Abstracts Biological Sciences

Regulation and expression of DNA methyltransferase in the mouse germline

by Carmen. Mertineit

The epigenetic modification of DNA by methylation at cytosine in CpG dinucleotides plays an important role in X-chromosome inactivation, genomic imprinting, host defense and cancer. In this thesis, I investigated the role of the predominant mammalian DNA methyltransferase, Dnmt1, in the establishment and maintenance of DNA methylation patterns in the germline. In a comparative study of the expression of Dnmt1 during male and female germ cell development, I identified putative windows in which Dnmt1 may set down sex-specific methylation patterns for imprinted genes, I also identified and characterized sex-specific exons in the 5' region of Dnmt1 which control the production and localization of enzyme during specific stages of gametogenesis by a mechanism that involves alternative splicing. An oocyte-specific 5' exon is associated with the production of a functional Dnmt1 protein that is truncated at the N-terminus, accumulates to high levels during oocyte growth and later persists through preimplantation development, while a spermatocyte-specific 5' exon leads to the production of a larger, nontranslated message. I further examined the dynamics in the expression of the oocyte-specific Dnmt1 isoform in populations of isolated oocytes obtained at different stages of development and their association with the methylation status of the maternally imprinted Snrpn gene. The data suggest that the tightly regulated expression of the oocyte-specific Dnmt1 during postnatal oogenesis may play a role in the establishment of maternal methylation imprints at this time, Furthermore, since the oocyte-specific isoform persists during preimplantation development, at a time when the somatic isoform of Dnmt1 is not detected, I propose that the oocyte isoform may also be important for the propagation of maternally- and paternally-derived imprints in the early embryo.