This study was aimed at the investigation of the mechanisms by which mutations on
proteins of the thin filaments could lead to cardiac and skeletal muscle diseases. The
contractile properties of reconstituted thin filaments were assessed using the
quantitative in vitro motility assay.
The cardiac disease investigated was the dilated cardiomyopathy (DCM). Pilot work
on 2 mutants indicated that, contrarily to what was believed in the past, the molecular
phenotype generated by mutations on thin filament proteins linked to the disease was
not a reduced absolute Ca2+-sensitivity and cross-bridge turnover rate, but an
uncoupling between the phosphorylation level of troponin I and the Ca2+-sensitivity.
In this thesis I will present the confirmation of the hypothesis with 7 DCM causing
mutations. At least one mutation on each of the thin filaments subunits was tested and
they all shared that molecular phenotype despite differences in the absolute Ca2+-
sensitivity.
Mutations on β- and γ-tropomyosin linked to 3 skeletal muscle diseases, Nemaline
myopathy, Cap disease and congenital fibre type disproportion (CFTD), were taken
into consideration. Some of these mutations were also reported to cause more than
one of these diseases. The β-tropomyosin mutations indicated that there is not a clear
correlation between molecular phenotype and disease, as different modifications of
the contractile properties seem to lead to the same disease. One of the β-tropomyosin
mutations (ΔK7), though, as another mutation on skeletal actin (K326N) that was
investigated, showed a new hypercontractile phenotype connected to a gain-offunction
molecular phenotype, which led to a change in the clinical diagnosis. The
experiments performed with the γ-tropomyosin mutants encountered many technical
issues but indicated that a common phenotype of decreased Ca2+-sensitivity might be
connected to CFTD, observation supported also by a β-tropomyosin mutant connected
only to that disease.