TGFß-Inducible Early Gene-1 (TIEG1): A Novel Hypertrophic Cardiomyopathy- Susceptibility Gene

Background: Hypertrophic cardiomyopathy (HCM) is the most common heritable cardiovascular disease and the most common cause of sudden cardiac death in the young. Over 24 genes have been implicated in the pathogenesis of HCM. However, for about half of patients with HCM, the genotypic substrate remains elusive. A recent study showed that male TGF Inducible Early Gene-1 (TIEG1) knock-out (TIEG1) mice develop HCM after 16 months. Microarray analysis on the mice hearts showed a 13-fold up-regulation of PTTG1-encoded pituitary tumor-transforming gene 1. We therefore speculated that TIEG1 could be a novel candidate gene in the pathogenesis of genotype negative HCM, possibly through a loss of its repression on PTTG1 expression. Methods: For this study, we analyzed a cohort of 923 unrelated patients from two independent cohorts of patients with HCM (664 male, age at diagnosis 47.6 ± 18, mean left ventricular wall thickness (MLVWT) 20.0 ± 7mm). All patients were genotype negative with respect to the 9 genes responsible for myofilament/sarcomeric-HCM. Open reading frame/splice site mutational analysis of TIEG’s 4 translated exons was performed using DHPLC and direct DNA-sequencing. Site directed mutagenesis was performed to clone novel variants. The effect of wild type and mutant TIEG1 on the PTTG1 and SMAD7 promoters was studied using transient transfection and luciferaseassays. Cardiac HCM tissue was studied by immunohistochemistry to determine levels of PTTG1 protein expression. Results: Six novel missense mutations (A12T, M27T, T216A, E137K, A204T and S225N) in TIEG1 were discovered in 6/923 patients (2 males/4 females, mean age at diagnosis 56.2 ± 23 years, MLVWT 20.8 ± 4 mm). Each missense mutation was absent in 800 ethnically-matched reference alleles and involved residues that were conserved across species. Compared to the 50% repression of PTTG1 promoter function by wild type TIEG1, 5 TIEG1 mutants had this repression significantly attenuated resulting in marked accentuation of PTTG1 promoter function similar to the TIEG1 KO-mice. One TIEG1 mutant significantly altered TIEG1-function on SMAD7expression. By immunohistochemistry, PTTG1-protein expression was increased in myectomy specimens from all patients with HCM, irrespective of TIEG1 mutation status, compared to normal hearts.