E. Viégas-Pèquignot

We have identified a novel human gene of the Ig superfamily, designated LAG-3. Expression of this gene is undetectable in resting PBL, while it is found (a 2-kb message) in activated T and NK cells. The LAG-3 gene includes eight exons; the corresponding cDNA encodes a 498-amino acid membrane protein with four extracellular IgSF domains. The first one belongs to the V-SET; it is particular since it includes an extra loop in the middle of the domain and an unusual intrachain disulphide bridge. The three other domains belong to the C2-SET. Strong internal homologies are found in the LAG-3 molecule between domains 1 and 3, as well as between domains 2 and 4. It is therefore likely that LAG-3 has evolved by duplication of a pre-existing gene encoding a two IgSF-domain structure. The compared analysis of LAG-3 and CD4, with respect to both their peptidic sequence as well as their exon/intron organization, indicated that the two molecules are closely related. This point is strengthened by the finding that both genes are located on the distal part of the short arm of chromosome 12.

DNA methylation patterns were evaluated during preimplantation mouse development by analyzing the binding of monoclonal antibody to 5-methylcytosine (5-MeC) on metaphase chromosomes. Specific chromosome patterns were observed in each cell stage. A banding pattern predominated in chromosomes at the one-cell stage. Banding was replaced at the two-cell stage by an asymmetrical labeling of the sister chromatids. Then, the proportion of asymmetrical chromosomes decreased by one-half at each cell division until the blastocyst stage, and chromosomes became progressively symmetrical and weakly labeled. Our results indicate that chromosome demethylation is associated with each DNA replication and suggest that a passive mechanism predominates during early development.