Suppressed apoptosis of pre-B cells in bone marrow of pre-leukemic p190bcr/abl transgenic mice

The DNA damage checkpoint protein RAD9A is essential for male meiosis in the mouse

In mitotic cells, RAD9A functions in repairing DNA double-strand breaks (DSBs) by homologous recombination and facilitates the process by cell cycle checkpoint control in response to DNA damage. DSBs occur naturally in the germline during meiosis but whether RAD9A participates in repairing such breaks is not known. In this study, we determined that RAD9A is indeed expressed in the male germ line with a peak of expression in late pachytene and diplotene stages, and the protein was found associated with the XY body. As complete loss of RAD9A is embryonic lethal, we constructed and characterized a mouse strain with Stra8-Cre driven germ cell-specific ablation of Rad9a beginning in undifferentiated spermatogonia in order to assess its role in spermatogenesis. Adult mutant male mice were infertile or sub-fertile due to massive loss of spermatogenic cells. The onset of this loss occurs during meiotic prophase, and there was an increase in the numbers of apoptotic spermatocytes as determined by TUNEL. Spermatocytes lacking RAD9A usually arrested in meiotic prophase, specifically in pachytene. The incidence of unrepaired DNA breaks increased, as detected by accumulation of γH2AX and DMC1 foci on the axes of autosomal chromosomes in pachytene spermatocytes. The DNA topoisomerase IIβ-binding protein 1 (TOPBP1) was still localized to the sex body, albeit with lower intensity, suggesting that RAD9A may be dispensable for sex body formation. We therefore show for the first time that RAD9A is essential for male fertility and for repair of DNA DSBs during meiotic prophase I.

Induction of apoptosis involving multiple pathways is a primary response to cyclin A1-deficiency in male meiosis

The meiotic arrest in male mice null for the cyclin A1 gene (Ccna1) was associated with apoptosis of spermatocytes. To determine whether the apoptosis in spermatocytes was triggered in response to the arrest at G2/M phase, as opposed to being a secondary response to overall disruption of spermatogenesis, we examined testes during the first wave of spermatogenesis by terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling (TUNEL) staining. We observed enhanced apoptosis coinciding with the arrest point in postnatal day 22 tubules, with no overt degeneration. Along with activation of caspase-3, an increase in the levels and change of subcellular localization of Bax protein was observed in cyclin A1-deficient spermatocytes, which coincided with the detection of apoptosis. As p53 is implicated in the activation of Bax-mediated cell death, we generated mice lacking both cyclin A1 and p53. Although the absence of p53 did not rescue the meiotic arrest, there was a decrease in the number of apoptotic cells in the double-mutant testes. This finding suggested that p53 may be involved in the process by which the arrested germ cells are removed from the seminiferous tubules but that other pathways function as well to ensure removal of the arrested spermatocytes.

Fusion tyrosine kinase mediated signalling pathways in the transformation of haematopoietic cells

The fusion tyrosine kinases (FTKs) are generated by chromosomal translocations creating bipartite proteins in which the kinase is hyperactivated by an adjoining oligomerization domain. Autophosphorylation of the FTK generates a 'signalosome', an ensemble of signalling proteins that transduce signals to downstream pathways. At the earliest stages of oncogenesis, FTKs can mimic mitogenic cytokine signalling pathways involving the GAB-2 adaptor protein and signal transducers and activators of transcription (STAT) factors, generating replicative stress and thereby promoting a mutator phenotype. In parallel, FTKs couple to survival pathways that upregulate prosurvival proteins such as Bcl-xL, so preventing DNA-damage-induced apoptosis. Following transformation, FTKs induce resistance to genotoxic attack by upregulating DNA repair mechanisms such as STAT5-dependent RAD51 transcription. The phenomenon of 'oncogene addiction' reflects the continued requirement of an active FTK 'signalosome' to mediate survival and mitogenic signals involving the PI 3-kinase and mitogen-activated protein stress-activated protein kinase pathways, and the nuclear factor-kappa B, activator protein 1 and STAT transcription factors. The available data so far suggest that FTKs, with some possible exceptions, induce and maintain the transformed state using similar panoplies of signals, a finding with important therapeutic implications. The FTK signalling field has matured to an exciting phase in which rapid advances are facilitating rational drug design.

Resistance of bcr-abl-positive acute lymphoblastic leukemia to daunorubicin is not mediated by mdr1 gene expression

In vitro resistance to anthracyclines is thought to be a poor prognosis in achieving long-term remission in patients with acute lymphoblastic leukemia (ALL). Expression of a multidrug resistance gene (mdr1) that codes for 170 Kd transmembrane glycoprotein is responsible for conferring resistance to malignant cells to anthracyclines. The t(9:22) translocation, resulting in bcr-abl fusion gene, is commonly found in B-lineage ALL and is known to be a poor prognostic factor for long-term remission. To investigate whether resistance to anthracyclines contributes to poor prognosis in bcr-abl-positive ALL, we studied daunorubicin sensitivity by an in vitro colorimetric methyl tetrazolium (MTT) assay in B-lineage ALL patients who were bcr-abl-positive and compared them with the B-lineage, age-matched bcr-abl-negative group. We also looked for and compared the presence of mdr1 gene expression in these two groups of patients by RT-PCR. Of the 46 patients included in the study, 16 (34.7%) were positive for the bcr-abl fusion gene. mdr1 gene expression was seen in 14 of these 46 patients (30.4%). However, the expression of the mdr1 gene was relatively lower in the bcr-abl-positive group (3 out of 16, 18.7%) compared to the bcr-abl-negative group (11 out of 30, 36.6%). The median LD(50) of daunorubicin (concentration lethal to 50% of the leukemic blasts) differed significantly between bcr-abl-positive and -negative patients (P = 0.018). This in vitro study suggests that bcr-abl-positive ALL is relatively resistant to daunorubicin, but this resistance is not mediated through mdr1 gene expression.