The tumor suppressor RASSF1A in human carcinogenesis: an update.

Loss of heterozygosity of the small arm of chromosome 3 is one of the most common alterations in human cancer. Most notably, a segment in 3p21.3 is frequently lost in lung cancer and several other carcinomas. We and others have identified a novel Ras effector at this segment, which was termed Ras Association Domain family 1 (RASSF1A) gene. RASSF1 consists of two main variants (RASSF1A and RASSF1C), which are transcribed from distinct CpG island promoters. Aberrant methylation of the RASSF1A promoter region is one of the most frequent epigenetic inactivation events detected in human cancer and leads to silencing of RASSF1A. Hypermethylation of RASSF1A was commonly observed in primary tumors including lung, breast, pancreas, kidney, liver, cervix, nasopharyngeal, prostate, thyroid and other cancers. Moreover, RASSF1A methylation was frequently detected in body fluids including blood, urine, nipple aspirates, sputum and bronchial alveolar lavages. Inactivation of RASSF1A was associated with an advanced tumor stage (e.g. bladder, brain, prostate, gastric tumors) and poor prognosis (e.g. lung, sarcoma and breast cancer). Detection of aberrant RASSF1A methylation may serve as a diagnostic and prognostic marker. The functional analyses of RASSF1A reveal an involvement in apoptotic signaling, microtubule stabilization and mitotic progression. The tumor suppressor RASSF1A may act as a negative Ras effector inhibiting cell growth and inducing cell death. Thus, RASSF1A may represent an epigenetically inactivated bona fide tumor suppressor in human carcinogenesis.

Epigenetic inactivation of the Ras-association domain family 1 (RASSF1A) gene and its function in human carcinogenesis.

The Ras GTPases are a superfamily of molecular switches that regulate cellular proliferation and apoptosis in response to extra-cellular signals. The regulation of these pathways depends on the interaction of the GTPases with specific effectors. Recently, we have cloned and characterized a novel gene encoding a putative Ras effector: the Ras-association domain family 1 (RASSF1) gene. The RASSF1 gene is located in the chromosomal segment of 3p21.3. The high allelic loss in a variety of cancers suggested a crucial role of this region in tumorigenesis. At least two forms of RASSF1 are present in normal human cells. The RASSF1A isoform is highly epigenetically inactivated in lung, breast, ovarian, kidney, prostate, thyroid and several other carcinomas. Re-expression of RASSF1A reduced the growth of human cancer cells supporting a role for RASSF1 as a tumor suppressor gene. RASSF1A inactivation and K-ras activation are mutually exclusive events in the development of certain carcinomas. This observation could further pinpoint the function of RASSF1A as a negative effector of Ras in a pro-apoptotic signaling pathway. In malignant mesothelioma and gastric cancer RASSF1A methylation is associated with virus infection of SV40 and EBV, respectively, and suggests a causal relationship between viral infection and progressive RASSF1A methylation in carcinogenesis. Furthermore, a significant correlation between RASSF1A methylation and impaired lung cancer patient survival was reported, and RASSF1A silencing was correlated with several parameters of poor prognosis and advanced tumor stage (e.g. poor differentiation, aggressiveness, and invasion). Thus, RASSF1A methylation could serve as a useful marker for the prognosis of cancer patients and could become important in early detection of cancer.

[Noninduced single-stranded breaks in DNA in murine F9 teratocarcinoma cells]. / Neindutsirovannye odnonitevye razryvy v DNK kletok linii F9 teratokartsinomy myshi.

Our previous study demonstrated the high incidence of non-induced DNA single strand breaks (SSB) in preimplantation mouse embryo genom (Patkin et al., 1994). F9 mouse teratocarcinoma cell line is an in vitro model for early embryonal differentiation, since F9 cells remind in many respects the inner cell mass cells of mouse blastocyst and are capable of differentiation under retinoic acid (RA) and dibutyryl cAMP (db-cAMP) treatment. Using gap filling reaction of F9 metaphase chromosomes and single-cell DNA electrophoresis, we have observed multiple SSB in undifferentiated F9 cells as well as in F9 cells at the early steps of RA-induced differentiation (days of RA treatment), but not in terminally differentiated F9 cells and in mouse embryonal fibroblasts. Rad51 nuclear protein that binds specifically single stranded DNA is highly expressed in all cells of undifferentiated F9 population and is not expressed in terminally differentiated F9 population. Multiple SSB could lead to enhanced rate of sister chromatid exchanges (SCE) in F9 cells. In undifferentiated F9 population the level of SCE was 9.6 +/- 0.44 per metaphase, that was not higher than in NIH 3T3 cell line. However, RA treatment for 48 h led to rising the SCE level up to 16.68 +/- 0.72 followed by its decrease to the initial rate by 72 h of RA treatment. Since the enhanced level of SSB in undifferentiated F9 cells and in mouse blastocyst does not normally lead to chromosomal instability, we consider SSB to be a natural consequence of fast-going DNA replication in these cells.