Alessandra Ruggieri

Glycogen hyperphosphorylation underlies lafora body formation

Glycogen, the largest cytosolic macromolecule, acquires solubility, essential to its function, through extreme branching. Lafora bodies are aggregates of polyglucosan, a long, linear, poorly branched, and insoluble form of glycogen. Lafora bodies occupy vast numbers of neuronal dendrites and perikarya in Lafora disease in time‐dependent fashion, leading to intractable and fatal progressive myoclonus epilepsy. Lafora disease is caused by deficiency of either the laforin glycogen phosphatase or the malin E3 ubiquitin ligase. The 2 leading hypotheses of Lafora body formation are: (1) increased glycogen synthase activity extends glycogen strands too rapidly to allow adequate branching, resulting in polyglucosans; and (2) increased glycogen phosphate leads to glycogen conformational change, unfolding, precipitation, and conversion to polyglucosan. Recently, it was shown that in the laforin phosphatase‐deficient form of Lafora disease, there is no increase in glycogen synthase, but there is a dramatic increase in glycogen phosphate, with subsequent conversion of glycogen to polyglucosan. Here, we determine whether Lafora bodies in the malin ubiquitin ligase‐deficient form of the disease are due to increased glycogen synthase or increased glycogen phosphate.

VMA21 deficiency prevents vacuolar ATPase assembly and causes autophagic vacuolar myopathy

X-linked Myopathy with Excessive Autophagy (XMEA) is a childhood onset disease characterized by progressive vacuolation and atrophy of skeletal muscle. We show that XMEA is caused by hypomorphic alleles of the VMA21 gene, that VMA21 is the diverged human ortholog of the yeast Vma21p protein, and that like Vma21p, VMA21 is an essential assembly chaperone of the vacuolar ATPase (V-ATPase), the principal mammalian proton pump complex. Decreased VMA21 raises lysosomal pH which reduces lysosomal degradative ability and blocks autophagy. This reduces cellular free amino acids which leads to downregulation of the mTORC1 pathway, and consequent increased macroautophagy resulting in proliferation of large and ineffective autolysosomes that engulf sections of cytoplasm, merge, and vacuolate the cell. Our results uncover a novel mechanism of disease, namely macroautophagic overcompensation leading to cell vacuolation and tissue atrophy.

Increased Laforin and Laforin Binding to Glycogen Underlie Lafora Body Formation in Malin-deficient Lafora Disease

Background: Laforin deficiency causes glycogen hyperphosphorylation, which converts glycogen to aggregate-prone poorly branched polyglucosans. Malin deficiency also causes polyglucosans. Results: Malin deficiency increases total and glycogen-associated laforin, which renders glycogen aggregate-prone and poorly branched. Conclusion: Malin deficiency causes polyglucosans through increased laforin. Significance: Phosphate and laforin (and perhaps other proteins) must be cleared to enable normal glycogen construction and to prevent fatal polyglucosan disease. The solubility of glycogen, essential to its metabolism, is a property of its shape, a sphere generated through extensive branching during synthesis. Lafora disease (LD) is a severe teenage-onset neurodegenerative epilepsy and results from multiorgan accumulations, termed Lafora bodies (LB), of abnormally structured aggregation-prone and digestion-resistant glycogen. LD is caused by loss-of-function mutations in the EPM2A or EPM2B gene, encoding the interacting laforin phosphatase and malin E3 ubiquitin ligase enzymes, respectively. The substrate and function of malin are unknown; an early counterintuitive observation in cell culture experiments that it targets laforin to proteasomal degradation was not pursued until now. The substrate and function of laforin have recently been elucidated. Laforin dephosphorylates glycogen during synthesis, without which phosphate ions interfere with and distort glycogen construction, leading to LB. We hypothesized that laforin in excess or not removed following its action on glycogen also interferes with glycogen formation. We show in malin-deficient mice that the absence of malin results in massively increased laforin preceding the appearance of LB and that laforin gradually accumulates in glycogen, which corresponds to progressive LB generation. We show that increasing the amounts of laforin in cell culture causes LB formation and that this occurs only with glycogen binding-competent laforin. In summary, malin deficiency causes increased laforin, increased laforin binding to glycogen, and LB formation. Furthermore, increased levels of laforin, when it can bind glycogen, causes LB. We conclude that malin functions to regulate laforin and that malin deficiency at least in part causes LB and LD through increased laforin binding to glycogen.

CAOS—Episodic Cerebellar Ataxia, Areflexia, Optic Atrophy, and Sensorineural Hearing Loss: A Third Allelic Disorder of the ATP1A3 Gene

We describe the molecular basis of a distinctive syndrome characterized by infantile stress-induced episodic weakness, ataxia, and sensorineural hearing loss, with permanent areflexia and optic nerve pallor. Whole exome sequencing identified a deleterious heterozygous c.2452 G>A, p.(E818K) variant in the ATP1A3 gene and structural analysis predicted its protein-destabilizing effect. This variant has not been reported in context with rapid-onset dystonia parkinsonism and alternating hemiplegia of childhood, the 2 main diseases associated with ATP1A3. The clinical presentation in the family described here differs categorically from these diseases in age of onset, clinical course, cerebellar over extrapyramidal movement disorder predominance, and peripheral nervous system involvement. While this paper was in review, a highly resembling phenotype was reported in additional patients carrying the same c.2452 G>A variant. Our findings substantiate this variant as the cause of a unique inherited autosomal dominant neurologic syndrome that constitutes a third allelic disease of the ATP1A3 gene.

The genetic basis of undiagnosed muscular dystrophies and myopathies Results from 504 patients

Objective: To apply next-generation sequencing (NGS) for the investigation of the genetic basis of undiagnosed muscular dystrophies and myopathies in a very large cohort of patients. Methods: We applied an NGS-based platform named MotorPlex to our diagnostic workflow to test muscle disease genes with a high sensitivity and specificity for small DNA variants. We analyzed 504 undiagnosed patients mostly referred as being affected by limb-girdle muscular dystrophy or congenital myopathy. Results: MotorPlex provided a complete molecular diagnosis in 218 cases (43.3%). A further 160 patients (31.7%) showed as yet unproven candidate variants. Pathogenic variants were found in 47 of 93 genes, and in more than 30% of cases, the phenotype was nonconventional, broadening the spectrum of disease presentation in at least 10 genes. Conclusions: Our large DNA study of patients with undiagnosed myopathy is an example of the ongoing revolution in molecular diagnostics, highlighting the advantages in using NGS as a first-tier approach for heterogeneous genetic conditions.

Fukutin gene mutations in an Italian patient with early onset muscular dystrophy but no central nervous system involvement

Hypoglycosylation of α‐dystroglycan characterizes a subgroup of muscular dystrophies of variable severity, including Fukuyama congenital muscular dystrophy. We found fukutin gene mutations in a 4.5‐year‐old Italian patient, with reduced α‐dystroglycan expression, dystrophic features on muscle biopsy, hypotonia since birth, mild myopathy, but no brain involvement. Mutations in the fukutin gene can be associated with much milder phenotypes than classical Fukuyama congenital muscular dystrophy, and, although rare, can occur in non‐Japanese. Muscle Nerve, 2009

DNAJB6 Myopathies: Focused Review on an Emerging and Expanding Group of Myopathies

Mutations in the DNAJB6 gene have been associated with the autosomal dominant limb girdle muscular dystrophy type 1D (LGMD1D), a disorder characterized by abnormal protein aggregates and rimmed vacuoles in muscle fibers. DNAJB6 is a ubiquitously expressed Hsp40 co-chaperone characterized by a J domain that specifies Hsp70 functions in the cellular environment. DNAJB6 is also a potent inhibitor of expanded polyglutamine (polyQ) aggregation preventing aggregate toxicity in cells. In DNAJB6-mutated patients this anti-aggregation property is significantly reduced, albeit not completely lost. To elucidate the pathogenetic mechanisms underlying the DNAJB6-related myopathy, animal models have been created showing that, indeed, conditional muscular expression of a DNAJB6 mutant in the mouse causes a LGMD1D myofibrillary muscle tissue phenotype. Both mutations and phenotypes reported until recently were rather homogeneous, being exclusively missense mutations of a few amino acids of the protein G/F domain, and with a phenotype characterized by adult-onset slowly progressive muscular dystrophy predominantly affecting proximal muscles. Lately, several novel mutations and new phenotypes of DNAJB6 have been described. These mutations once more affect the G/F domain of DNAJB6 with missense changes and a splice site mutation; and the phenotypes include childhood onset and distal involvement of muscles, or childhood-onset LGMD1D with loss of ambulation in early adulthood and respiratory involvement. Thus, the spectrum of DNAJB6-related phenotypes is widening. Although our knowledge about the role of DNAJB6 in the pathogenesis of muscle diseases has made great progression, several questions remain unsolved, including why a ubiquitous protein affects only, or predominantly, skeletal muscle; why only the G/F domain is involved; and what is the possible role of the DNAJB6a isoform. Clarification of these issues will provide clues to implement possible therapeutic strategies for DNAJB6-related myopathies.

A CASQ1 founder mutation in three Italian families with protein aggregate myopathy and hyperCKaemia

Background Protein aggregate myopathies are increasingly recognised conditions characterised by a surplus of endogenous proteins. The molecular and mutational background for many protein aggregate myopathies has been clarified with the discovery of several underlying mutations. Familial idiopathic hyperCKaemia is a benign genetically heterogeneous condition with autosomal dominant features in a high proportion of cases. Methods In 10 patients from three Italian families with autosomal dominant benign vacuolar myopathy and hyperCKaemia, we performed linkage analysis and exome sequencing as well as morphological and biochemical investigations. Results and conclusions We show, by Sanger and exome sequencing, that the protein aggregate myopathy with benign evolution and muscle inclusions composed of excess CASQ1, affecting three Italian families, is due to the D244G heterozygous missense mutation in the CASQ1 gene. Investigation of microsatellite markers revealed a common haplotype in the three families indicating consanguinity and a founder effect. Results from immunocytochemistry, electron microscopy, biochemistry and transfected cell line investigations contribute to our understanding of pathogenetic mechanisms underlining this defect. The mutation is common to other Italian patients and is likely to share a founder effect with them. HyperCKaemia in the CASQ1-related myopathy is common and sometimes the sole overt manifestation. It is likely that CASQ1 mutations may remain undiagnosed if a muscle biopsy is not performed, and the condition could be more common than supposed.

Late adult-onset of X-linked myopathy with excessive autophagy

Introduction: X‐linked myopathy with excessive autophagy (XMEA) is characterized by autophagic vacuoles with sarcolemmal features. Mutations in VMA21 result in insufficient lysosome acidification, causing progressive proximal weakness with onset before age 20 years and loss of ambulation by middle age. Methods: We describe a patient with onset of slowly progressive proximal weakness of the lower limbs after age 50, who maintains ambulation with the assistance of a cane at age 71. Results: Muscle biopsy at age 66 showed complex muscle fiber splitting, internalized capillaries, and vacuolar changes characteristic of autophagic vacuolar myopathy. Vacuoles stained positive for sarcolemmal proteins, LAMP2, and complement C5b‐9. Ultrastructural evaluation further revealed basal lamina reduplication and extensive autophagosome extrusion. Sanger sequencing identified a known pathologic splice site mutation in VMA21 (c.164–7T>G). Conclusions: This case expands the clinical phenotype of XMEA and suggests VMA21 sequencing be considered in evaluating men with LAMP2‐positive autophagic vacuolar myopathy. Muscle Nerve 50: 138–144, 2014

Congenital autophagic vacuolar myopathy is allelic to X-linked myopathy with excessive autophagy

X-linked myopathy with excessive autophagy (XMEA) is characterized by weakness and wasting primarily of the proximal muscles of the lower extremities. Onset is usually after age 5 and progression is extremely slow with ambulation maintained well into the 50s. The heart, CNS, peripheral nervous system, and other organs are clinically spared. Pathology reveals large autophagic vacuoles enclosing incompletely degraded cytoplasmic components, which translocate to the myofiber surface and extrude their contents, forming a field of cell debris between multiplied layers of basal lamina.1 XMEA is caused by mutations of the VMA21 gene, which reduce, but do not eliminate, expression of the chief assembly chaperone (VMA21) of the main proton pump (V-ATPase [vacuolar-type H+–adenosine triphosphatase]) of all mammalian cells.2