic events such as mutation, loss of heterozygosity, or deletion, or by epigenetic modifications such as DNA methylation or chromatin remodeling. Oncogenes can be activated through overexpression due to gene amplification, increased transcription, or changes in structure due to mutations that lead to increased transforming activity. This review focuses on the molecular mechanisms of oral carcinogenesis and the use of biologic therapy to specifically target molecules altered in OSCC. The rapid progress that has been made in our understanding of the molecular alterations contributing to the development of OSCC is leading to improvements in the early diagnosis of tumors and the refinement of biologic treatments individualized to the specific characteristics of a patient's tumor.

SnoRNA U50 is a candidate tumor-suppressor gene at 6q14.3 with a mutation associated with clinically significant prostate cancer

Deletion of chromosome 6q14-q22 is common in multiple human cancers including prostate cancer, and chromosome 6 transferred into cancer cells induces senescence and reduces cell growth, tumorigenicity and metastasis, indicating the existence of one or more tumor-suppressor genes in 6q. To identify the 6q tumor-suppressor gene, we first narrowed the common region of deletion to a 2.5 Mb interval at 6q14-15. Of the 11 genes located in this minimal deletion region and expressed in normal prostates, only snoRNA U50 was mutated, demonstrated transcriptional downregulation and inhibited colony formation in prostate cancer cells. The mutation, a homozygous 2 bp (TT) deletion, was found in two of 30 prostate cancer cell lines/xenografts and nine of 89 localized prostate cancers (eleven of 119 or 9% cancers). Two of 89 (2%) patients with prostate cancer also showed the same mutation in their germline DNA, but none of 104 cancer-free control men did. The homozygous deletion abolished U50 function in a colony formation assay. Analysis of 1371 prostate cancer cases and 1371 matched control men from a case-control study nested in a prospective cohort showed that, although a germline heterozygous genotype of the deletion was detected in both patients and controls at similar frequencies, the homozygosity of the deletion was significantly associated with clinically significant prostate cancer (odds ratio 2.9; 95% confidence interval 1.17-7.21). These findings establish snoRNA U50 as a reasonable candidate for the 6q tumor-suppressor gene in prostate cancer and likely in other types of cancers.

Deletion, mutation, and loss of expression of KLF6 in human prostate cancer

Kruppel-like factors (KLFs) are a group of transcription factors that appear to be involved in different biological processes including carcinogenesis. In a recent study, KLF6 was reported as a tumor suppressor gene in prostate cancer because of its frequent loss of heterozygosity (LOH) and mutation as well as functional suppression of cell proliferation. Loss of chromosomal locus spanning KLF6 is relatively infrequent in other published studies of prostate cancer, however. To clarify the role of KLF6 in prostate cancers, particularly those that are high grade, we examined KLF6 for deletion, mutation, and loss of expression in 96 prostate cancer samples including 21 xenografts/cell lines. Loss of heterozygosity occurred in 4 (19%) of 21 xenografts/cell lines and 8 (28%) of 29 informative tumors. Fourteen of the 96 (15%) samples showed 15 somatic sequence changes in the KLF6 gene, including 7 that changed KLF6 peptide sequences, 4 that did not, and 4 that were located in untranslated regions. Expression levels of KLF6 were significantly lost in 4 of 20 (20%) xenografts/cell lines of prostate cancer, as detected by RT-PCR and Northern blot analysis. These findings indicate that significant genetic alterations of KLF6 occur in a minority of high-grade prostate cancers.

Strand asymmetry of CpG transitions as indicator of G1 phase-dependent origin of multiple tumorigenic p53 mutations in stem cells

In dividing cells, expression of mutations is DNA strand symmetric. Of all mutations originating de novo in nondividing cells, only those in the transcribed (noncoding) strand are immediately expressed in mRNA and protein. In contrast, any new mutation in the nontranscribed (coding) strand remains unexpressed until the cells enter S phase and begin proliferation. This previously unrecognized difference enables us to examine the cell cycle-dependent origin of multiple tumorigenic mutations in stem cells. The human p53 gene, which acts as a gatekeeper in the control of G1 to S phase transition, was chosen for the analysis. Of all multiple mutations contained in p53 databases, we have tested in detail CpG transitions. Three features of CpG sites dictate this choice: C --> T transitions at methylated mCpG are the direct product of mC deamination and are replication-independent; it is easy to identify the strand bearing a primary mC --> T event because C --> T on the transcribed strand appears as G --> A on the nontranscribed strand; and CpG transitions are the most frequent (as both singular and multiple occurrences) tumor-related p53 mutations. The origin of double nonsilent CpG transitions in nondividing cells predicts a significant excess of the heterostrand (C --> T, G --> A) doublets over the homostrand (C --> T, C --> T and G --> A, G --> A) doublets. For p53, we found such an excess. Based on this result, along with the results of three other tests reported here, we conclude that the majority of multiple p53 mutations from human tumors occurred in quiescent stem cells.

Stem cell origins of leukaemia and curability

It is suggested that most childhood acute lymphoblastic leukaemias and some other paediatric cancers are chemo-curable because they arise in stem cell populations that are functionally transient, chemosensitive and programmed for apoptosis. Most adult acute leukaemias are chemo-incurable at least in part because they originate in relatively drug resistant stem cells with extensive self-renewal capacity. The latter property in turn increases the probability of clones evolving with multi-drug resistance. Particular mutations may superimpose additional adverse features on leukaemic cells.

Identification of a single chromosome in the normal human genome essential for suppression of hamster cell transformation

Normal human fibroblasts were fused to carcinogen-transformed baby hamster kidney (BHK) cells and found to be able to suppress the anchorage-independent transformed phenotype of the hamster cells. This suppression was not due to interspecies incompatibility, for transformation could be effectively expressed in hybrids if either the human or the BHK parent had initially been transformed by a dominantly acting viral genome. Upon growth of suppressed hybrids, loss of human chromosomes was accompanied by the re-expression of transformation. Karyotype analysis indicated that only human chromosome 1 was retained in all hybrids that were suppressed and was lost in all hybrids in which transformation was re-expressed. Cytological evidence for the presence or absence of chromosome 1 was confirmed by electrophoretic identification of the human isozyme for phosphoglucomutase 1. Clones re-expressing transformation were isolated from two suppressed hybrids and in both cases loss of suppression was accompanied by the loss of human chromosome 1. Thus, the maintenance of suppression in these cross-species hybrids appears to require the continued presence of normal human chromosome 1. These findings raise the possibility that the frequent involvement of human chromosome 1 in potentially inactivating aberrations in human tumors may reflect a suppressor role for this chromosome in human malignancy.

Mechanisms of neoplastic transformation

This paper introduces a series of invited essays on current controversies in basic cancer research. The initial group of essays focuses on the detection and interpretation of molecular and cellular changes suspected to be of importance in the cause and pathogenesis of cancer. There are two formats: (i) differing viewpoints are presented in parallel, or (ii) the author(s) evaluate a hypothesis in light of available data. Each type of paper aims to critically evaluate current hypotheses and supporting data, while avoiding pronouncements on validity. Recent advances in molecular biology now permit us to consider genes as chemical entities. Individual genes can be isolated, cloned to produce multiple copies, sequenced, and assayed for biological function. This new molecular technology is being applied to fundamental questions in cancer research. The controversies resulting from these pioneering studies are the topics of the initial papers in this series. Forthcoming essays will concern the mechanism(s) of tumor promotion; a search for cancer genes by DNA transfection; the role of DNA rearrangements as initiating events in carcinogenesis; the O6 position of guanine as a critical target of carcinogens; and metals as mutagens and carcinogens.

Chemical carcinogens transform BHK cells by inducing a recessive mutation

Treatment of BHK cells with mutagenic carcinogens induced neoplastic transformation in a single step. This transformation displayed the characteristics expected for a recessive mutation. Increasing doses of carcinogens induced transformants with kinetics similar to the kinetics with which they induced 6-thioguanine-resistant or ouabain-resistant mutants in the same population of cells. Transformants with temperature-restricted phenotypes were easily induced by carcinogens which cause mutations by base changes, but when ICR frameshift mutagens were used, the proportion of temperature-limited transformants was inversely related to the frequency with which a particular mutagen induced frameshift mutations. In hybrids between pseudodiploid isogenic strains of normal and transformed BHK cells, transformation was expressed as a dominant trait when the transformed parent was induced by a papovavirus, but was suppressed as a recessive trait when the transformed parent arose spontaneously or was chemically induced. Segregation of transformation was observed upon growth of suppressed normal hybrids, and the transformed phenotype which was reexpressed was in most cases characteristics of the original transformed parent.

Chemical carcinogens transform BHK cells by inducing a recessive mutation

Treatment of BHK cells with mutagenic carcinogens induced neoplastic transformation in a single step. This transformation displayed the characteristics expected for a recessive mutation. Increasing doses of carcinogens induced transformants with kinetics similar to the kinetics with which they induced 6-thioguanine-resistant or ouabain-resistant mutants in the same population of cells. Transformants with temperature-restricted phenotypes were easily induced by carcinogens which cause mutations by base changes, but when ICR frameshift mutagens were used, the proportion of temperature-limited transformants was inversely related to the frequency with which a particular mutagen induced frameshift mutations. In hybrids between pseudodiploid isogenic strains of normal and transformed BHK cells, transformation was expressed as a dominant trait when the transformed parent was induced by a papovavirus, but was suppressed as a recessive trait when the transformed parent arose spontaneously or was chemically induced. Segregation of transformation was observed upon growth of suppressed normal hybrids, and the transformed phenotype which was reexpressed was in most cases characteristics of the original transformed parent.

Clinical manifestations of familial 13;18 translocation

Female first cousins, aged 21 and 2 1/2 years, with many of the characteristic features of trisomy 18, were found to have identical unbalanced translocations, 46,XX,--13, + der(13)t(13;18) (p13;q12)mat. Clinical features of another cousin, two uncles, and an aunt suggested that they, too, had a partial trisomy 18 phenotype. The long survival and normal menstrual and secondary sexual development in one case are remarkable. A heritable balanced translocation, 46,XX or XY, t(13;18) (p13;q12), was detected in the mothers of the cases, a sib, an aunt, and two uncles. Translocation carriers had abnormalities in gonadal structure or function, with aspermia in males and polycystic ovaries with infertility in several females, suggesting that some gene controlling reproductive development occurs on the long arm of chromosome 18, with normal function interrupted at the breakpoint. Balanced translocation carriers may also be at greater risk for both benign and malignant neoplasms, which included acute leukaemia in an uncle and adenocarcinoma of the stomach at an early age in the grandmother. Although aetiological laboratory studies identified no premalignant state, the clinical findings suggest a defect that may predispose to cytogenetic abnormalities and malignancy.