Long term follow-up and further molecular and histopathological studies in the LGMD1F sporadic TNPO3-mutated patient
KeywordsTNPO3 Transportin 3 LGMD1F
Limb girdle muscular dystrophies (LGMD) are a large group of muscular disorders, with progressive shoulder and pelvic muscle weakness as the most relevant feature. They are classified as autosomal dominant (LGMD1) and autosomal recessive (LGMD2) forms. Up to now, eight genetically defined LGMD1 subtypes (LGMD1 A-H) have been identified [1, 9].
In 2001, the clinical and morphological phenotype of a novel form of LGMD type 1, affecting 32 subjects in a large Spanish family, was described . According to subsequent molecular studies, the disease was demonstrated to be linked to the novel chromosomal locus 7q32.1–32.2. This genetically distinct form of autosomal dominant-LGMD was classified as LGMD1F  (OMIM #608423). Recently, using a whole genome sequencing approach, the causative mutation of the LGMD1F was identified in the termination codon of TNPO3, the gene coding for transportin 3. Molecular results at DNA, RNA and protein levels as well as morphological findings supported the pathogenic role of this mutation in LGMD1F . Investigation by next-generation sequencing in further 4 members of the Spanish family, originating from Italy, confirmed the mutation in TNPO3 . Up to now, beside this Italo-Spanish family, only one sporadic LGMD patient has been identified with a heterozygous point mutation in the TNPO3 gene .
In this patient we now report the long term clinical and radiological follow-up, morphological and immunochemical studies on patient muscle biopsy, and molecular studies by Real Time PCR and by cell transfection with the mutant cDNA.
The patient was investigated at our institute when he was 38 years old because of slowly progressive difficulty in walking and climbing stairs presenting since the age of 35 years. No familial occurrence of neuromuscular disorders or consanguinity was referred.
On neurological examination scapular, anterior and posterior thigh muscle atrophy were observed. Assessment of muscle strength using the British Medical Research Council (MRC) scale, showed weakness of shoulder girdle muscles (with arm flexion and abduction possible against gravity until 90°), without scapular winging, inferior trapezius (2/5), arm extensors (2/5), elbow flexors (3/5), hip flexors (3/5), hip extensors (2/5), knee flexors and extensors (4/5), dorsal foot extensors (4/5). No cranial nerve involvement was observed. Joint contractures and skeletal deformities were not detected. The patient presented a waddling gait with an increased lumbar lordosis and was unable to get up from the floor. Functional ability of upper limbs was 3 according to Brooke scale (from 1: normal; to 6: no function for upper extremity)  and lower limb function was 3 according to Vignos scale (1: able to climb stairs without help; to 10: bedridden for lower limb function) . CK was only slightly increased (253 UI/l). EMG showed myopathic findings in all tested muscles with fibrillation potentials and positive sharp waves. On muscle CT scan, moderate fatty changes were found in bilateral quadriceps and hamstrings and medial gastrocnemius. Respiratory and cardiac functions were normal. Symptoms progressively worsened in the following years, loosing the ability to climb stairs at the age of 45.
A muscle biopsy from the left quadriceps, taken at age 38, displayed fibre size variability, a few central nuclei, scattered degenerative fibres (Fig. 2), few cytochrome oxidase-negative fibres, and ragged red appearing fibres that, although rare (about 1%) were above the expected number in a 38 years old man. Immunostaining for dystrophin, sarcoglycans, caveolin 3, and alpha-dystroglycan, was normal, as well as dysferlin and calpain 3 immunoblotting. Respiratory chain activity and mitochondrial DNA analysis by Southern blot were normal.
By next generation sequencing analysis, a heterozygous G > A transition (c.G2453A) in exon 20 of the TNPO3 gene was found (reported in exon 21 in the original paper) . The G > A point mutation changes the arginine in position 818 with a glutamine in a highly conserved residue, predicted to be damaging by all the used bioinformatic tools. This mutation is now listed in dbSNP (rs587777431) and it is present in gnomAD (The Genome Aggregation Database) with a population frequency of 0.00004215. This variant was not found in the two healthy sisters.
After publication of the original report , we extensively reassessed muscle biopsy, clinical features and radiologic findings in the patient and performed transfection studies to characterize the mutation.
Western blot analysis of muscle homogenate using antibodies against transportin 3 showed that the band corresponding to the protein was of greatly reduced intensity in the patient compared to a control subject (Fig. 2).
A Real Time PCR assay, carried out to quantify transcript levels of TNPO3 in the patient, revealed a reduction of more than 50% in the patient mRNA compared to controls (Fig. 2).
LGMD1F, reported so far only in the large Italo-Spanish kindred, is clinically characterized by pelvic and shoulder girdle weakness, with a wide variability in the age at onset, spanning from 1 to 58 years. Individuals with juvenile onset presented severe and rapid progression of the disease involving proximal and distal limb muscles and leading to early loss of autonomous walking. Patients with adult onset disease manifested a slow progression of symptoms and persistent ability to walk. Other aspects of the clinical phenotype considered as specific indicators of LGMD1F are dysphagia, arachnodactyly with or without finger contractures, and dysarthria . Our sporadic case is similar to the patients of the Italo-Spanish family with adult onset of symptoms and moderate progression of weakness. Differently from the family patients, he does not manifest any of the adjunctive symptoms described and suggested as specific of LGMD1F. Some differences were also observed at muscle MRI. Compared to most affected patients reported by Melià and colleagues , our patient showed a more diffuse involvement of thigh muscles, with relative and selective sparing of gracilis and rectus femoris, and less severe involvement of lower leg muscles.
Like in the Italo-Spanish family, myofibrillar abnormalities, although minor, as well as mitochondrial abnormalities [3, 4], were observed in muscle biopsies of our patient. No progression of histopathological features was observed in the patient muscle, however the interval between the two biopsies was only two years; furthermore being both biopsies from long time ago, the histological features cannot be correlated to the clinical features observed in the most recent clinical follow-up.
A possible mitochondrial dysfunction has been hypothesized in myofibrillar myopathies such as the desmin- or the filamin-mutated myopathies and mitochondrial abnormalities have been interpreted as a secondary phenomenon and an early histological sign [6, 12]. A role for transportin 3 in mitochondrial function cannot be ruled out and will need further studies and in patients with different TNPO3 mutations to shed light on its function in muscle disease. Transportin 3, being implicated in the translocation of splice regulators to the nucleoplasm and in pre-mRNA processing , could indeed play a role in the maturation of RNAs coding for mitochondrial or myofibrillar proteins.
The missense change in our patient, unlike the reported c.2771del, does not affect protein localization, but causes a reduction in TNPO3 transcript, likely due to messenger instability, and a consequent reduction of the protein. The affected residue R818 is in a helix region towards the C-terminus of the protein, the role of which is not precisely defined. The overall efficiency of transportin 3 is likely insufficient to mediate nuclear translocation of its ligands in our patient.
On the whole our study provides further histopathological, molecular and clinical insights on this still imprecise muscular dystrophy.
This work was supported by the Italian Ministry of Health. The EuroBioBank and Telethon Network of Genetic Biobanks (GTB18001) are gratefully acknowledged for providing biological samples.
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