Novel TRIM32 mutation in sarcotubular myopathy

Abstract

Tripartite motif-containing protein 32 (TRIM32) is a member of the TRIM ubiquitin E3 ligases which ubiquitinates different substrates in muscle including sarcomeric proteins. Mutations in TRIM32 are associated with Limb-Girdle Muscular Dystrophy 2H. In a 66 old woman with disto-proximal myopathy, we identified a novel homozygous mutation of TRIM32 gene c.1781G > A (p. Ser594Asn) localised in the c-terminus NHL domain. Mutations of this domain have been also associated to Sarcotubular Myopathy (STM), a form of distal myopathy with peculiar features in muscle biopsy, now considered in the spectrum of LGMD2H. Muscle biopsy revealed severe abnormalities of the myofibrillar network with core like areas, lobulated fibres, whorled fibres and multiple vacuoles. Desmin and Myotilin stainings also pointed to accumulation as in Myofibrillar Myopathy. This report further confirms that STM and LGMD2H represent the same disorder and suggests to consider TRIM32 mutations in the genetic diagnosis of Sarcotubular Myopathy and Myofibrillar Myopathy.

Keywords: LGMD2H; TRIM32; desmin; myotilin; sarcotubular myopathy; spheroids bodies.

Figure 1.

Muscle biopsy. a-c) Representative images of H&E (a), NADH (b) and COX (c) stainings from patient muscle biopsy are shown. Lobulated fibres, whorled fibres and multiple vacuoles containing amorphous material are evident. In small boxes pictures from a normal control biopsy with the same staining are presented for comparison.

Figure 2.

Muscle MRI. Muscle MRI of lower limbs showed a severe involvement of adductors longus, magno and brevis (Score 4), glutei and tight posterior muscles (Score 3).

Figure 3.

TRIM32 and its substrates. a) TRIM32 modeling of WT and mutated protein was performed YASARA. Mutation p.Ser594Asn alters specifically the correct conformation of the NHL domain. b-c) Desmin and Myotilin stainings pointed out accumulation of these proteins within the muscle fibers. In small boxes pictures from a normal control biopsy immunofluorescence with same antibody d) TRIM32 protein levels were analyzed by Western blot. In the patient, the amount of TRIM32 protein in muscle was only slightly reduced compared to control