DNA damage induced mating type switching in Saccharomyces cerevisiae

Induction of fusion-competent myoblast-specific gene expression during myogenic differentiation of Drosophila Schneider cells by DNA double-strand breaks or replication inhibition

Differentiation of Drosophila Schneider cells caused by DNA double-strand break (DSB)-inducing topoisomerase II (topo II) inhibitors were attenuated by ICRF-193, a non-DNA-damaging topo II inhibitor. ICRF-193 did not inhibit differentiation induced by neocarzinostatin (NCS), a drug that causes DNA DSBs independent of topo II. Schneider cells differentiated upon treatment with gamma-ray. These results suggest that DNA DSBs induce myogenic differentiation of Schneider cells. We also found DNA replication inhibitors, hydroxyurea (HU), aphidicolin, and ethylmethanesulfonate (EMS) induced myogenic differentiation of Schneider cells. HU-induced differentiation was inhibited upon pretreatment of cells with chemical inhibitors of PP 1/2A, p38 MAPK, JNK, and proteasome. RT-PCR analysis revealed that the expressions of fusion-competent myoblast-specific genes lmd, sns, and del were induced in Schneider cells upon treatment with NCS or HU, whereas expressions of three founder cell-specific genes, duf, ants, and rols, were undetectable. These results indicate that the expression of fusion competent-myoblast-specific genes is induced during myogenic differentiation of Drosophila Schneider cells by DNA DSBs or replication inhibition.

Myogenic differentiation of Drosophila Schneider cells by DNA double-strand break-inducing drugs

Drosophila melanogaster has been widely used as a model organism to study various aspects of development. Apart from the whole Drosophila embryo, there are a number of cultured cell lines derived from Drosophila embryo that have also been used for elucidating various aspects of development. Drosophila Schneider line 2 cells were derived from the late stages of the embryo (Schneider, 1972). We found that the Schneider cells undergo myogenic differentiation upon treatment with neocarzinostatin (NCS), DNA double-strand break (DSB)-inducing drug, as indicated by elongated morphology, myosin heavy chain protein expression, multinucleation and exit from the cell cycle. No induction of differentiation was observed when cell proliferation was inhibited with drugs that do not cause DNA DSBs. Pre-treatment of Schneider cells with inhibitors of PKC, PP 1/2A, p38 MAPK, JNK and proteasomes resulted in the inhibition of morphological differentiation induced by NCS. These results indicate that DNA DSBs can turn on the myogenic program in Drosophila Schneider cells and the process is dependent on PK C-, PP 1/2A-, p38 MAPK-, and JNK- mediated signaling and proteasomal activity. The molting hormone, 20-hydroxyecdysone (20-HE), also showed an anti-myogenic effect on the process. This is the first report of insect cells undergoing differentiation by DNA DSB-inducing drugs as far as we know, and it provides a very useful and convenient in vitro system to study various aspects of Drosophila myogenesis.

Gene conversion can create new MHC alleles

It is possible to measure gene conversion of MHC genes with the help of a semi-nested PCR assay. Several considerations are of utmost importance when such an assay is set up. Using this assay, we have found that gene conversion occurs in MHC class II genes in mouse sperm, but not in somatic cells tested. Although this gene conversion occurs in germline cells, it is already completed in spermatogonia, and consequently is mitotic event unlinked to meiosis. The frequency of gene conversion events in MHC class II genes varies strongly from one allele to another, with the highest detected frequencies as high as 1/40,000 for an individual heterozygous for both donor and acceptor sequences. Deletions or insertions in one gene relative to the other seem to lower the efficiency of gene conversion considerably. Stretches within MHC genes amenable to gene conversion are located in CpG clusters, whereas MHC genes not involved in gene conversion have background CpG levels. DNA damage, either chemical or radiation induced, increases the frequency of gene conversion of MHC class II genes in cultured cells of the fibroblastoid lineage. The effect of chemical DNA damage seems roughly dose dependent, whereas irradiation has a maximal effect at low doses.

DNA damage caused by etoposide and gamma-irradiation induces gene conversion of the MHC in a mouse non-germline testis cell line

We have explored the effects of gamma-irradiation and etoposide on the gene conversion frequency between the endogenous major histocompatibility complex class II genes Abk and Ebd in a mouse testis cell line of non-germline origin with a polymerase chain reaction assay. Both gamma-rays and etoposide were shown to increase the gene conversion frequency with up to 15-fold compared to untreated cells. Etoposide, which is an agent that stabilise a cleavable complex between DNA and DNA topoisomerase II, shows an increased induction of gene conversion events with increased dose of etoposide. Cells treated with gamma-rays, which induce strand breaks, had an increased gene conversion frequency when they were subjected to low doses of irradiation, but increasing doses of irradiation did not lead to an increase of gene conversion events, which might reflect differences in the repair process depending on the extent and nature of the DNA damage. These results where DNA damage was shown to be able to induce gene conversion of endogenous genes in mouse testis cells suggests that the DNA repair system could be involved in the molecular genetic mechanism that results in gene conversion in higher eukaryotes like mammals.

Preferential incision of interstrand crosslinks induced by 8-methoxypsoralen plus UVA in yeast during the cell cycle

Interstrand crosslink (ICL) induction by 8-methoxypsoralen plus UVA and the incision step of the repair have been investigated during the mitotic cell cycle of haploid Saccharomyces cerevisiae. Cells were synchronised by elutriation and events were examined at the level of the MAT alpha and the HML alpha loci in a SIR strain. The DNA sequence of these two loci is identical, but the MAT alpha locus may be replicated earlier in S phase and is transcriptionally active while the HML alpha locus may be replicated later in S phase and is transcriptionally inactive because of Sir repression that creates a heterochromatin-like structure at this locus. ICL were induced to similar extents in both loci during the stages of the cell cycle examined, and these levels were identical to those reported for asynchronous cultures. Preferential incisions occurred for ICL in the MAT alpha locus compared to those in the HML alpha locus, independently of the cell cycle phase studied. The levels of incision were comparable for events in the early G1 phase (eG1), late G1 phase (lG1), early S phase (eS), middle S phase (mS), late S phase (lS) or G2 phase (G2). Thus the preferential incision of ICL observed previously in asynchronous cell culture is maintained throughout the cell cycle and, surprisingly, occurs equally well in G1. Here the opportunities for recombination to further process the incised damaged are substantially limited compared to those in the S and G2 phases.

A recombination-based transgenic mouse system for genotoxicity testing

It is well established that mutagens induce recombination in cultured cells and experimental organisms. Presumably, this is a consequence of the DNA-damage-triggering cellular-repair mechanisms. The relationship between recombination and mutagenicity has been exploited in submammalian organisms, such as yeast, to assay the ability of chemical agents and radiation to induce a form of recombination called gene conversion--the non-reciprocal transfer of genetic information. This work has demonstrated the efficacy of predicting mutagenicity on the basis of recombination induction. Here, we describe the utilization of a transgenic mouse system for efficient detection of germ-line gene-conversion events as a mutagen-screening tool. These mice contain two mutually defective reporter (lacZ) genes under the regulatory control of a spermatogenesis-specific promoter. A particular intrachromosomal gene conversion event must occur for the generation of functional lacZ activity. Conversion events are visualized by histochemical staining or flow cytometric analysis of transgenic spermatids. The highly mutagenic compound chlorambucil induced a several fold percentage-wise increase of lacZ-positive spermatids, whereas acrylamide, a weak genotoxin, produced no marked increase in converted spermatids. The results indicate that recombination-based transgenic mouse models for genotoxin screening present a viable option for inexpensive and rapid whole-animal mutagen testing. The particular mice we describe may ultimately prove to be a useful tool for identifying agents which can cause heritable genetic mutations in humans.