Mechanistic insights into the homo-dimerization of HOIL-1L and SHARPIN

HOIL-1L and SHARPIN are two essential regulatory subunits of the linear ubiquitin chain assembly complex (LUBAC), which is the only known E3 ligase complex generating linear ubiquitin chains. In addition to their LUBAC-dependent functions, HOIL-1L and SHARPIN alone play crucial roles in many LUBAC-independent cellular processes. Importantly, deficiency of HOIL-1L or SHARPIN leads to severe disorders in humans or mice. However, the mechanistic bases underlying the multi-functions of HOIL-1L and SHARPIN are still largely unknown. Here, we uncover that HOIL-1L and SHARPIN alone can form homo-dimers through their LTM motifs. We solve two crystal structures of the dimeric LTM motifs of HOIL-1L and SHARPIN, which not only elucidate the detailed molecular mechanism underpinning the dimer formations of HOIL-1L and SHARPIN, but also reveal a general mode shared by the LTM motifs of HOIL-1L and SHARPIN for forming homo-dimer or hetero-dimer. Furthermore, we elucidate that the polyglucosan body myopathy-associated HOIL-1L A18P mutation disturbs the structural folding of HOIL-1L LTM, and disrupts the dimer formation of HOIL-1L. In summary, our study provides mechanistic insights into the homo-dimerization of HOIL-1L and SHARPIN mediated by their LTM motifs, and expands our understandings of the multi-functions of HOIL-1L and SHARPIN as well as the etiology of relevant human disease caused by defective HOIL-1L.

Glycogen storage disease type IV without detectable polyglucosan bodies: importance of broad gene panels

Glycogen storage disease type IV (GSD IV) is caused by mutations in the glycogen branching enzyme 1 (GBE1) gene and is characterized by accumulation of polyglucosan bodies in liver, muscle and other tissues. We report three cases with neuromuscular forms of GSD IV, none of whom had polyglucosan bodies on muscle biopsy. The first case had no neonatal problems and presented with delayed walking. The other cases presented at birth: one with arthrogryposis, hypotonia, and respiratory distress, the other with talipes and feeding problems. All developed a similar pattern of axial weakness, proximal upper limb weakness and scapular winging, and much milder proximal lower limb weakness. Our cases expand the phenotypic spectrum of neuromuscular GSD IV, highlight that congenital myopathy and limb girdle weakness can be caused by mutations in GBE1, and emphasize that GSD IV should be considered even in the absence of characteristic polyglucosan bodies on muscle biopsy.

Polyglucosan body myopathy 1 may cause cognitive impairment: a case report from China

Background

Polyglucosan body myopathy 1 (PGBM1) is a type of glycogen storage disease that can cause skeletal muscle myopathy and cardiomyopathy with or without immunodeficiency due to a pathogenic mutation in the RBCK1 gene. PGBM1 has been reported in only 14 European and American families, and no cognitive impairment phenotype was reported. Its prevalence in Asia is unknown.

Case presentation:

We report a Chinese boy with teenage onset of skeletal muscle myopathy and mild cognitive impairment. Whole-exome sequencing analysis identified a homozygous missense mutation in RBCK1 (c.1411G > A:p.Glu471Lys). A muscle biopsy indicated the accumulation of periodic acid-Schiff-positive material, which could be ubiquitinated by immunohistochemistry with an anti-ubiquitin antibody. In skeletal muscle tissue, HOIL-1 and HOIP protein levels were lower than those in the control, confirming the phenotype of an RBCK1 mutation. MRI revealed abnormal cerebral white matter signals. Immune system and cardiac examination found no abnormalities. The patient was diagnosed with PGBM1 with no effective treatment.

Conclusions

This case from China with a novel homozygous missense mutation in RBCK1 extends the phenotypic spectrum and geographical distribution of PGBM 1, which may cause cerebral white matter changes and cognitive impairment.

GYG1 causing progressive limb girdle myopathy with onset during teenage years (polyglucosan body myopathy 2)

An 84-year-old lady with slowly progressive limb and axial muscle weakness with onset in her teens was referred for genetic investigations. Targeted next generation sequencing (NGS) revealed a homozygous mutation GYG1 in exon5:c.487delG:p.D163fs, confirming the diagnosis of Polyglucosan Body Myopathy 2 (PGBM2). Retrospective review of muscle pathology revealed a florid vacuolar myopathy with histochemical and ultrastructural features consistent with a polyglucosan storage myopathy. No cardiac symptoms were reported. Our case is consistent with the core phenotype of GYG1-related PGBM2 apart from an early onset of weakness without cardiac symptoms. The presence of α-amylase resistant PAS-positive material in skeletal muscle biopsy of patients with slowly progressive limb girdle muscle weakness should prompt the search for GYG1 mutations. This case highlights the combined role of muscle pathology and NGS in the molecular resolution of patients with undiagnosed neuromuscular conditions.

Mutations outside the N-terminal part of RBCK1 may cause polyglucosan body myopathy with immunological dysfunction: expanding the genotype-phenotype spectrum

A subset of patients with polyglucosan body myopathy was found to have underlying mutations in the RBCK1 gene. Affected patients may display diverse symptoms ranging from skeletal muscular weakness, cardiomyopathy to chronic autoinflammation and immunodeficiency. It was suggested that the exact localization of the mutation within the gene might be responsible for the specific phenotype, with N-terminal mutations causing severe immunological dysfunction and mutations in the middle or C-terminal part leading to a myopathy phenotype. We report the clinical, immunological and genetic findings of two unrelated individuals suffering from a childhood-onset RBCK1-asscociated disease caused by the same homozygous truncating mutation (NM_031229.2:c.896_899del, p.Glu299Valfs*46) in the middle part of the RBCK1 gene. Our patients suffered from a myopathy with cardiac involvement, but in contrast to previous reports on mutations in this part of the gene, also displayed signs of autoinflammation and immunodeficiency. Our report suggests that RBCK1 mutations at locations that were previously thought to lack immunological features may also present with immunological dysfunction later in the disease course. This notably broadens the genotype–phenotype correlation of RBCK1-related polyglucosan body myopathy.