Functional analysis of Tsc2 gene product by conditional gene targeting.

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In this study, we established a conditional gene targeting system for the mouse homologue of human tuberous sclerosis 2 (Tsc2) gene. Initially, a mouse line carrying a silent mutant allele (S-allele) of Tsc2 gene, in which exons 3 and 4 were flanked with a pair of loxP sequences, was generated through gene targeting technique. A deletion mutant allele (D-allele) lacking exons 3 and 4, generated from the S-allele by Cre-mediated recombination, was functionally inactivated, since mice heterozygous for D-allele developed renal tumors as Tsc2 knockout mice previously reported. Next, we crossed mice carrying S-allele with "knock-in" mice which express Cre recombinase under the promoter of ventricular-specific myosin light chain gene (Mlc2v) to carry out the cardiac muscle-specific Tsc2 knockout in vivo.Although mice homozygous for S-allele carrying Mlc2v-Cre allele (CKO mice) were born and grown normally, they were dead by 1 year of age after exhibiting abnormal breathing. None of the control mice showed such mortality. In hearts of CKO mice, D-allele was generated by Cre-mediated recombination. The hearts of CKO mice were markedly enlarged and showed histological anomalies such as hypertrophy and nuclear atypia of cardiac muscle cells. In addition, the hearts of CKO mice showed dilation of left ventricle by echocardiography and expressed maker genes of failing heart such as atrial natriuretic polypeptide (Anp) gene. Thus, phenotype resembling the dilated cardiomyopathy was induced by the cardiac muscle-specific Tsc2 knockout in mice. Now we further analyzed the function of Tsc2 product in growth and differentiation of cardiac muscle cells. The conditional gene targeting of Tsc2 established in this study will be a useful experimental system to elucidate the function of Tsc2 product as well as the mechanism of tumorigenesis associated with Tsc2 mutation.