Delayed early embryonic lethality following disruption of the murine cyclin A2 gene.

In higher eukaryotes, cell cycle progression is controlled by cyclin dependent kinases (Cdks) complexed with cyclins. A-type cyclins are involved at both G1/S and G2/M transitions of the cell cycle. Cyclin A2 activates cdc2 (Cdk1) on passage into mitosis and Cdk2 at the G1/S transition. Antisense constructs, or antibodies directed against cyclin A2 block cultured mammalian cells at both of these transitions. In contrast, overexpression of cyclin A2 appears to advance S phase entry and confer anchorage-independent growth, and can lead to apoptosis. A second A-type cyclin, cyclin A1 has been described recently which, in the mouse, is expressed in germ cells but not somatic tissues. To address the possible redundancy between different cyclins in vivo and also the control of early embryonic cell cycles, we undertook the targeted deletion of the murine cyclin A2 gene. The homozygous null mutant is embryonically lethal, demonstrating that the cyclin A2 gene is essential. Surprisingly, homozygous null mutant embryos develop normally until post-implantation, around day 5.5 p.c. This observation may be explained by the persistence of a maternal pool of cyclin A2 protein until at least the blastocyst stage, or an unexpected role for cyclin A1 during early embryo development.

Differential expression of the cyclin-dependent kinase inhibitor P27 in primary hepatocytes in early-mid G1 and G1/S transitions.

P27, an inhibitor of cyclin-dependent kinases, plays an important role in the control of cell adhesion and contact inhibition-dependent cell cycle regulation. Hepatocytes, maintained in primary culture, offer a model of synchronized primary epithelial cells which retain a differentiated profile while stimulated to proliferate. We therefore investigated the pattern of endogenous p27 expression in cyclin rat hepatocytes isolated by collagenase perfusion followed by mitogenic stimulation. P27 was expressed in whole normal liver and freshly isolated hepatocytes. We then observed a sharp decrease in p27 levels, concomitant with the progression in early-mid G1, followed by reaccumulation in late G1 and the G1/S transition. Immunochemistry and BrdU labelling demonstrated nuclear localization of p27 and its expression in cells engaged in both G1 and S phase. P27 was detected in late G1 in complexes containing cyclins D1, E and A. Cyclin E- and A-associated kinase activities, however, were detected at the G1/S transition and depletion experiments confirmed that most active complexes were free of p27. Phosphorylated forms of p27 were detected in unstimulated and stimulated hepatocytes in both early-mid G1 and G1/S. Finally, two-dimensional gel electrophoresis showed evidence for several forms of p27 with a distinct profile of distribution in quiescent and stimulated hepatocytes. Collectively, our data offer a model in which p27 shows a biphasic profile of accumulation, with the early decrease possibly involved in the progression through early and mid G1. In contrast with most cell types tested so far, the late G1 accumulation did not impair formation of active cyclin E- and A associated kinases, and thus G1/S transition.

Alterations of cyclin-dependent kinase 4 inhibitor (p16INK4A/MTS1) gene structure and expression in acute lymphoblastic leukemias.

The cyclin-dependent kinase 4 (cdk4) inhibitor (p16INK4/MTS1/CDKN2) gene has been recently identified as a putative tumor suppressor gene because of the high frequency of homozygous deletion observed in numerous human tumor cell lines, including leukemias. However, results obtained from uncultured tumor samples have led to discussion of the relevance of these findings. Using reverse transcriptase polymerase chain reaction (RT-PCR) and Southern blot analysis, we have investigated p16INK4A gene at both RNA and genomic levels in various types of leukemias: acute myeloid leukemia (AML) (n = 23); acute lymphocytic leukemia (ALL) (n = 22) and B cell chronic lymphoproliferative disorders (CLPD) (n = 33). p16INK4A mRNA expression was not found in only 1/20 AML and 2/23 CLPD samples. Conversely, p16INK4A mRNA was not detected in 5/17 ALL cases, and intensity of PCR products were barely detectable in seven additional cases, possibly related to the contamination by normal cells in some cases. By Southern blotting, a homozygous deletion of p16INK4A gene was found in 6/17 ALL cases (35%) among which 4/6 were negative or weakly positive by RT-PCR assay. None of the five AML and 20 CLL samples studied had p16INK4A deletion. Sequence analysis of p16INK4A exon 2 did not show point mutation in two of these cases lacking mRNA expression. Our data provide further evidence that among hematological malignancies, ALL are the most likely to be associated with p16INK4A inactivation, mainly by homozygous gene deletion. Since most hematological malignancies-except ALL-are infrequently associated with p16INK4A and retinoblastoma (Rb) gene alteration it seems worthwhile to explore cdk4 and cdk6 expression to determine whether or not the disruption of the p16INK4A/Rb/cdk4/cdk6 regulatory loop might play a role in their pathogenesis.

Spectrum of germline mutations in the RB1 gene: a study of 232 patients with hereditary and non hereditary retinoblastoma.

Germline mutations in the RB1 gene confer hereditary predisposition to retinoblastoma. We have performed a mutation survey of the RB1 gene in 232 patients with hereditary or non hereditary retinoblastoma. We systematically explored all 27 exons and flanking sequences as well as the promotor. All types of point mutations are represented and are found unequally distributed along the RB1 gene sequence. In the population we studied, exons 3, 8, 18 and 19 are preferentially altered. The range of frequency of detection of germline mutations is about 20%, indicating that other mechanisms of inactivation of RB1 should be involved. The spectrum of mutations presented here should help to improve the clinical management of retinoblastoma and to understand the molecular mechanisms leading to tumorigenesis.

cAMP-dependent positive control of cyclin A2 expression during G1/S transition in primary hepatocytes.

cAMP positively and negatively regulates hepatocyte proliferation but its molecular targets are still unknown. Cyclin A2 is a major regulator of the cell cycle progression and its synthesis is required for progression to S phase. We have investigated whether cyclin A2 and cyclin A2-associated kinase might be one of the targets for the cAMP transduction pathway during progression of hepatocytes through G1 and G1/S. We show that stimulation of primary cultured hepatocytes by glucagon differentially modulated the expression of G1/S cyclins. Glucagon indeed upregulated cyclin A2 and cyclin A2-associated kinase while cyclin E-associated kinase was unmodified. In conclusion, our study identifies cyclin A2 as an important effector of the cAMP transduction network during hepatocyte proliferation.

Early development of mouse embryos null mutant for the cyclin A2 gene occurs in the absence of maternally derived cyclin A2 gene products.

Progression through the mammalian cell cycle is regulated by the sequential activation and inactivation of the cyclin-dependent kinases. In adult cells, cyclin A2-dependent kinases are required for entry into S and M phases, completion of S phase, and centrosome duplication. However, mouse embryos lacking the cyclin A2 gene nonetheless complete preimplantation development, but die soon after implantation. In this report, we investigated whether a contribution of maternal cyclin A2 mRNA and protein to early embryonic cell cycles might explain these conflicting observations. Our data show that a maternal stock of cyclin A2 mRNA is present in the oocyte and persists after fertilization until the second mitotic cell cycle, when it is degraded to undetectable levels coincident with transcriptional activation of the zygotic genome. A portion of maternally derived cyclin A2 protein is stable during the first mitosis and persists in the cytoplasm, but is completely degraded at the second mitosis. The ability of cyclin A2-null mutants to develop normally from the four-cell to the postimplantation stage in the absence of detectable cyclin A2 gene product indicates therefore that cyclin A2 is dispensable for cellular progression during the preimplantation nongrowth period of mouse embryo development.