The Chinese hamster genome contains a total of 18 cytologically detectable arrays of interstitial telomeric sequences. A combination of G-banding and two-colour fluorescence in situ hybridization revealed that 25 out of 27 (93%) breakpoints of spontaneously occurring terminal deletions in four immortalized Chinese hamster cell lines were located in chromosomal regions containing interstitial telomeric sequences. Each of the four immortalized Chinese hamster cell lines expressed telomerase. Radiation experiments revealed the sensitivity of interstitial telomeric sequences to radiation-induced chromosomal breakage in all telomerase-positive cell lines. However, radiation-induced chromosomal breakage at interstitial telomeric sites in non-transformed, primary Chinese hamster cells was almost non-existent. Telomerase activity in primary Chinese hamster cells was not detected. These results indirectly suggest that interstitial telomeric sites represent a favourable substrate for chromosomal healing.
Genotoxic agents induce chromosomal alterations, such as aberrations, micronuclei, and sister chromatid exchanges as well as mutations both in vivo and in vitro. Ionizing radiation and typical radiomimmetic agents such as bleomycin are very efficient inducers of chromosomal aberrations. The type of aberrations induced by these agents are cell-cycle dependent, i.e., chromosome type in pre-replication stages and chromatid type in post-replication stages of the cell cycle. Under optimal DNA repair conditions, DNA double-strand breaks (DSBs) appear to be the most important lesion responsible for the production of aberrations. In human lymphocytes, fast-repairing DSBs lead to exchange-type aberrations. The fact that the dose-response curves for induction of exchange aberrations induced by ionizing radiation are similar in vitro and in vivo allows one to use the yield of induced aberrations to estimate absorbed radiation dose in the case of accidents. In this respect, frequencies of translocations detected by the chromosome painting technique appear to be more sensitive. Mutations do not express immediately after exposure and require an expression time before they can be detected. In humans, it is estimated that for the mutations induced in bone marrow, it takes about 2 months for them to express and to be detected in peripheral blood lymphocytes. Hence, frequency of mutations is of limited value for estimating radiation doses immediately after an accident. This holds true for chemical exposure as well.(ABSTRACT TRUNCATED AT 250 WORDS)
We report a case of cerebral phaeohyphomycosis caused by Scytalidium dimidiatum (synanamorph Nattrassia mangiferae) in a young, apparently immunocompetent Indian male. Etiological diagnosis was made by recovery of the fungus in culture and histopathological examination. The infection proved fatal despite aggressive antifungal therapy.
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