A novel recessive TTN founder variant is a common cause of distal myopathy in the Serbian population

Abstract

Variants in the TTN gene have been associated with distal myopathies and other distinctive phenotypes involving skeletal and cardiac muscle. Through whole-exome sequencing we identified a novel stop-gain variant (c.107635C>T, p.(Gln35879Ter)) in the TTN gene, coding a part of the M-line of titin, in 14 patients with autosomal recessive distal myopathy and Serbian ancestry. All patients share a common 1 Mb core haplotype associated with c.107635C>T, suggesting a founder variant. In compound heterozygotes, nine other TTN variants were identified: four stop-gain, three frameshift, one missense and one splice donor variant. Patients homozygous for the common variant did not show significant clinical differences to the compound heterozygous patients. The clinical presentation of all patients was an adult onset distal myopathy with predominant lower limb involvement. In addition, most patients had normal to mildly elevated serum creatine kinase levels, myopathic electromyograms, normal cardiologic and respiratory tests and muscle pathology consistent with a dystrophic process. In this study, we describe a distinct phenotype for patients with distal myopathy associated with novel recessive TTN variants including a Serbian founder variant. Our results expand the phenotypic and genetic spectrum of titinopathies and will facilitate the diagnosis of this condition in patients of Serbian origin.

Figure 1

Segregation analysis of TTN variants in the family of patient no. 01. Both unaffected parents carry one of the variants identified in the patient, which is in accordance with an autosomal recessive mode of inheritance. The unaffected brother is a carrier of the wild-type alleles. The pedigree symbol for the patient is shown in black.

Figure 2

Identification of the core founder haplotype. Haplotypes segregating with the common c.107635C>T variant defined by genome-wide SNP array data and microsatellite markers. All analysed chromosomes carrying the c.107635C>T variant (in orange) share the 1 Mb core haplotype, between rs334613 and rs7591863 (in grey). SNPs indicating a breakpoint are shown in red. Genotypes that could not be fully phased are shown in italics. Genomic locations are from the Human Genome Assembly GRCh37. Microsatellite allele size is shown in bp. First, last and every tenth SNP are shown.

Figure 3

Lower limb involvement in patients with TTN variants. Muscle atrophy is most pronounced in the anterior compartment of the lower legs, but can also be seen in the posterior compartment of the lower legs and in the hamstring muscles. The two upper images show the legs of patient no. 01 and the lower images the legs of patient no. 11.

Figure 4

Histological findings in muscles biopsies of patients with TTN variants. Haematoxylin and eosin staining of muscle tissue from patient nos 01, 10 and 14. Biopsies showed an increase in internal nuclei (a), dystrophic changes with significant fibrous and fat tissue replacement, but without predominance of muscle fibre types (b). For comparison, the biopsy of the youngest patient shows minor fibre size variation and an increase in connective tissue only (c).

Figure 5

Thigh and calf muscles of six patients with titinopathy assessed by T1w MRI. For all patients one axial image of the lower legs and two images of the thigh muscles are shown. (a) In patient no. 01, the hamstring muscles were severely affected (asterisk) and there was notable asymmetry in pathology of the calf muscles (arrow). (b) Patient no. 03. showed both hamstring (asterisk) and femoral quadriceps muscle involvement (arrow). Both anterior and posterior compartments of the lower legs were asymmetrically affected. (c) There was marked atrophy in an obese patient. (d) In patient no. 09, the semimembranosus muscle was mildly affected (arrow), while in the lower legs the tibialis anterior, extensor digitorum longus and peroneus muscles were severely affected (asterisk) and the posterior compartment spared. (e) Patient no.13 showed a typical pattern of affected hamstring (asterisk) and tibialis anterior muscles (arrow). (f) The youngest patient, no. 14, with short disease course and normal MRI of muscles.