P.3.11 Atypical phenotypes in titinopathies explained by second titin mutations and compound heterozygosity

Several patients with previously reported titin gene (TTN) mutations causing tibial muscular dystrophy (TMD) have more complex, severe or unusual phenotypes. This study aimed to clarify the molecular cause of these variant phenotypes in proband patients of six European families. Clinical, histopathological and muscle imaging data of patients and family members was reanalyzed and muscle biopsies of the patients were studied by titin Western blotting and RT-PCR. Western blotting showed a more pronounced C-terminal titin abnormality than expected for heterozygous mutants in all six probands, suggesting the existence of additional TTN mutations. RT-PCR indicated unequal mRNA expression of the two TTN alleles in biopsies of four patients, two of them with an LGMD2J phenotype. TTN was analyzed by Sanger sequencing from genomic DNA of the patients. Three patients proved to be compound heterozygotes with novel TTN frameshift mutations combined with previously reported TMD mutations. One LGMD2J patient was heterozygous for the FINmaj TMD mutation with a likely second yet undiscovered mutation. The unequal expression levels of TTN transcripts in these four probands suggested that the expression of the frameshifted allele was severely reduced, probably through nonsense-mediated decay, explaining the observed more severe phenotypes. One Portuguese patient was homozygote for a previously known Spanish TMD mutation. This TMD mutation seems to cause a more severe TMD rather than LGMD2J when homozygous. One Finnish patient had FINmaj mutation combined with a novel missense mutation in the A-band region of titin causing a new phenotype completely different from previously described titinopathies. Our results further expand the complexity of muscular dystrophies caused by TTN mutations and suggest that the coexistence of second mutations may constitute a more common general mechanism of phenotype variability in muscular dystrophies. Several patients with previously reported titin gene (TTN) mutations causing tibial muscular dystrophy (TMD) have more complex, severe or unusual phenotypes. This study aimed to clarify the molecular cause of these variant phenotypes in proband patients of six European families. Clinical, histopathological and muscle imaging data of patients and family members was reanalyzed and muscle biopsies of the patients were studied by titin Western blotting and RT-PCR. Western blotting showed a more pronounced C-terminal titin abnormality than expected for heterozygous mutants in all six probands, suggesting the existence of additional TTN mutations. RT-PCR indicated unequal mRNA expression of the two TTN alleles in biopsies of four patients, two of them with an LGMD2J phenotype. TTN was analyzed by Sanger sequencing from genomic DNA of the patients. Three patients proved to be compound heterozygotes with novel TTN frameshift mutations combined with previously reported TMD mutations. One LGMD2J patient was heterozygous for the FINmaj TMD mutation with a likely second yet undiscovered mutation. The unequal expression levels of TTN transcripts in these four probands suggested that the expression of the frameshifted allele was severely reduced, probably through nonsense-mediated decay, explaining the observed more severe phenotypes. One Portuguese patient was homozygote for a previously known Spanish TMD mutation. This TMD mutation seems to cause a more severe TMD rather than LGMD2J when homozygous. One Finnish patient had FINmaj mutation combined with a novel missense mutation in the A-band region of titin causing a new phenotype completely different from previously described titinopathies. Our results further expand the complexity of muscular dystrophies caused by TTN mutations and suggest that the coexistence of second mutations may constitute a more common general mechanism of phenotype variability in muscular dystrophies.