SUSTAINED CARDIOMYOCYTE DNA-SYNTHESIS IN WHOLE-EMBRYO CULTURES LACKING THE TSC2 GENE-PRODUCT

Tuberous sclerosis complex (TSC) is characterized by the appearance of nonmalignant tumors that affect a wide spectrum of organs, including the heart. TSC disease-causing genes have been identified on chromosom es 9 (TSC1) and 16 (TSC2). This study examined the impact of the TSC2 gene product on cardiomyocyte proliferation and terminal differentiati on. We took advantage of the observation that Eker rats carry a germ-l ine TSC2 mutation. Rats heterozygous for the mutation (TSC2(EK/+)) are predisposed to renal carcinoma, whereas animals homozygous for the mu tation (TSC2(EK/EK)) die in utero during midgestation. Spontaneously c ontractile cardiomyocytes were observed after multiple passages of who le embryo cultures prepared from embryonic day 12.5 TSC2(EK/EK) fetuse s but not from TSC2(EK/+) or wild-type fetuses. The TSC2(EK/EK) cardio myocytes continued to actively synthesize DNA after as many as eight p assages. Cytological, ultrastructural, and molecular analyses indicate d that the TSC2(EK/EK) cardiomyocytes retained a highly differentiated phenotype similar to that observed for normal rat cardiomyocytes duri ng late embryonic and early neonatal life. These results suggested tha t the TSC2 gene product is required for normal cardiomyocyte cell-cycl e withdrawal and terminal differentiation.