Novel SIL1 mutations and exclusion of functional candidate genes in Marinesco-Sjögren syndrome

Exome sequencing revealed variants in SGCA and SIL1 genes underlying limb girdle muscular dystrophy and Marinesco-Sjögren syndrome patients

BACKGROUND

Inherited neuromuscular (NMD) and neurodegenerative diseases (NDD) belong to two distinct categories that disturb different components of the nervous system, leading to a variety of different symptoms and clinical manifestations. Both NMD and NDD are a heterogeneous group of genetic conditions. Genetic variations in the SGCA and SIL1 genes have been implicated in causing Limb Girdle Muscular Dystrophy (LGMD), a type of neuromuscular disorder, and Marinesco-Sjögren Syndrome (MSS) which is a neurodegenerative disorder.

METHODS

In the present study, we have investigated four patients presenting LGMD and five patients with MSS features. After collecting detailed clinical and family history, necessary laboratory investigations, including estimation of a skeletal muscle marker enzyme serum creatine kinase (CK), nerve conduction study (NCS), electromyography (EMG), echocardiography (Echo), Magnetic resonance imaging (MRI -brain), CT-brain and X-rays were performed. Whole exome followed by Sanger sequencing was employed to search for the disease-causing variants.

RESULTS

Physical examination in LGMD patients revealed poor muscle tone and facing difficulty in straightening up from the floor. Clinical history revealed frequent falls and strenuousness in climbing stairs. They started toe-walking in early childhood. Laboratory investigations confirmed elevated CK levels and abnormal NCS and EMG. The MSS patients showed abnormalities in gate and jerking movement, abnormal speech, and strabismus with cataract. MRI-brain showed cerebral atrophy in some MSS patients with elevated CK levels. Whole exome sequencing revealed a nonsense variant [c.C574T, p.(Arg192*)] in the SGCA gene and a frameshift [c.936dupG, p.(Leu313AlaFs*39)] in the SIL1 gene in LGMD and MSS patients, respectively.

CONCLUSION

Our study emphasizes the significance of integrating clinical and genetic analyses for precise diagnosis and tailored management strategies in inherited NMD and NDD disorders. To the best of our knowledge, this is the first study documenting SGCA and SIL1 recurrent variants in subcontinent populations with few rare clinical features. The recurrent mutations expanding the global understanding of the mutation's geographic and ethnic distribution and contributing valuable epidemiological data. The study will facilitate genetic counseling for families experiencing similar clinical features, both within Pakistani populations and in other regions.

A Review of Brain and Pituitary Gland MRI Findings in Patients with Ataxia and Hypogonadism

The association of cerebellar ataxia and hypogonadism occurs in a heterogeneous group of disorders, caused by different genetic mutations often associated with a recessive inheritance. In these patients, magnetic resonance imaging (MRI) plays a pivotal role in the diagnostic workflow, with a variable involvement of the cerebellar cortex, alone or in combination with other brain structures. Neuroimaging involvement of the pituitary gland is also variable. Here, we provide an overview of the main clinical and conventional brain and pituitary gland MRI imaging findings of the most common genetic mutations associated with the clinical phenotype of ataxia and hypogonadism, with the aim of helping neuroradiologists in the identification of these disorders.

Movement Disorders Associated with Hypogonadism

A variety of movement disorders can be associated with hypogonadism. Identification of this association may aid in guiding workup and reaching an accurate diagnosis. We conducted a comprehensive and structured search to identify the most common movement disorders associated with hypogonadism. Only Case Reports and Case Series articles were included. Ataxia was the most common movement disorder associated with hypogonadism, including entities such as Gordon-Holmes syndrome, Boucher-Neuhäuser, Marinesco-Sjögren and Perrault syndrome. Tremor was also commonly described, particularly with aneuploidies such as Klinefelter syndrome and Jacob's syndrome. Other rare conditions including mitochondrial disorders and Woodhouse-Sakati syndrome are associated with dystonia and parkinsonism and either hypo or hypergonadotropic hypogonadism. We also highlight those entities where a combination of movement disorders is present. Hypogonadism may be more commonly associated with movement disorders than previously appreciated. It is important for the clinician to be aware of this association, as well as accompanying symptoms in order to reach a precise diagnosis.

Genotype-phenotype correlations in ocular manifestations of Marinesco-Sjögren syndrome: Case report and literature review

PURPOSE

This study aims to present a family with two children with MSS who presented with different ophthalmic features. We also aim to review MSS patients' ocular manifestations to provide a basis for future clinical trials and improve MSS patients' ophthalmologic care.

CASE DESCRIPTION

Both patients presented with global developmental delay, microcephaly, cerebellar ataxia, and myopathy. The older sibling had developed bilateral cataracts at the age of six. Her 2 years younger sister interestingly showed bilateral hyperopic refractive error without cataracts yet. Mendeliome sequencing unraveled a novel homozygous frameshift mutation in the SIL1 gene (SIL1, NM_022464.5, c.1042dupG, p.E348Gfs*4), causing MSS. A systematic literature review revealed that cataracts appear in 96% of MSS cases with a mean onset at 3.2 years. Additional frequent ocular features were strabismus (51.6%) and nystagmus (45.2%).

CONCLUSION

SIL1-related MSS is associated with marked clinical variability. Cataracts can develop later than neuromuscular features and cognitive signs. Since cataract is a relatively late finding, patients may refer to ophthalmologists for other reasons such as refractive errors, strabismus, or nystagmus. Molecular genetic testing for SIL1 is essential to facilitate early diagnosis in patients with suspected MSS.

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.

Functional Role of SIL1 in Neurodevelopment and Learning

Background

Sil1 is the causative gene of Marinesco-Sjӧgren Syndrome (MSS). The mutated Sil1 generates shortened SIL1 protein which will form aggregation and be degraded rapidly. Mental retardation is a major symptom of MSS which suggests a role of SIL1 in the development of the central nervous system, but how SIL1 functions remains unclear.

Objectives

The aim of this study is to explore the role of SIL1 in regulating cerebral development and its underlying molecular mechanism.

Methods

The basic expression pattern of SIL1 in tissues and cultured cortical neurons is measured by immunostaining and Western blot. The expression of SIL1 is reduced in vitro and in vivo through RNA interference delivered by a lentivirus. The expression of NMDA receptor subunits and the function of the Reelin signaling pathway are then examined by surface biotinylation and Western blot subsequently. Finally, the spatial learning of young mice was assessed by the Barnes maze task.

Results

SIL1 deficiency caused a diminished expression of both Reelin receptors and therefore impaired the Reelin signaling pathway. It then inhibited the developmental expression of GluN2A and impaired the spatial learning of 5-week-old mice.

Conclusions

These results suggested that SIL1 is required for the development of the central nervous system which is associated with its role in Reelin signaling.

Identification of Cellular Pathogenicity Markers for SIL1 Mutations Linked to Marinesco-Sjögren Syndrome

Background and objective: Recessive mutations in the SIL1 gene cause Marinesco-Sjögren syndrome (MSS), a rare neuropediatric disorder. MSS-patients typically present with congenital cataracts, intellectual disability, cerebellar ataxia and progressive vacuolar myopathy. However, atypical clinical presentations associated with SIL1 mutations have been described over the last years; compound heterozygosity of SIL1 missense mutations even resulted in a phenotype not fulfilling the clinical diagnostic criteria of MSS. Thus, a read-out system to evaluate reliably the pathogenicity of amino acid changes in SIL1 is needed. Here, we aim to provide suitable cellular biomarkers enabling the robust evaluation of pathogenicity of SIL1 mutations. Methods: Five SIL1 variants including one polymorphism (p.K132Q), three known pathogenic mutations (p.V231_I232del, p.G312R, and p.L457P) and one ambiguous missense variant (p.R92W) were studied along with the wild-type proteins in Hek293 in vitro models by cell biological assays, immunoprecipitation, immunoblotting, and immunofluorescence as well as electron microscopy. Moreover, the SIL1-interactomes were interrogated by tandem-affinity-purification and subsequent mass spectrometry. Results: Our combined studies confirmed the pathogenicity of p.V231_I232del, p.G312R, and p.L457P by showing instability of the proteins as well as tendency to form aggregates. This observation is in line with altered structure of the ER-Golgi system and vacuole formation upon expression of these pathogenic SIL1-mutants as well as the presence of oxidative or ER-stress. Reduced cellular fitness along with abnormal mitochondrial architecture could also be observed. Notably, both the polymorphic p.K132Q and the ambiguous p.R92W variants did not elicit such alterations. Study of the SIL1-interactome identified POC1A as a novel binding partner of wild-type SIL1; the interaction is disrupted upon the presence of pathogenic mutants but not influenced by the presence of benign variants. Disrupted SIL1-POC1A interaction is associated with centrosome disintegration. Conclusions: We developed a combination of cellular outcome measures to evaluate the pathogenicity of SIL1 variants in suitable in vitro models and demonstrated that the p. R92W missense variant is a polymorphism rather than a pathogenic mutation leading to MSS.

Ataxia in children: early recognition and clinical evaluation

Background

Ataxia is a sign of different disorders involving any level of the nervous system and consisting of impaired coordination of movement and balance. It is mainly caused by dysfunction of the complex circuitry connecting the basal ganglia, cerebellum and cerebral cortex.

A careful history, physical examination and some characteristic maneuvers are useful for the diagnosis of ataxia. Some of the causes of ataxia point toward a benign course, but some cases of ataxia can be severe and particularly frightening.

Methods

Here, we describe the primary clinical ways of detecting ataxia, a sign not easily recognizable in children. We also report on the main disorders that cause ataxia in children.

Results

The causal events are distinguished and reported according to the course of the disorder: acute, intermittent, chronic-non-progressive and chronic-progressive.

Conclusions

Molecular research in the field of ataxia in children is rapidly expanding; on the contrary no similar results have been attained in the field of the treatment since most of the congenital forms remain fully untreatable. Rapid recognition and clinical evaluation of ataxia in children remains of great relevance for therapeutic results and prognostic counseling.

Brain in situ hybridization maps as a source for reverse-engineering transcriptional regulatory networks: Alzheimer's disease insights

Microarray data have been a valuable resource for identifying transcriptional regulatory relationships among genes. As an example, brain region-specific transcriptional regulatory events have the potential of providing etiological insights into Alzheimer Disease (AD). However, there is often a paucity of suitable brain-region specific expression data obtained via microarrays or other high throughput means. The Allen Brain Atlas in situ hybridization (ISH) data sets (Jones et al., 2009) represent a potentially valuable alternative source of high-throughput brain region-specific gene expression data for such purposes. In this study, Allen Brain Atlas mouse ISH data in the hippocampal fields were extracted, focusing on 508 genes relevant to neurodegeneration. Transcriptional regulatory networks were learned using three high-performing network inference algorithms. Only 17% of regulatory edges from a network reverse-engineered based on brain region-specific ISH data were also found in a network constructed upon gene expression correlations in mouse whole brain microarrays, thus showing the specificity of gene expression within brain sub-regions. Furthermore, the ISH data-based networks were used to identify instructive transcriptional regulatory relationships. Ncor2, Sp3 and Usf2 form a unique three-party regulatory motif, potentially affecting memory formation pathways. Nfe2l1, Egr1 and Usf2 emerge among regulators of genes involved in AD (e.g. Dhcr24, Aplp2, Tia1, Pdrx1, Vdac1, and Syn2). Further, Nfe2l1, Egr1 and Usf2 are sensitive to dietary factors and could be among links between dietary influences and genes in the AD etiology. Thus, this approach of harnessing brain region-specific ISH data represents a rare opportunity for gleaning unique etiological insights for diseases such as AD.

Cellular Signature of SIL1 Depletion: Disease Pathogenesis due to Alterations in Protein Composition Beyond the ER Machinery

SIL1 acts as nucleotide exchange factor for the endoplasmic reticulum chaperone BiP. Mutations of SIL1 cause Marinesco-Sjögren syndrome (MSS), a neurodegenerative disorder. Moreover, a particular function of SIL1 for etiopathology of amyotrophic lateral sclerosis (ALS) was highlighted, thus declaring the functional SIL1-BiP complex as a modifier for neurodegenerative disorders. Thereby, depletion of SIL1 was associated with an earlier manifestation and in strengthened disease progression in ALS. Owing to the absence of appropriate in vitro models, the precise cellular pathophysiological mechanisms leading to neurodegeneration in MSS and triggering the same in further disorders like ALS are still elusive. We found that SIL1 depletion in human embryonic kidney 293 (HEK293) cells led to structural changes of the endoplasmic reticulum (ER) including the nuclear envelope and mitochondrial degeneration that closely mimic pathological alterations in MSS and ALS. Functional studies revealed disturbed protein transport, cytotoxicity with reduced proliferation and viability, accompanied by activation of cellular defense mechanisms including the unfolded protein response, ER-associated degradation pathway, proteolysis, and expression of apoptotic and survival factors. Our data moreover indicated that proteins involved in cytoskeletal organization, vesicular transport, mitochondrial function, and neurological processes contribute to SIL1 pathophysiology. Altered protein expression upon SIL1 depletion in vitro could be confirmed in Sil1-deficient motoneurones for paradigmatic proteins belonging to different functional classes. Our results demonstrate that SIL1-depleted HEK293 cells are an appropriate model to identify proteins modulated by SIL1 expression level and contributing to neurodegeneration in MSS and further disorders like ALS. Thereby, our combined results point out that proteins beyond such involved ER-related protein processing are affected by SIL1 depletion.

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.

The role of HSP70 and its co-chaperones in protein misfolding, aggregation and disease

Molecular chaperones and their associated co-chaperones are essential in health and disease as they are key facilitators of protein folding, quality control and function. In particular, the HSP70 molecular chaperone networks have been associated with neurodegenerative diseases caused by aberrant protein folding. The pathogenesis of these disorders usually includes the formation of deposits of misfolded, aggregated protein. HSP70 and its co-chaperones have been recognised as potent modulators of inclusion formation and cell survival in cellular and animal models of neurodegenerative disease. In has become evident that the HSP70 chaperone machine functions not only in folding, but also in proteasome mediated degradation of neurodegenerative disease proteins. Thus, there has been a great deal of interest in the potential manipulation of molecular chaperones as a therapeutic approach for many neurodegenerations. Furthermore, mutations in several HSP70 co-chaperones and putative co-chaperones have been identified as causing inherited neurodegenerative and cardiac disorders, directly linking the HSP70 chaperone system to human disease.

BiP and its nucleotide exchange factors Grp170 and Sil1: mechanisms of action and biological functions

BiP (immunoglobulin heavy-chain binding protein) is the endoplasmic reticulum (ER) orthologue of the Hsp70 family of molecular chaperones and is intricately involved in most functions of this organelle through its interactions with a variety of substrates and regulatory proteins. Like all Hsp70 family members, the ability of BiP to bind and release unfolded proteins is tightly regulated by a cycle of ATP binding, hydrolysis, and nucleotide exchange. As a characteristic of the Hsp70 family, multiple DnaJ-like co-factors can target substrates to BiP and stimulate its ATPase activity to stabilize the binding of BiP to substrates. However, only in the past decade have nucleotide exchange factors for BiP been identified, which has shed light not only on the mechanism of BiP-assisted folding in the ER but also on Hsp70 family members that reside throughout the cell. We will review the current understanding of the ATPase cycle of BiP in the unique environment of the ER and how it is regulated by the nucleotide exchange factors, Grp170 (glucose-regulated protein of 170kDa) and Sil1, both of which perform unanticipated roles in various biological functions and disease states.

Sil1, a nucleotide exchange factor for BiP, is not required for antibody assembly or secretion

Sil1 is a nucleotide exchange factor for the ER chaperone BiP, and mutations in this gene lead to Marinesco–Sjögren syndrome, a multisystem disorder. Effects of loss of Sil1 on biosynthesis and secretion of antibodies, a well-characterized BiP client, are determined in a mouse model for this disease and patient-derived lymphoblastoid cell lines.

Sil1 is a nucleotide exchange factor for the endoplasmic reticulum chaperone BiP, and mutations in this gene lead to Marinesco–Sjögren syndrome (MSS), a debilitating autosomal recessive disease characterized by multisystem defects. A mouse model for MSS was previously produced by disrupting Sil1 using gene-trap methodology. The resulting Sil1^Gt mouse phenocopies several pathologies associated with MSS, although its ability to assemble and secrete antibodies, the best-characterized substrate of BiP, has not been investigated. In vivo antigen-specific immunizations and ex vivo LPS stimulation of splenic B cells revealed that the Sil1^Gt mouse was indistinguishable from wild-type age-matched controls in terms of both the kinetics and magnitude of antigen-specific antibody responses. There was no significant accumulation of BiP-associated Ig assembly intermediates or evidence that another molecular chaperone system was used for antibody production in the LPS-stimulated splenic B cells from Sil1^Gt mice. ER chaperones were expressed at the same level in Sil1^WT and Sil1^Gt mice, indicating that there was no evident compensation for the disruption of Sil1. Finally, these results were confirmed and extended in three human EBV-transformed lymphoblastoid cell lines from individuals with MSS, leading us to conclude that the BiP cofactor Sil1 is dispensable for antibody production.

The nucleotide exchange factor SIL1 is required for glucose-stimulated insulin secretion from mouse pancreatic beta cells in vivo

AIMS/HYPOTHESIS

Regulation of insulin secretion along the secretory pathway is incompletely understood. We addressed the expression of SIL1, a nucleotide exchange factor for the endoplasmic reticulum (ER) chaperone glucose-regulated protein 78 kD (GRP78), in pancreatic beta cells and investigated whether or not SIL1 is involved in beta cell function.

METHODS

SIL1 expression was analysed by immunoblotting and immunofluorescence. Metabolic and islet variables, including glucose tolerance, beta cell mass, insulin secretion, islet ultrastructure, insulin content and levels of ER stress marker proteins, were addressed in Sil1 knockout (Sil1 (-/-)) mice. Insulin, proinsulin and C-peptide release was addressed in Sil1 (-/-) islets, and SIL1 overexpression or knockdown was explored in MIN6 cells in vitro. Models of type 1 diabetes and insulin resistance were induced in Sil1 (-/-) mice by administration of streptozotocin (STZ) and a high-fat diet (HFD), respectively.

RESULTS

We show that SIL1 is expressed in pancreatic beta cells and is required for islet insulin content, islet sizing, glucose tolerance and glucose-stimulated insulin secretion in vivo. Levels of pancreatic ER stress markers are increased in Sil1 (-/-) mice, and Sil1 (-/-) beta cell ER is ultrastructurally compromised. Isolated Sil1 (-/-) islets show lower proinsulin and insulin content and impaired glucose-stimulated insulin secretion. Modulation of SIL1 protein levels in MIN6 cells correlates with changes in insulin content and secreted insulin. Furthermore, Sil1 (-/-) mice are more susceptible to STZ-induced type 1 diabetes with increased apoptosis. Upon HFD feeding, Sil1 (-/-) mice show markedly lower insulin secretion and exacerbated glucose intolerance compared with control mice. Surprisingly, however, HFD-fed Sil1 (-/-) mice display pronounced islet hyperplasia with low amounts of insulin in total pancreas.

CONCLUSIONS/INTERPRETATION

These results reveal a novel role for the nucleotide exchange factor SIL1 in pancreatic beta cell function under physiological and disease conditions such as diabetes and the metabolic syndrome.

SIL1, a causative cochaperone gene of Marinesco-Söjgren syndrome, plays an essential role in establishing the architecture of the developing cerebral cortex

Marinesco-Sjögren syndrome (MSS) is a rare autosomal recessively inherited disorder with mental retardation (MR). Recently, mutations in the SIL1 gene, encoding a co-chaperone which regulates the chaperone HSPA5, were identified as a major cause of MSS. We here examined the pathophysiological significance of SIL1 mutations in abnormal corticogenesis of MSS. SIL1-silencing caused neuronal migration delay during corticogenesis ex vivo. While RNAi-resistant SIL1 rescued the defects, three MSS-causing SIL1 mutants tested did not. These mutants had lower affinities to HSPA5 in vitro, and SIL1-HSPA5 interaction as well as HSPA5 function was found to be crucial for neuronal migration ex vivo. Furthermore time-lapse imaging revealed morphological disorganization associated with abnormal migration of SIL1-deficient neurons. These results suggest that the mutations prevent SIL1 from interacting with and regulating HSPA5, leading to abnormal neuronal morphology and migration. Consistent with this, when SIL1 was silenced in cortical neurons in one hemisphere, axonal growth in the contralateral hemisphere was delayed. Taken together, abnormal neuronal migration and interhemispheric axon development may contribute to MR in MSS.

Molecular diagnosis of autosomal recessive cerebellar ataxia in the whole exome/genome sequencing era

Autosomal recessive cerebellar ataxias (ARCA) are a clinically and genetically heterogeneous group of rare neurodegenerative disorders characterized by autosomal recessive inheritance and an early age of onset. Progressive ataxia is usually the prominent symptom and is often associated with other neurological or additional features. ARCA classification still remains controversial even though different approaches have been proposed over the years. Furthermore, ARCA molecular diagnosis has been a challenge due to phenotypic overlap and increased genetic heterogeneity observed within this group of disorders. Friedreich’s ataxia and ataxia telangiectasia have been reported as the most frequent and well-studied forms of ARCA. Significant progress in understanding the genetic etiologies of the ARCA has been achieved during the last 15 years. The methodological revolution that has been observed in genetics over the last few years has contributed significantly to the molecular diagnosis of rare diseases including the ARCAs. Development of high throughput technologies has resulted in the identification of new ARCA genes and novel mutations in known ARCA genes. Therefore, an improvement in the molecular diagnosis of ARCA is expected. Moreover, based on the fact that many patients still remain undiagnosed, additional forms of ataxia are expected to be identified. We hereby review the current knowledge on the ARCAs, focused on the genetic findings of the most common forms that were molecularly characterized before the whole exome/genome era, as well as the most recently described forms that have been elucidated with the use of these novel technologies. The significant contribution of whole-exome sequencing or whole-genome sequencing in the molecular diagnosis of ARCAs is discussed.

SIL1 mutations and clinical spectrum in patients with Marinesco-Sjogren syndrome

Marinesco-Sjögren syndrome is a rare autosomal recessive multisystem disorder featuring cerebellar ataxia, early-onset cataracts, chronic myopathy, variable intellectual disability and delayed motor development. More recently, mutations in the SIL1 gene, which encodes an endoplasmic reticulum resident co-chaperone, were identified as the main cause of Marinesco-Sjögren syndrome. Here we describe the results of SIL1 mutation analysis in 62 patients presenting with early-onset ataxia, cataracts and myopathy or combinations of at least two of these. We obtained a mutation detection rate of 60% (15/25) among patients with the characteristic Marinesco-Sjögren syndrome triad (ataxia, cataracts, myopathy) whereas the detection rate in the group of patients with more variable phenotypic presentation was below 3% (1/37). We report 16 unrelated families with a total of 19 different SIL1 mutations. Among these mutations are 15 previously unreported changes, including single- and multi-exon deletions. Based on data from our screening cohort and data compiled from the literature we found that SIL1 mutations are invariably associated with the combination of a cerebellar syndrome and chronic myopathy. Cataracts were observed in all patients beyond the age of 7 years, but might be missing in infants. Six patients with SIL1 mutations had no intellectual disability, extending the known wide range of cognitive capabilities in Marinesco-Sjögren syndrome to include normal intelligence. Modestly constant features were somatic growth retardation, skeletal abnormalities and pyramidal tract signs. Examination of mutant SIL1 expression in cultured patient lymphoblasts suggested that SIL1 mutations result in severely reduced SIL1 protein levels irrespective of the type and position of mutations. Our data broaden the SIL1 mutation spectrum and confirm that SIL1 is the major Marinesco-Sjögren syndrome gene. SIL1 patients usually present with the characteristic triad but cataracts might be missing in young children. As cognitive impairment is not obligatory, patients without intellectual disability but a Marinesco-Sjögren syndrome-compatible phenotype should receive SIL1 mutation analysis. Despite allelic heterogeneity and many families with private mutations, the phenotype related to SIL1 mutations is relatively homogenous. Based on SIL1 expression studies we speculate that this may arise from a uniform effect of different mutations on protein expression.

Phenotype-genotype correlations in patients with Marinesco-Sjögren syndrome

Marinesco-Sjögren syndrome (MSS; MIM 248800) is an autosomal recessive disorder characterized by congenital cerebellar ataxia, early cataracts, developmental delay, myopathy and short stature. Alterations in the gene SIL1 cause MSS in some patients with typical findings. In this study, molecular investigations including sequencing of the SIL1 gene, western blotting and microscopic investigations in fibroblast cultures were carried out in a cohort of 15 patients from 14 unrelated families, including the large, inbred family reported by Superneau et al., having the clinical features of MSS to provide insights into the pathophysiology of the disorder. A total of seven different mutations were found in eight of the patients from seven families. The mutations caused loss of the BIP-associated protein (BAP) protein in four patients by western blot. Novel clinical features such as dental abnormalities, iris coloboma, eczema and hormonal abnormalities were noticed in some patients, but there was no clear way to distinguish those with and without SIL1 mutations. Cultured fibroblasts contained numerous cytoplasmic inclusion bodies, similar to those identified in the brain of the whoozy mouse in five unrelated patients, three with and two without SIL1 mutations, suggesting some SIL1 negative patients share a common cellular pathogenesis with those who are SIL1 positive.

Marinesco-Sjögren syndrome due to SIL1 mutations with a comment on the clinical phenotype

BACKGROUND

Marinesco-Sjögren syndrome is an autosomal recessive cerebellar ataxia, characterised by cerebellar ataxia, myopathy, cataracts and intellectual disability, due to mutations in the SIL1 gene.

METHODS

The clinical features and two novel SIL1 mutations of four Dutch patients with Marinesco-Sjögren syndrome are described and compared to the literature on genetically proven Marinesco-Sjögren patients.

RESULTS

The core phenotype of this syndrome appears homogeneous, but: [1] cataract can develop later than the motor and cognitive signs; [2] myopathy is an early feature that seems progressive during the course of the disease; [3] serum creatine kinase is normal or only mildly elevated; [4] peripheral neuropathy is absent; and [5] a variable degree of intellectual disability is present in most Marinesco-Sjögren patients.

CONCLUSIONS

Because the late appearance of some hallmarks and the uncertainty as to whether incomplete phenotypes occur, SIL1 mutation analysis is helpful early in the diagnostic work-up of children with suspected inherited ataxias.

C-terminal mutations destabilize SIL1/BAP and can cause Marinesco-Sjögren syndrome

Marinesco-Sjögren syndrome (MSS) is an autosomal recessive, neurodegenerative, multisystem disorder characterized by severe phenotypes developing in infancy. Recently, mutations in the endoplasmic reticulum (ER)-associated co-chaperone SIL1/BAP were identified to be the major cause of MSS. SIL1 acts as a nucleotide exchange factor for BiP, the ER Hsp70 orthologue, which plays an essential role in the folding and assembly of nascent polypeptide chains in the ER. SIL1 facilitates the release of BiP from unfolded protein substrates, enabling the subsequent folding and transport of the protein. Although most mutations leading to MSS result in deletion of the majority of the protein, three separate mutations have been identified that disrupt only the last five or six amino acids of the protein, which were assumed to encode a divergent ER retention motif. This study presents an in depth analysis of two of these mutants and reveals that the phenotype in the affected individuals is not likely to be due to depletion of SIL1 from the ER via secretion. Instead, our analyses show that the mutant proteins are particularly unstable and either form large aggregates in the ER or are rapidly degraded via the proteasome. In agreement with our findings, homology modeling suggests that the very C-terminal residues of SIL1 play a role in its structural integrity rather than its localization. These new insights might be a first step toward a possible pharmacological treatment of certain types of MSS by specifically stabilizing the mutant SIL1 protein.

Past, present and future therapeutics for cerebellar ataxias

Cerebellar ataxias are a group of disabling neurological disorders. Patients exhibit a cerebellar syndrome and can also present with extra-cerebellar deficits, namely pigmentary retinopathy, extrapyramidal movement disorders, pyramidal signs, cortical symptoms (seizures, cognitive impairment/behavioural symptoms), and peripheral neuropathy. Recently, deficits in cognitive operations have been unraveled. Cerebellar ataxias are heterogeneous both at the phenotypic and genotypic point of view. Therapeutical trials performed during these last 4 decades have failed in most cases, in particular because drugs were not targeting a deleterious pathway, but were given to counteract putative defects in neurotransmission. The identification of the causative mutations of many hereditary ataxias, the development of relevant animal models and the recent identifications of the molecular mechanisms underlying ataxias are impacting on the development of new drugs. We provide an overview of the pharmacological treatments currently used in the clinical practice and we discuss the drugs under development.

Alteration of the unfolded protein response modifies neurodegeneration in a mouse model of Marinesco-Sjögren syndrome

Endoplasmic reticulum (ER) stress has been linked to the onset and progression of many diseases. SIL1 is an adenine nucleotide exchange factor of the essential ER lumen chaperone HSPA5/BiP that senses ER stress and is involved in protein folding. Mutations in the Sil1 gene have been associated with Marinesco-Sjögren syndrome, hallmarks of which include ataxia and cerebellar atrophy. We have previously shown that loss of SIL1 function in mouse results in ER stress, ubiquitylated protein inclusions, and degeneration of specific Purkinje cells in the cerebellum. Here, we report that overexpression of HYOU1/ORP150, an exchange factor that works in parallel to SIL1, prevents ER stress and rescues neurodegeneration in Sil1(-/-) mice, whereas decreasing expression of HYOU1 exacerbates these phenotypes. In addition, loss of DNAJC3/p58(IPK), a co-chaperone that promotes ATP hydrolysis by BiP, ameliorates ER stress and neurodegeneration in Sil1(-/-) mice. These findings suggest that alterations in the nucleotide exchange cycle of BiP cause ER stress and neurodegeneration in Sil1-deficient mice. Our results present the first evidence of important genetic modifiers of Marinesco-Sjögren syndrome, and provide additional pathways for therapeutic intervention for this, and other ER stress-induced, diseases.