Premature sister chromatid separation in HIV-1-infected peripheral blood lymphocytes

Epigenetic displacement of HP1 from heterochromatin by HIV-1 Vpr causes premature sister chromatid separation

Although pericentromeric heterochromatin is essential for chromosome segregation, its role in humans remains controversial. Dissecting the function of HIV-1-encoded Vpr, we unraveled important properties of heterochromatin during chromosome segregation. In Vpr-expressing cells, hRad21, hSgo1, and hMis12, which are crucial for proper chromosome segregation, were displaced from the centromeres of mitotic chromosomes, resulting in premature chromatid separation (PCS). Interestingly, Vpr displaced heterochromatin protein 1-α (HP1-α) and HP1-γ from chromatin. RNA interference (RNAi) experiments revealed that down-regulation of HP1-α and/or HP1-γ induced PCS, concomitant with the displacement of hRad21. Notably, Vpr stimulated the acetylation of histone H3, whereas p300 RNAi attenuated the Vpr-induced displacement of HP1-α and PCS. Furthermore, Vpr bound to p300 that was present in insoluble regions of the nucleus, suggesting that Vpr aberrantly recruits the histone acetyltransferase activity of p300 to chromatin, displaces HP1-α, and causes chromatid cohesion defects. Our study reveals for the first time centromere cohesion impairment resulting from epigenetic disruption of higher-order structures of heterochromatin by a viral pathogen.

Chromosomal radiosensitivity of HIV positive individuals

PURPOSE

Radiosensitivity in relation to the human immunodeficiency virus (HIV) status is important in South Africa as the prevalence of HIV infections is high. In this study the in vitro chromosomal radiosensitivity of HIV positive individuals was investigated and compared with that of HIV negative individuals.

MATERIALS AND METHODS

Blood samples from 59 HIV positive and 39 HIV negative individuals were exposed in vitro to doses of 6MV X-rays ranging from 1-4 Gy. Chromosomal radiosensitivity was assessed with the micronucleus assay. Micronuclei are a measure of chromosomal damage and were quantified in at least 500 binucleated lymphoblasts (BN) per sample. Un-irradiated control samples from each donor were also analysed.

RESULTS

In 47% of HIV positive individuals difficulties with cell stimulation by adding phytohaemagglutinin (PHA) to blood cultures were noticed which resulted in insufficient yield of BN for microscopic analysis. Micronuclei frequencies were consistently higher in irradiated lymphocytes obtained from HIV positive individuals compared to that observed in cells from HIV negative donors. Data for both groups were fitted to the linear-quadratic equation Y = alphaD + betaD(2) where Y is the number of micronuclei in 500 binucleated cells and D is the dose in Gy. The fitted parameters for respectively HIV positive and HIV negative lymphocytes are alpha = 80.17 Gy(-1), beta = 14 Gy(-2) and alpha = 54.5 Gy(-1), beta = 16.2 Gy(-2). The confidence ellipses of these parameters are separated indicating that the increase in radiosensitivity is statistically significant.

CONCLUSION

T-lymphocytes of HIV infected individuals were considerably more sensitive to X-rays compared to that of HIV negative donors. This may have implications for normal tissue tolerance during radiotherapy as well as for the radiological health of radiation workers.

Premature chromatid separation is not a useful diagnostic marker for Cornelia de Lange syndrome

Cornelia de Lange syndrome (CdLS) is a rare, multiple congenital anomaly/mental retardation syndrome characterized by clinical variability and caused by mutations in the NIPBL (50-60%), SMC1L1 and SMC3 genes (5%), which encode for proteins involved in sister chromatid cohesion. Almost all of the studies of premature chromatid separation (PCS) in CdLS patients have failed to demonstrate that it is specific to CdLS, thus making its diagnostic use controversial. In order to verify the diagnostic usefulness of PCS screening in CdLS, we analysed metaphase spreads from 29 CdLS patients and 24 controls using a rigorous protocol to induce PCS, and precise criteria to score the affected chromosomes. Following exclusion of significant intra-sample variation we scored under blind conditions 150 spreads from a single preparation of each case and computed the ratio between the number of prematurely separated chromatids and the total number of chromatids. The results indicate the extreme variability of PCS in both cohorts (CdLS: mean 2.8 +/- 2.8%; controls: mean 4.0 +/- 5.4%) and highlight the difficulty of PCS monitoring, especially when selecting the control population. The absence of any difference in the frequency of PCS between the patients and controls, or between patients with different clinical or genetic backgrounds, precludes its potential use as an additional diagnostic tool.

Premature chromatid separation in a woman with carcinoma in situ of the uterine cervix and in her son with keratoacanthoma

An increased frequency of cells with premature chromatid separation (PCS) involving most chromosomes of a metaphase was observed in blood cultures of both a woman and her son with tumor. The mother had carcinoma in situ of the cervix, diagnosed at age of 42 years, and her son had keratoacanthoma, diagnosed at age of 23 years. To our knowledge, these are the first reported cases of carcinoma in situ of the uterine cervix and keratoacanthoma in patients with PCS. These findings provide additional evidence for a possible association of PCS and predisposition to malignancies.

HIV-1 Vpr induces ATM-dependent cellular signal with enhanced homologous recombination

An ATM-dependent cellular signal, a DNA-damage response, has been shown to be involved during infection of human immunodeficiency virus type-1 (HIV-1), and a high incidence of malignant tumor development has been observed in HIV-1-positive patients. Vpr, an accessory gene product of HIV-1, delays the progression of the cell cycle at the G2/M phase, and ATR-Chk1-Wee-1, another DNA-damage signal, is a proposed cellular pathway responsible for the Vpr-induced cell cycle arrest. In this study, we present evidence that Vpr also activates ATM, and induces expression of gamma-H2AX and phosphorylation of Chk2. Strikingly, Vpr was found to stimulate the focus formation of Rad51 and BRCA1, which are involved in repair of DNA double-strand breaks (DSBs) by homologous recombination (HR), and biochemical analysis revealed that Vpr dissociates the interaction of p53 and Rad51 in the chromatin fraction, as observed under irradiation-induced DSBs. Vpr was consistently found to increase the rate of HR in the locus of I-SceI, a rare cutting-enzyme site that had been introduced into the genome. An increase of the HR rate enhanced by Vpr was attenuated by an ATM inhibitor, KU55933, suggesting that Vpr-induced DSBs activate ATM-dependent cellular signal that enhances the intracellular recombination potential. In context with a recent report that KU55933 attenuated the integration of HIV-1 into host genomes, we discuss the possible role of Vpr-induced DSBs in viral integration and also in HIV-1 associated malignancy.