• NEUROMUSCULAR
• MYOPATHY

Polyglucosan bodies (PBs) are deposits of amylopectin-like polysaccharides, detected in muscles of patients affected with glycogenoses-like branching enzyme (GBE) and phosphofructokinase deficiencies.1

Recently, mutations in RBCK1 have been associated with a skeletal PBs myopathy,2 as well as mutations in GYG1, encoding for glycogenyn-1.3 All these conditions have autosomal recessive inheritance. Only seven unrelated cases with mutations in GYG1 have been reported, characterised by variable age at onset (from childhood to 7th decade), mainly proximal weakness, normal creatine kinases (CKs) levels and myopathic electromyography (EMG).3

We have been, for the past 35 years, following two affected sisters now aged 71 and 64 years (P.IV-5 and P.IV-10) (see online supplementary figure S1A). Their parents were both healthy first cousins from a little village in the Italian Alps. Both sisters had normal motor development.

At the age of 30 years, P.IV-5 showed weakness in arm abduction, more prominent on the right side. The disease course was slowly progressive over the decades, with early involvement of proximal limb muscles. Waddling gait with hyperlordosis was first observed when she was in her late 40s. She started to use a wheelchair outdoors at the age of 59 years. In the following years she became completely a wheelchair user and dependent for her daily activities. Her last examination, at the age of 71 years, showed facial weakness, which was initially absent, with muscle atrophy and hypotonia, absent deep tendon reflexes (DTR) and severely compromised muscle power (Medical Research Council (MRC) score: neck flexors 3, neck extensors 2, shoulder abductors 0, forearm extensors/flexors 0, wrist extensors 0, wrist flexors and hand grip 2, hip flexors/extensors 0, thigh extensors/flexors 2, foot plantar flexors and foot evertors 3, and foot dorsal flexors 2).

At the age of 53 years, P.IV-10 began feeling fatigue while climbing stairs and developed impaired, mainly right-sided, arm abduction. Over the years, she became unable to run and could only climb stairs with double support. Her last examination, at the age of 64 years, showed scapular winging, positive Gowers’ sign, waddling-type gait, preserved facial muscles and reduced DTR. At manual testing, muscle power was mildly impaired at both limb girdles (MRC 5-), but severely compromised at shoulder abduction (MRC 0).

Both sisters had a myopathic EMG, normal CK levels and no pulmonary abnormalities (normal spirometry and nocturnal oximetry). They underwent echocardiogram, Holter ECG, ECG and cardiac valuation over the previous year, with no evidence of cardiac involvement.

Their skeletal muscle biopsies showed a nearly identical vacuolar myopathy (see online supplementary figure S1). P.IV-5 underwent left and right biceps muscle biopsies, at the age of 36 and 50 years, respectively, which did not show any significant modifications. PBs were detected in vacuoles in up to 50% of muscle fibres in P.IV-5 and 40% in P.IV-10. These deposits were localised in both subsarcolemmal and intracytoplasmic areas, stained faintly with Gömöri trichrome and showed a diffuse periodic acid-Schiff positivity with overall resistance to diastase reaction (see online supplementary figure S1B–G). Electron microscopy showed that many vacuoles were filled with mostly homogeneous or slightly granular filamentous inclusions, consistent with PBs (see online supplementary figure S1H,I), intracytoplasmic collections of free glycogen, as well as a few autophagic vacuoles.

Biochemical analysis of enzymes involved in glycolysis/glycogen metabolism did not reveal any deficiencies (see online supplementary table S1). Glycogen spectrum was normal. GBE1 and RBCK1 were ruled-out. GYG1 sequencing revealed that both sisters were homozygous for c.143+3G>C mutation.

The sisters underwent shoulder girdle/arm and hip girdle/thigh muscle MRI (figure 1): deltoids and glutaei were the most affected muscles in P.IV-10, all muscles being almost substituted in P.IV-5; pseudohypertrophy of gracilis, sartorius and rectus femoris was evident in both patients.

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Figure 1

Muscle MRI from P.IV-5, performed at the age of 71 years, showing overall fatty replacement of muscles of hip (A), thigh (B and C), calf (D), and shoulder and arm (E): gracilis, sartorius and rectus femoris were partially spared and pseudohypertrophic at thigh level (B and C). Medial head of gastrocnemius and both tibialis anterior and posterior showed very partial sparing at the calf (D); fibroadipose substitution appears heterogeneous inside gastrocnemii (D). Muscle MRI from P.IV-10, performed at the age of 64 years, showing a severe involvement of glutaei at hip scanning (F) and of deltoids at shoulder level (I). Perineal muscles were relatively preserved (F). At thigh scanning (G and H) of P.IV-10, adductor longus, long head of biceps femoris, vastus intermedius and lateralis were involved to a lesser extent when compared with glutaei and deltoids; gracilis, sartorius, rectus femoris were pseudohypertrophic in P.IV-10, as observed in P.IV-5. Deltoids (I), semimembranosus and biceps (G and H), were only partially infiltrated. MRI protocol description is fully described in online supplementary materials.

We report two siblings affected with a PBs myopathy due to homozygous c.143+3G>C mutation in GYG1. This mutation affects the donor splice site in intron 2, causing the skipping of exon 2, and creates a premature stop codon thus causing lack of protein. This variant was detected in 4/7 cases of different ethnicities, two homozygous with disease onset in childhood/adolescence, and two compound heterozygous with onset in the fifth–seventh decade.3 Our findings confirm that this mutation is common in the Italian population and is associated with adult onset, even though homozygous.

Pelvic girdle weakness and atrophy were reported in all patients with mutations in GYG1, half of them presenting also shoulder girdle involvement.3 Our cases confirm that early and asymmetric weakness in arm abduction is common. Of note, this asymmetric motor syndrome overlaps one previously described in a middle-aged woman with a genetically unsolved PBs myopathy.4 Disease progression in our siblings showed variable severity: from mild weakness to wheelchair-dependency, the latter phenotype representing the most severe spectrum reported so far. Cardiac and respiratory muscles are spared.3 Muscle morphology shows PBs as the most striking finding.3

Nevertheless, it is unknown how lack of glycogenin-1 causes PB aggregation in muscle: these amylase-resistant polysaccharides are also observed in GBE deficiency, suggesting an impairment in glycogen synthesis process.

Among PB myopathies, muscle MRI was reported only in one congenital case with mutations in GBE1 with evidence of ubiquitous fibroadipose replacement.5 We observed a correlation between clinical severity and degree of fibroadipose substitution, with early involvement of deltoids and glutaei. Upper girdle muscle MRI has been investigated in a few neuromuscular conditions; our siblings’ pattern does not resemble that of other diseases affecting arm abduction, including facioscapulohumeral muscular dystrophy, in which trapezius and serratus anterior are mostly affected and are the earliest to be involved, with deltoids being spared.6 MRI pattern at lower girdle muscles resembles that observed in late-onset Pompe disease, which, however, shows prominent subscapularis and anterior serratus replacement in the upper girdle.7

In conclusion, our findings demonstrate that a myopathy with PB and early asymmetric impairment of arm abduction is consistent with mutations in GYG1. We outline an unknown variability of clinical phenotype within the same genetic background. At muscle MRI, early replacement of deltoids and glutaei muscles seems to be the hallmark, at least for the c.143+3G>C mutation, an observation to be validated by the analysis of a larger cohort with different GYG1 mutations.