PTEN is among the most frequently inactivated tumour suppressor genes in sporadic cancer. PTEN has dual protein and lipid phosphatase activity, and its tumour suppressor activity is dependent on its lipid phosphatase activity, which negatively regulates the PI3K-AKT-mTOR pathway. Germline mutations in PTEN have been described in a variety of rare syndromes that are collectively known as the PTEN hamartoma tumour syndromes (PHTS). Cowden syndrome is the best-described syndrome within PHTS, with approximately 80% of patients having germline PTEN mutations. Patients with Cowden syndrome have an increased incidence of cancers of the breast, thyroid and endometrium, which correspond to sporadic tumour types that commonly exhibit somatic PTEN inactivation. Pten deletion in mice leads to Cowden syndrome-like phenotypes, and tissue-specific Pten deletion has provided clues to the role of PTEN mutation and loss in specific tumour types. Studying PTEN in the continuum of rare syndromes, common cancers and mouse models provides insight into the role of PTEN in tumorigenesis and will inform targeted drug development.

G 1 ͞S and G 2 ͞M cell cycle checkpoints maintain genomic stability in eukaryotes in response to genotoxic stress. We report here both genetic and functional evidence of a Gadd45-mediated G 2 ͞M checkpoint in human and murine cells. Increased expression of Gadd45 via microinjection of an expression vector into primary human fibroblasts arrests the cells at the G 2 ͞M boundary with a phenotype of MPM2 immunopositivity, 4n DNA content and, in 15% of the cells, centrosome separation. The Gadd45-mediated G 2 ͞M arrest depends on wild-type p53, because no arrest was observed either in p53-null Li-Fraumeni fibroblasts or in normal fibroblasts coexpressed with p53 mutants. Increased expression of cyclin B1 and Cdc25C inhibited the Gadd45-mediated G 2 ͞M arrest in human fibroblasts, indicating that the mechanism of Gadd45-mediated G 2 ͞M checkpoint is at least in part through modulation of the activity of the G 2 -specific kinase, cyclin B1͞p34 cdc2 . Genetic and physiological evidence of a Gadd45-mediated G 2 ͞M checkpoint was obtained by using GADD45-deficient human or murine cells. Human cells with endogenous Gadd45 expression reduced by antisense GADD45 expression have an impaired G 2 ͞M checkpoint after exposure to either ultraviolet radiation or methyl methanesulfonate but are still able to undergo G 2 arrest after ionizing radiation. Lymphocytes from gadd45-knockout mice (gadd45 ؊͞؊) also retained a G 2 ͞M checkpoint initiated by ionizing radiation and failed to arrest at G 2 ͞M after exposure to ultraviolet radiation. Therefore, the mammalian genome is protected by a multiplicity of G 2 ͞M checkpoints in response to specific types of DNA damage.Mammalian cells have evolved an intricate defense network to maintain genomic integrity by preventing the fixation of permanent damage from endogenous and exogenous mutagens. Cellcycle checkpoints, a major genomic surveillance mechanism, exist at the G 1 ͞S and G 2 ͞M transitions that are regulated in response to DNA damage (1). Defects in these steps may result in a mutator phenotype that is associated with tumorigenesis.Tumor suppressor gene product p53 is implicated to be one of the essential components of cell-cycle checkpoints (2-5). p53 is a transcription factor that up-regulates a number of important cell cycle-modulating genes, including p21 WAF1͞CIP1͞SDI1

More than 20 different cDNA clones encoding DNA-damage-inducible transcripts in rodent cells have recently been isolated by hybridization subtraction (A. J. Fornace, Jr., I. Alamo, Jr., and M. C. Hollander, Proc. Natl. Acad. Sci. USA 85:8800-8804, 1988). In most cells, one effect of DNA damage is the transient inhibition of DNA synthesis and cell growth. We now show that five of our clones encode transcripts that are increased by other growth cessation signals: growth arrest by serum reduction, medium depletion, contact inhibition, or a 24-h exposure to hydroxyurea. The genes coding for these transcripts have been designated gadd (growth arrest and DNA damage inducible). Two of the gadd cDNA clones were found to hybridize at high stringency to transcripts from human cells that were induced after growth cessation signals or treatment with DNA-damaging agents, which indicates that these responses have been conserved during mammalian evolution. In contrast to results with growth-arrested cells that still had the capacity to grow after removal of the growth arrest conditions, no induction occurred in HL60 cells when growth arrest was produced by terminal differentiation, indicating that only certain kinds of growth cessation signals induce these genes. All of our experiments suggest that the gadd genes are coordinately regulated: the kinetics of induction for all five transcripts were similar; in addition, overexpression of gadd genes was found in homozygous deletion c14CoS/c14CoS mice that are missing a small portion of chromosome 7, suggesting that a trans-acting factor encoded by a gene in this deleted portion is a negative effector of the gadd genes. The gadd genes may represent part of a novel regulatory pathway involved in the negative control of mammalian cell growth.