Expression of an alternative Dnmt1 isoform during muscle differentiation.

The methylation pattern of genomic DNA undergoes dramatic changes during mammalian development, with extensive de novo methylation occurring during gametogenesis and after implantation. We identified an alternative Dnmt1 transcript in skeletal muscle by Northern blot analysis and cloned the corresponding cDNA by rapid amplification of cDNA ends and reverse transcription-PCR. Using an in vitro skeletal muscle differentiation system, we show that this alternative Dnmt1 isoform is specifically expressed in differentiated myotubes, whereas the ubiquitously expressed isoform is down-regulated during myogenesis. Sequence analysis showed that this skeletal Dnmt1 isoform is identical to the one present in testis, which had been described as untranslatable. Here we present evidence that this alternative Dnmt1 transcript present in testis and skeletal muscle is translated despite the presence of several out-of-frame upstream ATGs and gives rise to a shorter Dnmt1 isoform, which could play an active role in the change of DNA methylation patterns during gametogenesis and myogenesis.

Structure and function of the mouse DNA methyltransferase gene: Dnmt1 shows a tripartite structure.

Dnmt1 is the predominant DNA methyltransferase (MTase) in mammals. The C-terminal domain of Dnmt1 clearly shares sequence similarity with many prokaryotic 5mC methyltransferases, and had been proposed to be sufficient for catalytic activity. We show here by deletion analysis that the C-terminal domain alone is not sufficient for methylating activity, but that a large part of the N-terminal domain is required in addition. Since this complex structure of Dnmt1 raises issues about its evolutionary origin, we have compared several eukaryotic MTases and have determined the genomic organization of the mouse Dnmt1 gene. The 5' most part of the N-terminal domain is dispensible for enzyme activity, includes the major nuclear import signal and comprises tissue-specific exons. Interestingly, the functional subdivision of Dnmt1 correlates well with the structure of the Dnmt1 gene in terms of intron/exon size distribution as well as sequence conservation. Our results, based on functional, structural and sequence comparison data, suggest that the gene has evolved from the fusion of at least three genes.

A novel isoform of the smooth muscle cell differentiation marker smoothelin.

Studies on smooth muscle cell differentiation and those on vascular development in mouse and humans have long been hampered by the lack of suitable markers. Here we describe a novel, large isoform of smoothelin, a structural protein of differentiated, contractile smooth muscle cells. The protein, which is highly conserved in mouse and humans, shows homology with other cytoskeleton-associated smooth muscle cell proteins and contains an actinin-type actin-binding domain. Northern blot analysis from various mouse organs identified short and long smoothelin mRNA forms, which exhibit distinct tissue expression patterns. The short form is highly expressed in visceral muscle tissues such as intestine and stomach and is not detectable in brain, while the long mRNA form is expressed in all vascularized organs. These results may provide new tools and approaches to study both smooth muscle cell differentiation and proliferative vascular disease.