Muscle pathology in Marinesco-Sjogren syndrome: a unique ultrastructural feature

Role of the HSP70 Co-Chaperone SIL1 in Health and Disease

Cell surface and secreted proteins provide essential functions for multicellular life. They enter the endoplasmic reticulum (ER) lumen co-translationally, where they mature and fold into their complex three-dimensional structures. The ER is populated with a host of molecular chaperones, associated co-factors, and enzymes that assist and stabilize folded states. Together, they ensure that nascent proteins mature properly or, if this process fails, target them for degradation. BiP, the ER HSP70 chaperone, interacts with unfolded client proteins in a nucleotide-dependent manner, which is tightly regulated by eight DnaJ-type proteins and two nucleotide exchange factors (NEFs), SIL1 and GRP170. Loss of SIL1′s function is the leading cause of Marinesco-Sjögren syndrome (MSS), an autosomal recessive, multisystem disorder. The development of animal models has provided insights into SIL1′s functions and MSS-associated pathologies. This review provides an in-depth update on the current understanding of the molecular mechanisms underlying SIL1′s NEF activity and its role in maintaining ER homeostasis and normal physiology. A precise understanding of the underlying molecular mechanisms associated with the loss of SIL1 may allow for the development of new pharmacological approaches to treat MSS.

In-depth phenotyping of lymphoblastoid cells suggests selective cellular vulnerability in Marinesco-Sjögren syndrome

SIL1 is a ubiquitous protein of the Endoplasmic Reticulum (ER) acting as a co-chaperone for the ER-resident chaperone, BiP. Recessive mutations of the corresponding gene lead to vulnerability of skeletal muscle and central nervous system in man (Marinesco-Sjögren syndrome; MSS) and mouse. However, it is still unclear how loss of ubiquitous SIL1 leads to selective vulnerability of the nervous system and skeletal muscle whereas other cells and organs are protected from clinical manifestations. In this study we aimed to disentangle proteins participating in selective vulnerability of SIL1-deficient cells and tissues: morphological examination of MSS patient-derived lymphoblastoid cells revealed altered organelle structures (ER, nucleus and mitochondria) thus showing subclinical vulnerability. To correlate structural perturbations with biochemical changes and to identify proteins potentially preventing phenotypical manifestation, proteomic studies have been carried out. Results of proteomic profiling are in line with the morphological findings and show affection of nuclear, mitochondrial and cytoskeletal proteins as well as of such responsible for cellular viability. Moreover, expression patterns of proteins known to be involved in neuromuscular disorders or in development and function of the nervous system were altered. Paradigmatic findings were confirmed by immunohistochemistry of splenic lymphocytes and the cerebellum of SIL1-deficient mice. Ataxin-10, identified with increased abundance in our proteome profile, is necessary for the neuronal survival but also controls muscle fiber apoptosis, thus declaring this protein as a plausible candidate for selective tissue vulnerability. Our combined results provide first insights into the molecular causes of selective cell and tissue vulnerability defining the MSS phenotype.

Ataxia

Ataxia is a disorder of balance and coordination resulted from dysfunctions involving cerebellum and its afferent and efferent connections. While a variety of disorders can cause secondary ataxias, the list of genetic causes of ataxias is growing longer. Genetic abnormalities may involve mitochondrial dysfunction, oxidative stress, abnormal mechanisms of DNA repair, possible protein misfolding, and abnormalities in cytoskeletal proteins. Few ataxias are fully treatable while hope for efficacious gene therapy and pharmacotherapy is emerging. A discussion of the ataxias is presented here with brief mention of acquired ataxias, and a greater focus on inherited ataxias.

Myopathy in Marinesco-Sjögren syndrome links endoplasmic reticulum chaperone dysfunction to nuclear envelope pathology

Marinesco-Sjögren syndrome (MSS) features cerebellar ataxia, mental retardation, cataracts, and progressive vacuolar myopathy with peculiar myonuclear alterations. Most MSS patients carry homozygous or compound heterozygous SIL1 mutations. SIL1 is a nucleotide exchange factor for the endoplasmic reticulum resident chaperone BiP which controls a plethora of essential processes in the endoplasmic reticulum. In this study we made use of the spontaneous Sil1 mouse mutant woozy to explore pathomechanisms leading to Sil1 deficiency-related skeletal muscle pathology. We found severe, progressive myopathy characterized by alterations of the sarcoplasmic reticulum, accumulation of autophagic vacuoles, mitochondrial changes, and prominent myonuclear pathology including nuclear envelope and nuclear lamina alterations. These abnormalities were remarkably similar to the myopathy in human patients with MSS. In particular, the presence of perinuclear membranous structures which have been reported as an ultrastructural hallmark of MSS-related myopathy could be confirmed in woozy muscles. We found that these structures are derived from the nuclear envelope and nuclear lamina and associate with proliferations of the sarcoplasmic reticulum. In line with impaired function of BiP secondary to loss of its nucleotide exchange factor Sil1, we observed activation of the unfolded protein response and the endoplasmic-reticulum-associated protein degradation-pathway. Despite initiation of the autophagy-lysosomal system, autophagic clearance was found ineffective which is in agreement with the formation of autophagic vacuoles. This report identifies woozy muscle as a faithful phenocopy of the MSS myopathy. Moreover, we provide a link between two well-established disease mechanisms in skeletal muscle, dysfunction of chaperones and nuclear envelope pathology.

Heterogeneity of Marinesco-Sjögren syndrome: report of two cases

Marinesco-Sjögren syndrome is an autosomal recessive, multiorgan disorder with cardinal features of cerebellar ataxia, congenital or early childhood cataracts, psychomotor retardation, myopathy, and short stature. Mutations in the SIL1 gene on chromosome 5q31 were demonstrated to cause Marinesco-Sjögren syndrome. We describe two Turkish patients with clinical characteristics of Marinesco-Sjögren syndrome, but without mutations in SIL1. These two patients also manifested cerebral white matter involvement in cranial imaging, which was previously described in Marinesco-Sjögren syndrome. Marinesco-Sjögren syndrome is genetically heterogeneous, and mutations of SIL1 are often not evident. Consequently, we presume that new genes for Marinesco-Sjögren syndrome await discovery. New genes hold the promise of furthering the mechanistic understanding of the condition, enabling clinically meaningful genetic classification schemes to be designed.

Differential diagnosis of congenital muscular dystrophies

Congenital muscular dystrophies (CMDs) are defined by signs of muscle weakness in the first 6 months of life with myopathic changes in muscle biopsy. The progress in the last decade has helped to make molecular and genetic diagnoses in the majority of patients fulfilling these criteria. In a number of patients a definite diagnosis cannot be reached and these individuals are often grouped together as "merosin positive" congenital muscular dystrophy. In the last 5 years, 25 patients referred for assessment as possible congenital muscular dystrophy have been found to have alternative diagnoses. This paper aims to highlight these conditions as the common differentials or more difficult to diagnoses to consider in patients presenting as CMD.

Myopathy is a prominent feature in Marinesco-Sjögren syndrome: A muscle computed tomography study

BACKGROUND

Marinesco-Sjögren syndrome (MSS) is an autosomal recessive multiorgan disorder showing clinical and genetic heterogeneity. The key features of MSS include cerebellar ataxia, early bilateral cataracts, delayed motor development, and varying degrees of mental retardation. Patients with a subtype of MSS with myoglobinuria and neuropathy have been linked to chromosome 18qter, and recently a locus for classical MSS has been localized on chromosome 5q31.

OBJECTIVES

To determine the importance of myopathy in this disorder apart from the CNS based disability and to establish the pattern of muscle involvement and degree of its severity.

METHODS

Muscle computed tomography (CT) investigations were carried out in nine Finnish MSS patients homozygous for markers around the MSS locus on chromosome 5q31.

RESULTS

Patients with severe clinical disability showed severe and generalized muscle degeneration. Muscle CT findings in patients with relatively severe clinical picture were characterized by severe involvement of the posterior thoracic and pelvic muscles, and almost all thigh muscles. In the legs the peronei and posterior compartment muscles were severely degenerated. The group of patients with moderate severity of disease showed the same pattern of involved muscle, albeit with lower degree of muscle degeneration.

CONCLUSIONS

Patients with MSS linked to chromosome 5q31 have a severe progressive myopathy, the extent of which may remain largely unrecognized because of the CNS involvement.

Homozygosity mapping of Marinesco-Sjögren syndrome to 5q31

Marinesco-Sjögren syndrome (MSS), first described in 1931, is an autosomal recessive condition characterised by somatic and mental retardation, congenital cataracts and cerebellar ataxia. Progressive myopathy was later reported to be also a cardinal sign of MSS, with myopathic changes on muscle biopsies. Hypergonadotrophic hypogonadism and skeletal deformities related to pronounced hypotonia were also reported. The major differential diagnosis of MSS is the syndrome defined by congenital cataracts, facial dysmorphism and peripheral neuropathy (CCFDN), which is localised to 18qter. Using homozygosity mapping strategy in two large consanguineous families of Turkish and Norwegian origin, respectively, we have identified the MSS locus on chromosome 5q31. LOD score calculation, including the consanguinity loops, gave a maximum value of 2.9 and 5.6 at theta=0 for the Turkish and the Norwegian families, respectively, indicating linkage between the disease and the D5S1995-D5S436 haplotype spanning a 9.3 cM interval. Patients of the two families presented with the strict clinical features of MSS. On the other hand, the study of two smaller French and Italian families, initially diagnosed as presenting an atypical MS syndrome, clearly excluded linkage from both the MSS locus on 5q31 and the CCFDN locus in 18qter. Patients of the two excluded families had all MSS features (but the myopathic changes) plus peripheral neuropathy and optic atrophy, and various combinations of microcornea, hearing impairment, seizures, Type I diabetes, cerebral atrophy and leucoencephalopathy, indicating that only the pure MSS syndrome is a homogeneous genetic entity.

Congenital muscular dystrophy with adducted thumbs, ptosis, external ophthalmoplegia, mental retardation and cerebellar hypoplasia: a novel form of CMD

At least six different forms of congenital muscular dystrophy are associated with structural changes of the central nervous system, and three of these have been mapped: merosin-deficient congenital muscular dystrophy on chromosome 6q2, Fukuyama congenital muscular dystrophy on chromosome 9q31, and muscle eye brain disease on chromosome 1p32. Walker-Warburg syndrome, congenital muscular dystrophy with calf hypertrophy, pontocerebellar hypoplasia, and normal eyes, and congenital muscular dystrophy with severe mental retardation and cerebellar cysts are nosologically distinct and have been excluded from the known congenital muscular dystrophy loci with structural changes of the central nervous system. Here, we describe a novel congenital muscular dystrophy syndrome which is phenotypically distinct from the recognized forms of congenital muscular dystrophy with brain involvement. Two siblings, a boy and a girl, were born to consanguineous parents from Sicily. Both children were born with adducted thumbs and toe contractures. They were floppy from birth, walked late, showed profound generalized muscle weakness including facial muscles, elevated creatine kinase levels of 200-700U/l, and histological changes compatible with muscular dystrophy. In addition, both showed ptosis, external ophthalmoplegia, mild mental retardation, and mild cerebellar hypoplasia on MRI. Immunocytochemistry showed normal expression of muscle membrane proteins including laminin alpha 2, laminin beta 2, and alpha-dystroglycan. Linkage analysis excluded the candidate loci on chromosomes 6q2, 9q31, and 1q32. The gene locus for congenital muscular dystrophy 1B, MDC 1B, on chromosome 1q42 was also excluded. Adducted thumbs are a distinct clinical sign that has not been reported in congenital muscular dystrophy before and should facilitate recognition of further patients with this disorder.

Nasu-Hakola disease: a review of its leukoencephalopathic and membranolipodystrophic features

The clinicopathological features of Nasu-Hakola disease are described by reviewing previously reported cases and adding consideration to newly disclosed evidence. This disease is an autosomal recessive disorder characterized by membranocystic lipodystrophy in the skeletal system and sclerosing leukoencephalopathy in the nervous system. The leukoencephalopathic alterations are demyelinization of the cerebral white matter, associated with conspicuous fibrillary gliosis and preservation of the subcortical arcuate fibers. Sudanophilic granules are focally scattered in the perivascular space or widely infiltrated in the affected white matter, and some neuronal loss with deposits of calcospherites is encountered in the basal ganglia and also in the thalamus. Spheroid formation with an increased number of neurofilaments in the neuronal axon is considered a possible pathogenesis, and a primary vascular mechanism is also suggested. Interestingly, most of the reported cases of Nasu-Hakola disease are from Japan and Finland which suggests heredofamilial background as a cause.

Congenital cataract and multisystem disorders

A knowledge of those syndromes associated with congenital cataract is essential for the paediatric ophthalmologist, as congenital cataracts are manifest in a large number of syndromes. It is important to have the correct diagnosis in such cases, not only for genetic and prognostic information, but also in order to help the parents to understand their child's condition. This paper describes the more common syndromes seen in association with congenital cataract, and emphasises the importance of looking at the whole child and family. We aim to provide a practical clinical guide to the diagnosis of hereditary and non-hereditary syndromes associated with congenital cataract.

Merosin-positive congenital muscular dystrophy with mental retardation and cataracts: a new entity in two families

Merosin-positive congenital muscular dystrophy is a heterogenous group of disorders with varying clinical presentations and severity. In general, central nervous system involvement is not present. There is also evidence for still unclassified forms. Here we report three cases in two families with merosin-positive congenital muscular dystrophy, mild mental retardation, bilateral cataracts and normal cranial magnetic resonance imaging. To our knowledge, such an association has not been reported previously, and thus is a new entity within congenital muscular dystrophy nosology.