Low-Dose Hyper-radiosensitivity is not Caused by a Failure to Recognize DNA Double-Strand Breaks

One of the earliest cellular responses to radiation-induced DNA damage is the phosphorylation of the histone variant H2AX (gamma-H2AX). gamma-H2AX facilitates the local concentration and focus formation of numerous repair-related proteins within the vicinity of DNA DSBs. Previously, we have shown that low-dose hyper-radiosensitivity (HRS), the excessive sensitivity of mammalian cells to very low doses of ionizing radiation, is a response specific to G(2)-phase cells and is attributed to evasion of an ATM-dependent G(2)-phase cell cycle checkpoint. To further define the mechanism of low-dose hyper-radiosensitivity, we investigated the relationship between the recognition of radiation-induced DNA double-strand breaks as defined by gamma-H2AX staining and the incidence of HRS in three pairs of isogenic cell lines with known differences in radiosensitivity and DNA repair functionality (disparate RAS, ATM or DNA-PKcs status). Marked differences between the six cell lines in cell survival were observed after high-dose exposures (>1 Gy) reflective of the DNA repair capabilities of the individual six cell lines. In contrast, the absence of functional ATM or DNA-PK activity did not affect cell survival outcome below 0.2 Gy, supporting the concept that HRS is a measure of radiation sensitivity in the absence of fully functional repair. No relationship was evident between the initial numbers of DNA DSBs scored immediately after either low- or high-dose radiation exposure with cell survival for any of the cell lines, indicating that the prevalence of HRS is not related to recognition of DNA DSBs. However, residual DNA DSB damage as indicated by the persistence of gamma-H2AX foci 4 h after exposure was significantly correlated with cell survival after exposure to 2 Gy. This observation suggests that the persistence of gamma-H2AX foci could be adopted as a surrogate assay of cellular radiosensitivity to predict clinical radiation responsiveness.

Mammalian spermatozoal mRNAs: tools for the functional analysis of male gametes

Mammalian spermatogenesis reflects a dynamic partnership between somatic genes which display altered expression patterns within the testes and germ cell specific genes expressed solely during male gametogenesis. Among the latter include the haploid-specific transition proteins and protamines which facilitate the molecular restructuring of the chromatin during the terminal differentiation of the spermatid nucleus. Previously, we have shown that the human protamines PRM1, PRM2 and transition protein TNP2 transcripts persist in mature spermatozoa subsequent to their functional role during spermiogenesis. We now demonstrate the conserved presence of these same spermatozoal mRNAs in mouse. While the basis for the persistence of these haploid-specific transcripts remains poorly defined, the results presented firmly establish that the presence of mRNA in mammalian spermatozoa is a general phenomenon. The potential use of these transcripts as a tool for investigating molecular pathologies is discussed.

Haploid transcripts persist in mature human spermatozoa

Mammalian spermiogenesis is marked by the morphological and functional differentiation of round haploid spermatids into mature spermatozoa. A molecular restructuring of the chromatin accompanies this process facilitated by the transition proteins and protamines which compact and condense the genetic material within the developing spermatid. Previous studies from this laboratory have demonstrated that human protamines PRM1, PRM2 and transition protein TNP2 transcripts are associated with round and elongating spermatids. Extending this investigation, we examined the occurrence of these transcripts in mature spermatozoa by in-situ hybridization analysis using [35S]-labelled cRNA probes. These results demonstrate that PRM1, PRM2 and TNP2 haploid-specific transcripts are present in mature spermatozoa. Quantitative analysis of the localized signal also indicates that the PRM1, PRM2 and TNP2 transcripts persist at a similar ratio to that previously described for these transcripts in human testes, i.e. PRM2 > PRM1 approximately equal to TNP2. The persistence of these transcripts in mature spermatozoa warrants further investigation.

A haploid expressed gene cluster exists as a single chromatin domain in human sperm

Mammalian spermiogenesis is marked by the initial disruption of the nuclear-histone-DNA complex by the transition proteins for ultimate replacement with protamines. The genes for three of these low molecular weight basic nuclear proteins exist as a single linear array of PRM1, PRM2, and TNP2 on human chromosome 16p13.2. To begin to address the mechanism governing their transcriptional potentiation, a region of approximately 40 kilo-bases of the human genome encompassing these genes was introduced into the germ line of mice. Fluorescence in situ hybridization and Southern analysis showed that this segment of the human genome integrated into independent chromosomal sites while maintaining its fidelity. Transcript analysis demonstrated that the expression of the endogenous mouse protamine Prm1 and Prm2 genes as well as the mouse transition protein Tnp2 gene were expressed along with their human transgene counterparts. The pattern of expression of these transgenic human genes within this multigenic cluster faithfully represented that observed in vivo. In addition, all members of this transgenic gene cluster were expressed in proportions similar to those in human testis. Copy number-dependent and position-independent expression of the transgenic construct demonstrated that the corresponding biological locus was contained within this segment of the human genome. Furthermore, DNase I sensitivity established that in sperm the human PRM1-->PRM2-->TNP2 genic domain was contained as an approximately 28.5-kilobase contiguous segment bounded by an array of nuclear matrix associated topoisomerase II consensus sites. This is the first description of a multigenic male gamete-specific domain as a fundamental gene regulatory unit. A model of haploid-specific gene determination is presented.

Coordinate expression of the PRM1, PRM2, and TNP2 multigene locus in human testis

Maintenance of the transcriptionally inert state of the mature human spermatozoon requires the expression of the various members of the human protamine gene cluster prior to the final stages of spermatogenesis. During this process, known as spermiogenesis, round spermatids morphologically differentiate into mature spermatozoa. The expression of the PRM1, PRM2, and TNP2 genes facilitates the compaction and condensation of the genetic material within the developing spermatid. To understand better the coordinate control governing this transformation, we have examined the localization and distribution of the human protamines PRM1 and PRM2 and transition protein TNP2 transcripts during human spermatogenesis. The stage-specific expression of these transcripts was determined by in situ hybridization analysis using [alpha-35S]-labeled cRNA probes. PRM1, PRM2, and TNP2 transcripts were abundant in association with round and elongating spermatids, located in the adluminal region of the seminiferous epithelium. They were not observed in association with spermatogonia, spermatocytes, Sertoli cells, or interstitial cells. These data indicate that the human PRM1, PRM2, and TNP2 transcripts are expressed postmeiotically in round and elongating spermatids. The quantitative evaluation of each transcript was determined as a function of the relative optical density per unit area. In all cases examined, the relative level of each transcript was consistent with the following pattern, PRM2 > PRM1 congruent to TNP2.

Comparative analysis of epidermal growth factor receptor gene expression and protein product in benign, premalignant, and malignant prostate tissue

In order to more clearly define the status of epidermal growth factor receptor (EGFR) in prostate cancer, expression of EGFR transcript and protein was analyzed in paired samples of benign and malignant tissues from 30 radical prostatectomy specimens. Prostate tumors and high grade prostatic intraepithelial neoplasias (PINs) expressed significantly less EGFR protein than benign tissues or low grade PINs (P < 0.001). Expression of EGFR mRNA was analyzed in a subset of the same samples, and was higher in more prostate tumors than benign specimens (P < 0.05). However, differences in mean mRNA expression between malignant and benign tissues were not significant. EGFR mRNA was expressed at moderate or low levels in equivalent numbers of PIN lesions. These results suggest that, although EGFR mRNA expression is somewhat elevated in prostate tumors, EGFR protein expression may be down-regulated in the same malignant tissues. Furthermore, our data demonstrate phenotypic similarity between prostate tumors and high grade PIN at the level of EGFR protein expression.

Concurrent abnormal expression of ERBB-2, EGFR, and p53 genes and clinical disease progression of breast carcinoma

Metastatic phenotype in human solid tumors is believed to follow stochastic acquisition of structural genetic aberrations-so-called multistep tumor progression. We tested this hypothesis in breast carcinoma by immunostaining 89 stage-heterogeneous cases for the products of three genes (p53, ERBB-2, and EGFR) which are frequently altered in this tumor system. Variable relationships were observed between advanced disease stage and immunostaining for individual gene products (ERBB-2 - p = 0.05, EGFR - p = 0.02, p53 - p = 0.12, Chi Square test). Regional or distant metastases at presentation correlated with multiple oncogene/tumor suppressor gene expression abnormalities: node negative -59% none positive, 29% one positive, 12% two or more positive, vs. node positive -37% none positive, 23% one positive, 39% two or more positive (p = 0.01). Only 2/12 (17%) of tumors with distant metastases at presentation were negative for abnormal expression of any of these gene products, and 7/12 (58%) were positive for two or three. Among axillary node negative patients who developed recurrences, 67% exhibited staining for at least one gene product, compared to only 27% of those without recurrences (p = 0.02). All 5 cases with abnormal staining for each gene product had regional or distant metastases at presentation and recurred. In multivariate analysis, individual expression of p53 outweighed expression of ERBB-2 and EGFR in correlation with outcome. These data suggest clinical neoplastic progression of breast carcinomas correlates with cumulative genetic events detectable by protein expression. Short term recurrence, however, may correlate more closely with abnormal expression of p53 than with EGFR or ERBB-2.

Prognostic significance of image morphometric microvessel enumeration in breast carcinoma

Using image morphometry, microvessel density was analyzed in 58 stage-heterogeneous breast carcinomas by counting vascular cross-sections in type IV collagen (basal lamina)-immunostained cryostat sections (methanol fixed, avidin-biotin method). Vascular density (mean, 19 vessels per square millimeter) did not correlate with metastatic disease at presentation (localized: 17.6 vessels per square millimeter vs. node/distant metastases: 20 vessels per square millimeter, P = NS). Short-term disease-free survival, however, was significantly related to intratumoral vascularity overall (no recurrence: mean = 14 vessels per square millimeter vs. recurred: mean = 26 vessels per square millimeter, P = .001) and in node-negative patients (no recurrence: mean = 12.1 vessels per square millimeter vs. recurred: mean = 27.8 vessels per square millimeter, P = .005; mean follow-up, 52 months). Further, 87% (7/8) of patients with microvascular counts greater than 30 vessels per square millimeter had recurrence vs. only 15% (2/13) with counts less than 10 vessels per square millimeter. We conclude that image morphometric quantitation of vascular basal lamina represents an objective means of assessing angiogenic capacity in breast tumors, which correlates strongly with disease aggressiveness in short-term follow-up.