Four nonmuscle tropomyosin isoforms have been reported to be produced from the rat Tm5 gene by alternative splicing (Beisel, K. W., and Kennedy, J. E. (1994) Gene (Amst.) 145, 251-256). In order to detect additional isoforms that might be expressed from that gene, we used reverse transcriptase-polymerase chain reaction assays and evaluated the presence of all product combinations of two alternative internal exons (6a and 6b) and four carboxyl-terminal exons (9a, 9b, 9c, and 9d) in developing and adult rat brain. We identified five different combinations for exon 9 (9a ؉ 9b, 9a ؉ 9c, 9a ؉ 9d, 9c, and 9d), and the exon combinations 9a ؉ 9c and 9a ؉ 9d were previously unreported. Each of these combinations existed with both exon 6a and exon 6b. Thus, the rat brain generates at least 10 different isoforms from the Tm5 gene. Northern blot hybridization with alternative exon-specific probes revealed that these isoforms were also expressed in a number of different adult rat tissues, although some exons are preferentially expressed in particular tissues. Studies of regulation of the 10 different Tm5 isoform mRNAs during rat brain development indicated that no two isoforms are coordinately accumulated. Furthermore, there is a developmental switch in the use of exon 6a to exon 6b from embryonic to adult isoforms. TM5 protein isoforms show a differential localization in the adult cerebellum.Tropomyosins are rod-like proteins that are associated with actin filament in muscle and nonmuscle cells. Multiple isoforms of tropomyosins exist in both muscle and nonmuscle cells (1). In muscle cells, tropomyosins play a pivotal role in regulating the interaction between the actin and myosin filaments. The role of tropomyosins in nonmuscle cells is beginning to be more defined. They are thought to differentially affect the stability of actin filaments (2) and have been shown to be implicated in various cellular functions including the regulation of cell transformation (3-5), cytokinesis (6), motility (7,8), and morphogenesis (9 -13).Alternative splicing accounts for the majority of tropomyosin isoform diversity. In mammals, four tropomyosin genes have been identified, ␣-Tm f (14), ␤-Tm (15), Tm4 (16), and Tm5 (17). Except for the Tm4 gene, it is now clear that the other three rat tropomyosin genes contain two alternate promoters (exons 1a and 1b), two alternative internal exons (6a and 6b), and four alternative COOH-terminal exons (9a, 9b, 9c, and 9d). Therefore, the tropomyosin genes can each theoretically generate mRNAs for at least 16 isoforms. However, only 20 isoforms have been characterized from all four rat tropomyosin genes (14 -24). This suggests that some exon combinations either do not occur or are unique to cell types that have not yet been studied.In the case of the rat Tm5 gene, one muscle isoform and four nonmuscle isoforms have been reported to date. The muscle product of this gene is known as ␣-Tm s since it is preferentially expressed in slow-twitch skeletal muscle (25). The four nonmuscle isoforms have been termed T...

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Familial hypertrophic cardiomyopathy (FHC) is a cardiac disorder transmitted as an autosomal dominant trait. FHC has been shown to be genetically heterogeneous with less than 50% of published pedigrees being associated with mutations in the beta myosin heavy chain (beta-MHC) gene on chromosome 14q11-q12. A second locus has recently been reported on chromosome 1. We examined the segregation of microsatellite markers in a French pedigree for which the disease is not linked to beta-MHC gene. We found significant linkage of the disease locus to several (CA)n repeats located on chromosome 11 (lod scores between +3.3 and +4.98). The data suggest the localization of the novel FHC gene in a region spanning 17 centiMorgans.

Familial hypertrophic cardiomyopathy (FHC) is a clinically and genetically heterogeneous disease. The first identified disease gene, located on chromosome 14qll-ql2, encodes the #-myosin heavy chain. We have performed linkage analysis of two French FHC pedigrees, 720 and 730, with two microsatellite markers located in the #-myosin heavy chain gene (MYO I and MYO II) and with four highly informative markers, recently mapped to chromosome 14qll-ql2. Significant linkage was found with MYO I and MYO II in pedigree 720, but results were not conclusive for pedigree 730. Haplotype analysis of the six markers allowed identification of affected individuals and of some unaffected subjects carrying the disease gene. Two novel missense mutations were identified in exon 13 by direct sequencing, 403 '" and 403ig-TriP in families 720 and 730, respectively. The 403ArgI' mutation was associated with incomplete penetrance, a high incidence of sudden deaths and severe cardiac events, whereas the consequences of the 403 r"T'P mutation appeared less severe. Haplotyping of polymorphic markers in close linkage to the #-myosin heavy chain gene can, thus, provide rapid analysis of non informative pedigrees and rapid detection of carrier status. Our results also indicate that codon 403 of the j-myosin heavy chain gene is a hot spot for mutations causing FHC. (J. Clin. Invest. 1993. 92:2807-2813.) Key words: familial hypertrophic cardiomyopathy * molecular genetics -mutation hot spot * -jmyosin heavy chain * microsatellite are diverse, including syncope, angina, heart failure, and sudden death (even in asymptomatic patients). The major common denominator of these various presentations is ventricular hypertrophy usually associated with microscopic evidence of myocardial fiber disarray (1, 2). Linkage of the disease with a locus on chromosome 14q11 -q 12 was found in approximately one third of the affected families (3-8). The #-myosin heavy chain gene (MYH7) is the disease gene of this locus, and 16 different missense mutations, localized in exons coding for the head and the head rod regions of the protein, were found in various pedigrees (9-21). The most common mutation is in exon 13, affecting the second nucleotide of codon 403 and results in the substitution ofan arginine by a glutamine residue. It was found in eight families of various ethnic origin (9-14), and it is associated with an early onset ofthe disease and a high frequency of sudden deaths.