Prediction of the Acute or Late Radiation Toxicity Effects in Radiotherapy Patients Using Ex Vivo Induced Biodosimetric Markers: A Review

A search for effective methods for the assessment of patients’ individual response to radiation is one of the important tasks of clinical radiobiology. This review summarizes available data on the use of ex vivo cytogenetic markers, typically used for biodosimetry, for the prediction of individual clinical radiosensitivity (normal tissue toxicity, NTT) in cells of cancer patients undergoing therapeutic irradiation. In approximately 50% of the relevant reports, selected for the analysis in peer-reviewed international journals, the average ex vivo induced yield of these biodosimetric markers was higher in patients with severe reactions than in patients with a lower grade of NTT. Also, a significant correlation was sometimes found between the biodosimetric marker yield and the severity of acute or late NTT reactions at an individual level, but this observation was not unequivocally proven. A similar controversy of published results was found regarding the attempts to apply G2- and γH2AX foci assays for NTT prediction. A correlation between ex vivo cytogenetic biomarker yields and NTT occurred most frequently when chromosome aberrations (not micronuclei) were measured in lymphocytes (not fibroblasts) irradiated to relatively high doses (4–6 Gy, not 2 Gy) in patients with various grades of late (not early) radiotherapy (RT) morbidity. The limitations of existing approaches are discussed, and recommendations on the improvement of the ex vivo cytogenetic testing for NTT prediction are provided. However, the efficiency of these methods still needs to be validated in properly organized clinical trials involving large and verified patient cohorts.

Human papillomavirus 16 promotes microhomology-mediated end-joining

Squamous cell carcinomas (SCCs) arising from aerodigestive or anogenital epithelium that are associated with the human papillomavirus (HPV) are far more readily cured with radiation therapy than HPV-negative SCCs. The mechanism behind this increased radiosensitivity has been proposed to be secondary to defects in DNA repair, although the specific repair pathways that are disrupted have not been elucidated. To gain insight into this important biomarker of radiosensitivity, we first examined genomic patterns reflective of defects in DNA double-strand break repair, comparing HPV-associated and HPV-negative head and neck cancers (HNSCC). Compared to HPV-negative HNSCC genomes, HPV+ cases demonstrated a marked increase in the proportion of deletions with flanking microhomology, a signature associated with a backup, error-prone double-strand break repair pathway known as microhomology-mediated end-joining (MMEJ). Then, using 3 different methodologies to comprehensively profile double-strand break repair pathways in isogenic paired cell lines, we demonstrate that the HPV16 E7 oncoprotein suppresses canonical nonhomologous end-joining (NHEJ) and promotes error-prone MMEJ, providing a mechanistic rationale for the clinical radiosensitivity of these cancers.

Ataxia Telangiectasia

Abstract SCI-7 Ataxia-telangiectasia (A-T) is the prototype for an expanded group of inherited radiation sensitive disorders that together define the XCIND syndrome: x-ray hypersensitivity, cancer, immunodeficiency, neurological dysfunction, and DNA repair deficiency. Although the clinical radiosensitivity of these disorders can be tested in the clinical laboratory, diagnostic methods remain limited and in need of further validation. Without exception, to date, sensitivity to ionizing radiation appears to be integrally associated with double strand break (DSB) repair defects and lymphoid cancer susceptibility, setting these disorders apart from single strand break repair disorders such as xeroderma pigmentosum. Responding within seconds to DSB damage are ATM kinase, the protein lacking in A-T, and the NMR complex (nibrin, Mre11, and Rad50). The latter three proteins are associated with three additional XCIND disorders (nibrin deficiency [aka nijmegen breakage syndrome], Mre11 deficiency [ATLD], and Rad50 deficiency). ATM kinase activates a myriad of other proteins that 1) halt DNA synthesis, replication, and the progression of the cell cycle; 2) form a complex protein “mesh” to physically stabilize the broken DNA strands; and 3) restore the integrity of the breaks before they unravel to create even larger chromosomal lesions and resulting malignancies. Another ATM-dependent cancer link involves the downregulation of ATM by microRNA-421. MicroRNA-421 is upregulated by the transcription factor N-myc. Despite this, neuroblastomas are not commonly observed in A-T or XCIND patients. Another subset of XCIND-associated disorders lacks proteins the drive the nonhomologous end joining pathway of DNA repair. Several of these diseases present in infancy as B−/T− severe combined immunodeficiency, or SCID, and are frequent candidates for stem cell transplantation. Attempts to ablate existing bone marrow prior to transplantation may further compromise such patients if they are inherently radiosensitive. Thus, attempts to preselect such patients and reduce radiation dosages may improve general post-transplantation survival. While most protein deficiencies can be diagnosed by immunoblots of appropriate cellular fractions, nonfunctional proteins are not detected by this platform. Colony survival assays (CSA) measure the ability of replicating cells (e.g., lymphoblasts or fibroblasts) to survive after exposure to radiation. Although causal proof that CSA can predict clinical radiosensitivity is lacking, the reduced percent survival fraction (i.e., radiosensitivity) of A-T, N-Bromosuccinimide, or Fanconi cell lines can be abrogated by introducing the mutated cognate gene. Other surrogate assays for radiosensitivity include kinetic studies, pre-irradiation and post-irradiation of γ-H2AX or SMC1 phosphorylation. Ultimately, DNA sequencing of a candidate gene can pinpoint the underlying pathogenesis of radiosensitivity in an XCIND disorder. Disclosures: No relevant conflicts of interest to declare.