Differences in DNA double strand breaks repair in male germ cell types: lessons learned from a differential expression of Mdc1 and 53BP1

In male germ cells the repair of DNA double strand breaks (DSBs) differs from that described for somatic cell lines. Irradiation induced immunofluorescent foci (IRIF's) signifying a double strand DNA breaks, were followed in spermatogenic cells up to 16 h after the insult. Foci were characterised for Mdc1, 53BP1 and Rad51 that always were expressed in conjecture with gamma-H2AX. Subsequent spermatogenic cell types were found to have different repair proteins. In early germ cells up to the start of meiotic prophase, i.e. in spermatogonia and preleptotene spermatocytes, 53BP1 and Rad51 are available but no Mdc1 is expressed in these cells before and after irradiation. The latter might explain the radiosensitivity of spermatogonia. Spermatocytes from shortly after premeiotic S-phase till pachytene in epithelial stage IV/V express Mdc1 and Rad51 but no 53BP1 which has no role in recombination involved repair during the early meiotic prophase. Mdc1 is required during this period as in Mdc1 deficient mice all spermatocytes enter apoptosis in epithelial stage IV when they should start mid-pachytene phase of the meiotic prophase. From stage IV mid pachytene spermatocytes to round spermatids, Mdc1 and 53BP1 are expressed while Rad51 is no longer expressed in the haploid round spermatids. Quantifying foci numbers of gamma-H2AX, Mdc1 and 53BP1 at various time points after irradiation revealed a 70% reduction after 16 h in pachytene and diplotene spermatocytes and round spermatids. Although the DSB repair efficiency is higher then in spermatogonia where only a 40% reduction was found, it still does not compare to somatic cell lines where a 70% reduction occurs in 2 h. Taken together, DNA DSBs repair proteins differ for the various types of spermatogenic cells, no germ cell type possessing the complete set. This likely leads to a compromised efficiency relative to somatic cell lines. From the evolutionary point of view it may be an advantage when germ cells die from DNA damage rather than risk the acquisition of transmittable errors made during the repair process.     

Pathways governing G1/S transition and their response to DNA damage

The ability to self-replicate is a fundamental feature of life, reflected at the cellular level by a highly regulated process initiated in G1 phase via commitment to a round of DNA replication and cell division. Here we briefly highlight recent advances in understanding the molecular pathways which govern the decision of mammalian somatic cells to enter S phase, and the so-called cell cycle checkpoints which guard the G1/S transition and S phase progression against potentially deleterious effects of genotoxic stress. Particular emphasis is put on the emerging parallel yet cooperative pathways of retinoblastoma protein (pRB)–E2F and Myc, their convergence to control the activity of the cyclin-dependent kinase 2 (Cdk2) at the G1/S boundary, as well as the two waves of checkpoint responses at G1/S: the rapid pathway(s) leading to Cdc25A degradation, and the delayed p53–p21 cascade, both silencing the Cdk2 activity upon DNA damage.     

Adiponectin concentrations as a criterion of metabolic control in persons with type 2 diabetes mellitus?

Adiponectin (ADP) is an adipocytokin with many antiatherogenic properties; its decreased level is associated with numerous atherogenic diseases and syndromes (e.g. diabetes mellitus (DM), dyslipidemia, endothelial dysfunction, hypertension, and obesity). Decreased ADP values in blood may be an independent risk factor of atherosclerotic (ATS) complications. AIM OF THE STUDY: 1) Do persons with type 2 diabetes have lower ADP values than individuals without DM but with a high risk of ATS complications? 2) Do ADP values differ between persons with well controlled and persons with uncontrolled type 2 diabetes? We examined 109 patients of the Metabolic Center of Hospital Sternberk. Out of them, 58 had type 2 diabetes, others were individuals with variously expressed risk factors of early atherosclerosis (obesity, hypertension, age, family history, smoking, dyslipidemia, etc.). In all persons under this study the following parameters were determined in peripheral venous blood: adiponectin, resistin, leptin, ObRe, cholesterol, HDL-cholesterol, triacylglycerols, glucose, HbA1c, creatinine, urea, ALT, AST, CRP, homocysteine, thrombocyte aggregation after CPG induction. The whole group was divided according to the presence of type 2DM into two subgroups; persons with diabetes were divided into the well controlled and uncontrolled subgroups. All data obtained were processed statistically using the software SPSS for Windows and Medcalc. The adiponectin/BMI index correlated negatively with HbA1c value (correlation coefficient -0.37, p = 0.00053), triacylglycerols (-0.4, p = 0.000001), P-glucose (-0.3, p = 0.0017), uricemia (-0.35, p = 0.0007) and positively with HDL-cholesterol value (0.6, p=0.00001). Women had higher adiponectin values than men. Persons with hypertension and with diabetes mellitus, individuals with atherogenic lipotype or persons with inflammation signs had lower values than individuals without these diseases and syndromes. Persons with wellcontrolled diabetes mellitus had higher values than persons with uncontrolled diabetes (medians of the adiponectin/BMI index 9.7 vs. 6.7, p < 0.01). Persons with type 2 diabetes mellitus have lower ADP values than persons with a high ATS risk without diabetes mellitus. Persons with wellcontrolled diabetes mellitus (DM) and with satisfactory compensation have significantly higher ADP levels (independently of other metabolic parameters of DM control). ADP may be a new marker of metabolic control in persons with a high risk of atherosclerotic complications.     

RNF8 ubiquitylates histones at DNA double-strand breaks and promotes assembly of repair proteins

Accumulation of repair proteins on damaged chromosomes is required to restore genomic integrity. However, the mechanisms of protein retention at the most destructive chromosomal lesions, the DNA double-strand breaks (DSBs), are poorly understood. We show that RNF8, a RING-finger ubiquitin ligase, rapidly assembles at DSBs via interaction of its FHA domain with the phosphorylated adaptor protein MDC1. This is accompanied by an increase in DSB-associated ubiquitylations and followed by accumulation of 53BP1 and BRCA1 repair proteins. Knockdown of RNF8 or disruption of its FHA or RING domains impaired DSB-associated ubiquitylation and inhibited retention of 53BP1 and BRCA1 at the DSB sites. In addition, we show that RNF8 can ubiquitylate histone H2A and H2AX, and that its depletion sensitizes cells to ionizing radiation. These data suggest that MDC1-mediated and RNF8-executed histone ubiquitylation protects genome integrity by licensing the DSB-flanking chromatin to concentrate repair factors near the DNA lesions.