Interphase chromosome positioning affects the spectrum of radiation-induced chromosomal aberrations.

In interphase, chromosomes occupy defined nuclear volumes known as chromosome territories. To probe the biological consequences of the described nonrandom spatial positioning of chromosome territories in human lymphocytes, we performed an extensive FISH-based analysis of ionizing radiation-induced interchanges involving chromosomes 1, 4, 18 and 19. Since the probability of exchange formation depends strongly on the spatial distance between the damage sites in the genome, a preferential formation of exchanges between proximally positioned chromosomes is expected. Here we show that the spectrum of interchanges deviates significantly from one expected based on random chromosome positioning. Moreover, the observed exchange interactions between specific chromosome pairs as well as the interactions between homologous chromosomes are consistent with the proposed gene density-related radial distribution of chromosome territories. The differences between expected and observed exchange frequencies are more pronounced after exposure to densely ionizing neutrons than after exposure to sparsely ionizing X rays. These experiments demonstrate that the spatial positioning of interphase chromosomes affects the spectrum of chromosome rearrangements.

Importance of TP53 and RB in the repair of potentially lethal damage and induction of color junctions after exposure to ionizing radiation.

Repair of potentially lethal damage (PLD) was investigated in cells with functional G1-phase arrest with wild-type TP53 and wild-type RB and in cells in which G1-phase arrest was abrogated by inactivation of TP53 or RB. Confluent cultures of cells were plated for clonogenic survival assay either immediately or 24 h after irradiation. Induction of color junctions, an exchange between a painted and unpainted chromosome, was studied in chromosomes 18 and 19 after irradiation with 4 Gy gamma rays. Significant repair of PLD was found in cells carrying both wild-type TP53 and wild-type RB. In cells in which TP53 or RB was inactivated, the survival curves from immediately plated and delayed-plated cells were not significantly different. The numbers of radiation-induced color junctions in chromosomes 18 and 19 were similar in all cell lines. From this study we conclude that a functional G1-phase arrest is important for repair of PLD and that TP53 and RB do not affect the frequencies of induction of color junctions in chromosome 18 or 19.

Breakage of human chromosomes 4, 19 and Y in G0 cells immediately after exposure to gamma-rays.

One of several possible explanations for the fact that the initial number of ionizing radiation-induced DNA double-strand breaks (dsbs) per cell per Gy appears to be approximately 5-10 times greater than the number of excess fragments produced in prematurely condensed chromosomes (PCCs) is that dsbs in DNA tightly associated with protein, such as might be the case for heterochromatin, are held together and not expressed as discontinuities in the PCC assay. To test this idea the breakage frequencies in chromosome 4, 19 and the heterochromatic and more euchromatic portions of the Y chromosome of non-cycling human fibroblasts were measured and compared over a wide range of doses by inducing PCC immediately after irradiation with Cs-137 gamma-rays. Even for doses up to 400 Gy no evidence was seen that Y heterochromatin or large fragments of it remained unbroken. The only significant deviation from the expected initial breakage frequency per Gy per unit length of chromosome, in the dose range where a genome average for all chromosomes could be obtained, was that observed for the euchromatic portion of the Y chromosome (Y p + cen-->q11.2) where breakage was nearly twice that expected. There was no correlation between breakage per unit length and the (A + T)/(G + C) ratio for these chromosomes or regions.

Objective response to radiation therapy and long-term survival of patients with WHO grade II astrocytic gliomas with known LOH 1p/19q status.

BACKGROUND: WHO grade II gliomas are often approached by radiation therapy (RT). However, little is known about tumor response and its potential impact on long-term survival. PATIENTS AND METHODS: Patients subjected to RT were selected from the own database of WHO grade II gliomas diagnosed between 1991 and 2000. The volumetric tumor response after RT was assessed based on magnetic resonance imaging and graded according to standard criteria as complete, partial (PR, >or= 50%), or minor (MR, 25% to <50%). RESULTS: There were 24 astrocytomas and three oligoastrocytomas. 21 patients (78%) were dead at follow-up (mean survival 74 months). None of the patients had chemotherapy. Objective response occurred in 14 patients (52%, five PR and nine MR) but was not associated with overall survival. The vast majority of the tumors had no loss of heterozygosity (LOH) 1p and/or 19q (86%). CONCLUSION: Approximately 50% of patients with astrocytic WHO grade II gliomas respond to RT despite the absence of LOH for 1p/19q. The potential predictive factors for response and the impact of response on overall survival remain unclear.