Loss-of-function OGFRL1 variants identified in autosomal recessive cherubism families.

Cherubism (OMIM 118400) is a rare craniofacial disorder in children characterized by destructive jawbone expansion due to the growth of inflammatory fibrous lesions. Our previous studies have shown that gain-of-function mutations in SH3 domain-binding protein 2 (SH3BP2) are responsible for cherubism and that a knock-in mouse model for cherubism recapitulates the features of cherubism, such as increased osteoclast formation and jawbone destruction. To date, SH3BP2 is the only gene identified to be responsible for cherubism. Since not all patients clinically diagnosed with cherubism had mutations in SH3BP2, we hypothesized that there may be novel cherubism genes and that these genes may play a role in jawbone homeostasis. Here, using whole exome sequencing, we identified homozygous loss-of-function variants in the opioid growth factor receptor like 1 (OGFRL1) gene in 2 independent autosomal recessive cherubism families from Syria and India. The newly identified pathogenic homozygous variants were not reported in any variant databases, suggesting that OGFRL1 is a novel gene responsible for cherubism. Single cell analysis of mouse jawbone tissue revealed that Ogfrl1 is highly expressed in myeloid lineage cells. We generated OGFRL1 knockout mice and mice carrying the Syrian frameshift mutation to understand the in vivo role of OGFRL1. However, neither mouse model recapitulated human cherubism or the phenotypes exhibited by SH3BP2 cherubism mice under physiological and periodontitis conditions. Unlike bone marrow-derived M-CSF-dependent macrophages (BMMs) carrying the SH3BP2 cherubism mutation, BMMs lacking OGFRL1 or carrying the Syrian mutation showed no difference in TNF-ɑ mRNA induction by LPS or TNF-ɑ compared to WT BMMs. Osteoclast formation induced by RANKL was also comparable. These results suggest that the loss-of-function effects of OGFRL1 in humans differ from those in mice and highlight the fact that mice are not always an ideal model for studying rare craniofacial bone disorders.

The new missense G376V-TDP-43 variant induces late-onset distal myopathy but not amyotrophic lateral sclerosis.

TDP-43-positive inclusions in neurons are a hallmark of several neurodegenerative diseases including familial amyotrophic lateral sclerosis (fALS) caused by pathogenic TARDBP variants as well as more common non-Mendelian sporadic ALS (sALS). Here we report a G376V-TDP-43 missense variant in the C-terminal prion-like domain of the protein in two French families affected by an autosomal dominant myopathy but not fulfilling diagnostic criteria for ALS. Patients from both families presented with progressive weakness and atrophy of distal muscles, starting in their 5th-7th decade. Muscle biopsies revealed a degenerative myopathy characterized by accumulation of rimmed (autophagic) vacuoles, disruption of sarcomere integrity and severe myofibrillar disorganization. The G376 V variant altered a highly conserved amino acid residue and was absent in databases on human genome variation. Variant pathogenicity was supported by in silico analyses and functional studies. The G376 V mutant increased the formation of cytoplasmic TDP-43 condensates in cell culture models, promoted assembly into high molecular weight oligomers and aggregates in vitro, and altered morphology of TDP-43 condensates arising from phase separation. Moreover, the variant led to the formation of cytoplasmic TDP-43 condensates in patient-derived myoblasts and induced abnormal mRNA splicing in patient muscle tissue. The identification of individuals with TDP-43-related myopathy but not ALS implies that TARDBP missense variants may have more pleiotropic effects than previously anticipated and support a primary role for TDP-43 in skeletal muscle pathophysiology. We propose to include TARDBP screening in the genetic work-up of patients with late-onset distal myopathy. Further research is warranted to examine the precise pathogenic mechanisms of TARDBP variants causing either a neurodegenerative or myopathic phenotype.

Duchenne Muscular Dystrophy in Kazakhstan: A Journey from Diagnosis to the Treatment, the Biases and Achievements.

BACKGROUND: Neuro-muscular disorders constitutes a group of rare but heterogeneous conditions. The onset of these diseases ranges widely from birth to elderly. Many of them are life threatening and progressive. Neuromuscular science is a very specialised medical field for which specific knowledge and expertise are necessary. Such an expertise is available only partially in Kazakhstan where underdiagnosis, misdiagnosis and mismanagement of patients with muscle diseases are commonplace. Hopefully, times are changing. With the implementation of international guidelines for the diagnosis and treatment of Duchenne Muscular Dystrophy (DMD), patients are now given better care including pharmacological interventions (including steroids in DMD), respiratory and nutritional support. OBJECTIVES: To report on clinical data and genetic variants in a nationwide cohort of DMD patients. To describe and analyse management strategies applied in Kazakhstan in these patients. METHODS: The medical records of 84 patients recruited by the national expert-consulting board based at the national multidisciplinary centre of reference in neuro-muscular disorders in Astana, Kazakhstan, have been ascertained for the study. The national expert committee meets monthly to decide over the prescription of disease-modifying therapies in paediatric neuromuscular disorders. Data on the age of disease onset, the age at genetic testing, spectrum of genetic variants, the stage of disease and the serum CK levels have been collected.ResultsThe mean age of 84 examined patients was 10 years. In Kazakhstan, the average age of disease manifestation was 3 years and 3 months. The vast majority of patients passed through genetic test due to the clinical manifestations. The average age of genetic confirmation was 7 years and 6 months. There were 58,33%of gross variations, of which 55,95%were deletions and 2,38%were duplications. Nonsense mutations were identified in 29,7%. CONCLUSION: The authors contend that strictly keeping the clinical guides in the diagnosis of DMD is essential, as the genetic variations may affect the stage and feasibility of novel therapies. The way of management of neuro-muscular diseases used in Kazakhstan is strictly recommended for implementation in developing countries.

Marked Facial Weakness, Ptosis, and Hanging Jaw: A Case with RYR1 -Related Congenital Centronuclear Myopathy.

Congenital myopathies are an expanding spectrum of neuromuscular disorders with early infantile or childhood onset hypotonia and slowly or nonprogressive skeletal muscle weakness. RYR1 -related myopathies are the most common and frequently diagnosed class of congenital myopathies. Malignant hyperthermia susceptibility and central core disease are autosomal dominant or de novo RYR1 disorder, whereas multiminicore, congenital fiber type disproportion and centronuclear myopathy are autosomal recessive RYR1 disorders. The presence of ptosis, ophthalmoparesis, facial, and proximal muscles weakness, with the presence of dusty cores and multiple internal nuclei on muscle biopsy are clues to the diagnosis. We describe an 18-year-old male, who presented with early infantile onset ptosis, ophthalmoplegia, myopathic facies, hanging lower jaw, and proximal muscle weakness confirmed as an RYR1 -related congenital centronuclear myopathy on genetic analysis and muscle biopsy.

High-Throughput Digital Image Analysis Reveals Distinct Patterns of Dystrophin Expression in Dystrophinopathy Patients.

Duchenne muscular dystrophy (DMD) is an incurable disease caused by out-of-frame DMD gene deletions while in frame deletions lead to the milder Becker muscular dystrophy (BMD). In the last decade several antisense oligonucleotides drugs have been developed to induce a partially functional internally deleted dystrophin, similar to that produced in BMD, and expected to ameliorate the disease course. The pattern of dystrophin expression and functionality in dystrophinopathy patients is variable due to multiple factors, such as molecular functionality of the dystrophin and its distribution. To benchmark the success of therapeutic intervention, a clear understanding of dystrophin expression patterns in dystrophinopathy patients is vital. Recently, several groups have used innovative techniques to quantify dystrophin in muscle biopsies of children but not in patients with milder BMD. This study reports on dystrophin expression using both Western blotting and an automated, high-throughput, image analysis platform in DMD, BMD, and intermediate DMD/BMD skeletal muscle biopsies. Our results found a significant correlation between Western blot and immunofluorescent quantification indicating consistency between the different methodologies. However, we identified significant inter- and intradisease heterogeneity of patterns of dystrophin expression in patients irrespective of the amount detected on blot, due to variability in both fluorescence intensity and dystrophin sarcolemmal circumference coverage. Our data highlight the heterogeneity of the pattern of dystrophin expression in BMD, which will assist the assessment of dystrophin restoration therapies.

INPP5K and SIL1 associated pathologies with overlapping clinical phenotypes converge through dysregulation of PHGDH.

Marinesco-Sjögren syndrome is a rare human disorder caused by biallelic mutations in SIL1 characterized by cataracts in infancy, myopathy and ataxia, symptoms which are also associated with a novel disorder caused by mutations in INPP5K. While these phenotypic similarities may suggest commonalties at a molecular level, an overlapping pathomechanism has not been established yet. In this study, we present six new INPP5K patients and expand the current mutational and phenotypical spectrum of the disease showing the clinical overlap between Marinesco-Sjögren syndrome and the INPP5K phenotype. We applied unbiased proteomic profiling on cells derived from Marinesco-Sjögren syndrome and INPP5K patients and identified alterations in d-3-PHGDH as a common molecular feature. d-3-PHGDH modulates the production of l-serine and mutations in this enzyme were previously associated with a neurological phenotype, which clinically overlaps with Marinesco-Sjögren syndrome and INPP5K disease. As l-serine administration represents a promising therapeutic strategy for d-3-PHGDH patients, we tested the effect of l-serine in generated sil1, phgdh and inpp5k a+b zebrafish models, which showed an improvement in their neuronal phenotype. Thus, our study defines a core phenotypical feature underpinning a key common molecular mechanism in three rare diseases and reveals a common and novel therapeutic target for these patients.

The lncRNA 44s2 Study Applicability to the Design of 45-55 Exon Skipping Therapeutic Strategy for DMD.

In skeletal muscle, long noncoding RNAs (lncRNAs) are involved in dystrophin protein stabilization but also in the regulation of myocytes proliferation and differentiation. Hence, they could represent promising therapeutic targets and/or biomarkers for Duchenne and Becker muscular dystrophy (DMD/BMD). DMD and BMD are X-linked myopathies characterized by a progressive muscular dystrophy with or without dilatative cardiomyopathy. Two-thirds of DMD gene mutations are represented by deletions, and 63% of patients carrying DMD deletions are eligible for 45 to 55 multi-exons skipping (MES), becoming BMD patients (BMDΔ45-55). We analyzed the genomic lncRNA presence in 38 BMDΔ45-55 patients and characterized the lncRNA localized in introns 44 and 55 of the DMD gene. We highlighted that all four lncRNA are differentially expressed during myogenesis in immortalized and primary human myoblasts. In addition, the lncRNA44s2 was pointed out as a possible accelerator of differentiation. Interestingly, lncRNA44s expression was associated with a favorable clinical phenotype. These findings suggest that lncRNA44s2 could be involved in muscle differentiation process and become a potential disease progression biomarker. Based on these results, we support MES45-55 therapy and propose that the design of the CRISPR/Cas9 MES45-55 assay consider the lncRNA sequences bordering the exonic 45 to 55 deletion.

Novel Homozygous Pathogenic Mutations of LAMA 2 Gene in Patients with Congen ital Muscular Dystrophy.

The laminin α2 subunit is a protein encoded by the laminin α2 gene(LAMA2) which has the role of adhesion (attachment of cells to one another). Genetics consideration showed that mutation in LAMA2 caused a collection of muscle-wasting conditions called muscular dystrophy. This disorder causes disconnection of muscular cells and degeneration of the musculoskeletal system. In this study, we defined the molecular consideration of three patients with laminin α2 deficiency by clinical presentations of congenital muscular dystrophy. In this regard, 65 exons of the LAMA2 gene were amplified by polymerase chain reaction. Moreover, multiple ligation-dependent probe amplification and next generation sequencing (NGS) were carried out for all the patients. Because of NGS negativity, gene sequencing was performed. Results of searching for rearrangements of the LAMA2 gene enabled us to recognize homozygous pathogenic mutations c.2049_c.2050del, c.7156-2A>G, and c,1303C>T. These mutations produce an out-of-frame transcript that will be degraded by nonsense mediated decay. Therefore, we think these changes are pathogenic ones.

Current management of Duchenne muscular dystrophy in the Middle East: expert report.

Aim: Duchenne muscular dystrophy (DMD) is a severe and rare X-linked neuromuscular childhood disorder that results in functional decline, loss of ambulation and early death due to cardiac or respiratory failure. The objective of this paper is to address different aspects of the current management of DMD in the Middle East, north Africa (MENA) region, and to gather experts' recommendations on how to optimally diagnose and treat patients suffering from this disease. Methods: A group of experts (neuromuscular medicine, neuropediatricians and geneticists) convened to discuss the diagnosis and management of DMD in the MENA region. A list of practical statements was prepared by the chair of the meeting to guide the discussions around critical aspects relating to the current and future management of DMD. Results & conclusion: Ideally, DMD management should be a multidisciplinary approach. Nevertheless, few tertiary care hospitals in the region are currently able to provide the full spectrum of medical expertise and services needed by DMD patients. Clinical practice in the region remains heterogeneous. Specific guidelines for diagnosis and treatment are needed in the MENA region to improve outcomes. Disease awareness among the general public and the medical community is lacking. Now that mutation-specific therapies are being developed and more widely studied, general education programs regarding early signs and symptoms, a standardized referral and diagnosis pathway, patient registries and support groups will significantly improve the management of the disease.

Expanding the importance of HMERF titinopathy: new mutations and clinical aspects.

OBJECTIVE: Hereditary myopathy with early respiratory failure (HMERF) is caused by titin A-band mutations in exon 344 and considered quite rare. Respiratory insufficiency is an early symptom. A collection of families and patients with muscle disease suggestive of HMERF was clinically and genetically studied. METHODS: Altogether 12 new families with 19 affected patients and diverse nationalities were studied. Most of the patients were investigated using targeted next-generation sequencing; Sanger sequencing was applied in some of the patients and available family members. Histological data and muscle MRI findings were evaluated. RESULTS: Three families had several family members studied while the rest were single patients. Most patients had distal and proximal muscle weakness together with respiratory insufficiency. Five heterozygous TTN A-band mutations were identified of which two were novel. Also with the novel mutations the muscle pathology and imaging findings were compatible with the previous reports of HMERF. CONCLUSIONS: Our collection of 12 new families expands mutational spectrum with two new mutations identified. HMERF is not that rare and can be found worldwide, but maybe underdiagnosed. Diagnostic process seems to be complex as this study shows with mostly single patients without clear dominant family history.

GNE myopathy in the bedouin population of Kuwait: Genetics, prevalence, and clinical description.

INTRODUCTION: GNE myopathy is a rare recessive myopathy caused by mutations in the GNE gene. It is mainly a distal myopathy with relative sparing of the quadriceps muscle. METHODS: Patients with distal myopathies from Kuwait were examined and tested for the Middle Eastern GNE gene founder mutation, p.M743T. Patients were further studied for disease-associated features. RESULTS: GNE myopathy was confirmed in 14 of the 37 patients (37.8%) screened. All cases were caused by the p.M743T mutation. Age of onset and time from disease onset to loss of ambulation were variable. Both wasted and hypertrophied calf muscles were noted. Severely affected quadriceps were present in 1 patient, and ptosis, ophthalmoplegia, and tongue wasting in another. DISCUSSION: The scope of the p.M743T mutation now includes the Arabian Peninsula. Variations in age of onset, disease progression, and distribution in patients harboring the same mutation suggest the role of other genetic- and environment-modifying factors. Muscle Nerve 58: 700-707, 2018.

Gender as a Modifying Factor Influencing Myotonic Dystrophy Type 1 Phenotype Severity and Mortality: A Nationwide Multiple Databases Cross-Sectional Observational Study.

BACKGROUND: Myotonic Dystrophy type 1 (DM1) is one of the most heterogeneous hereditary disease in terms of age of onset, clinical manifestations, and severity, challenging both medical management and clinical trials. The CTG expansion size is the main factor determining the age of onset although no factor can finely predict phenotype and prognosis. Differences between males and females have not been specifically reported. Our aim is to study gender impact on DM1 phenotype and severity. METHODS: We first performed cross-sectional analysis of main multiorgan clinical parameters in 1409 adult DM1 patients (>18 y) from the DM-Scope nationwide registry and observed different patterns in males and females. Then, we assessed gender impact on social and economic domains using the AFM-Téléthon DM1 survey (n = 970), and morbidity and mortality using the French National Health Service Database (n = 3301). RESULTS: Men more frequently had (1) severe muscular disability with marked myotonia, muscle weakness, cardiac, and respiratory involvement; (2) developmental abnormalities with facial dysmorphism and cognitive impairment inferred from low educational levels and work in specialized environments; and (3) lonely life. Alternatively, women more frequently had cataracts, dysphagia, digestive tract dysfunction, incontinence, thyroid disorder and obesity. Most differences were out of proportion to those observed in the general population. Compared to women, males were more affected in their social and economic life. In addition, they were more frequently hospitalized for cardiac problems, and had a higher mortality rate. CONCLUSION: Gender is a previously unrecognized factor influencing DM1 clinical profile and severity of the disease, with worse socio-economic consequences of the disease and higher morbidity and mortality in males. Gender should be considered in the design of both stratified medical management and clinical trials.

A novel CRYAB mutation resulting in multisystemic disease.

Mutations in the CRYAB gene, encoding alpha-B crystallin, cause distinct clinical phenotypes including isolated posterior polar cataract, myofibrillar myopathy, cardiomyopathy, or a multisystemic disorder combining all these features. Genotype/phenotype correlations are still unclear. To date, multisystemic involvement has been reported only in kindred harboring the R120G substitution. We report a novel CRYAB mutation, D109H, associated with posterior polar cataract, myofibrillar myopathy and cardiomyopathy in a two-generation family with five affected individuals. Age of onset, clinical presentation, and muscle abnormalities were very similar to those described in the R120G family. Alpha-B crystallin may form dimers and acts as a chaperone for a number of proteins. It has been suggested that the phenotypic diversity could be related to the various interactions between target proteins of individual mutant residues. Molecular modeling indicates that residues D109 and R120 interact with each other during dimerization of alpha-B crystallin; interestingly, the two substitutions affecting these residues (D109H and R120G) are associated with the same clinical phenotype, thus suggesting a similar pathogenic mechanism. We propose that impairment of alpha-B crystallin dimerization may also be relevant to the pathogenesis of these disorders.

Congenital myasthenic syndrome with tubular aggregates caused by GFPT1 mutations.

Congenital myasthenic syndrome (CMS) is a clinically and genetically heterogeneous group of inherited disorders of the neuromuscular junction. A difficult to diagnose subgroup of CMS is characterised by proximal muscle weakness and fatigue while ocular and facial involvement is only minimal. DOK7 mutations have been identified as causing the disorder in about half of the cases. More recently, using classical positional cloning, we have identified mutations in a previously unrecognised CMS gene, GFPT1, in a series of DOK7-negative cases. However, detailed description of clinical features of GFPT1 patients has not been reported yet. Here we describe the clinical picture of 24 limb-girdle CMS (LG-CMS) patients and pathological findings of 18 of them, all carrying GFPT1 mutations. Additional patients with CMS, but without tubular aggregates, and patients with non-fatigable weakness with tubular aggregates were also screened. In most patients with GFPT1 mutations, onset of the disease occurs in the first decade of life with characteristic limb-girdle weakness and fatigue. A common feature was beneficial and sustained response to acetylcholinesterase inhibitor treatment. Most of the patients who had a muscle biopsy showed tubular aggregates in myofibers. Analysis of endplate morphology in one of the patients revealed unspecific abnormalities. Our study delineates the phenotype of CMS associated with GFPT1 mutations and expands the understanding of neuromuscular junction disorders. As tubular aggregates in context of a neuromuscular transmission defect appear to be highly indicative, we suggest calling this condition congenital myasthenic syndrome with tubular aggregates (CMS-TA).

Distal myopathy with rimmed vacuoles and inflammation: a genetically proven case.

Distal myopathy with rimmed vacuoles (DMRV) is a major entity of distal myopathy. It is an autosomal recessive disorder and is due to mutations in the GNE gene that regulates the synthesis of sialic acid. Although reported predominantly from Japan, cases have been reported from other parts of the world. We report the first genetically proven case of DMRV from India in a 23-year-old male with gradual onset, progressive distal weakness of both lower limbs with features of inflammation in muscle biopsy.

Hexosamine biosynthetic pathway mutations cause neuromuscular transmission defect.

Neuromuscular junctions (NMJs) are synapses that transmit impulses from motor neurons to skeletal muscle fibers leading to muscle contraction. Study of hereditary disorders of neuromuscular transmission, termed congenital myasthenic syndromes (CMS), has helped elucidate fundamental processes influencing development and function of the nerve-muscle synapse. Using genetic linkage, we find 18 different biallelic mutations in the gene encoding glutamine-fructose-6-phosphate transaminase 1 (GFPT1) in 13 unrelated families with an autosomal recessive CMS. Consistent with these data, downregulation of the GFPT1 ortholog gfpt1 in zebrafish embryos altered muscle fiber morphology and impaired neuromuscular junction development. GFPT1 is the key enzyme of the hexosamine pathway yielding the amino sugar UDP-N-acetylglucosamine, an essential substrate for protein glycosylation. Our findings provide further impetus to study the glycobiology of NMJ and synapses in general.

Peripheral Nerve Society Guideline on processing and evaluation of nerve biopsies.

Nerve biopsy is often the final step in the diagnostic work-up of neuropathies of unknown origin. The aim of this guideline was to prepare an evidence-based guideline on the methods for performing and evaluating nerve biopsy. The panel performed a search of MEDLINE, hand search of bibliographies of the references retrieved, review of the evidence, and reached agreement by consensus. There were not enough formal studies of diagnostic test accuracy to allow evidence-based recommendations of levels A-C for most questions. The panel summarized the class IV evidence and reached agreement by consensus on the following recommendations: (1) Nerve biopsy should not be performed before adequate clinical, electrophysiological, and laboratory investigation and only be performed with appropriate informed consent. (2) An interactive working relationship with the relevant disciplines involved and the provision of sufficient clinical information is encouraged. (3) Biopsies should be processed and read by professionals with adequate training and experience. (4) Optimal analysis of nerve biopsy is best performed by laboratories that have the facilities and expertise to prepare and evaluate frozen and fixed sections (cryostat, paraffin, and epoxy sections). (5) Immunohistochemistry, teased fiber analysis, electron microscopy, and morphometry may help clarify the diagnosis in some conditions and should be considered as additional studies.

Myopathy with hexagonally cross-linked crystalloid inclusions: delineation of a clinico-pathological entity.

A novel myopathy characterized by hexagonally cross-linked tubular arrays has been reported in five patients. We studied the clinical and histopathological features of five additional unrelated patients with this myopathy. Patients experienced exercise intolerance with exercise-induced myalgia and weakness, without rhabdomyolysis. One patient additionally presented mild permanent pelvic girdle muscle weakness. Age at onset varied between 13 and 56 years. The inclusions were eosinophilic on H and E, bright red with modified Gomori's trichrome stains, present in type 2 fibers, and revealed immunoreactivity selectively for a caveolin-3-antibody. Ultrastructurally, the inclusions showed a highly organized, hexagonally cross-linked crystalloid structure. Mutations in the caveolin-3 encoding gene were excluded. Biochemical assessment of glycogenolysis in muscle was normal. Inherited or sporadic myopathy with hexagonally cross-linked tubular arrays is associated with a homogeneous clinical and histopathological phenotype. This myopathy should be included in the differential diagnosis of patients with exercise intolerance and myalgia.

Spectrum of mutations in sarcoglycan genes in the Mumbai region of western India: high prevalence of 525del T.

BACKGROUND: While the clinical and immunocytochemical features of sarcoglycanopathies have been reported from India, genetic aspects have not been studied. There is large variation in the sarcoglycan mutations among the studied populations. AIM: To study the spectrum of mutations in sarcoglycan genes (SG). MATERIALS AND METHODS: Patients fulfilling Bushby's criteria for limb girdle muscular dystrophy were prospectively analyzed. Patients gave their medical history and underwent a clinical examination, serum creatine kinase estimation, electrophysiology, muscle biopsy with immunostaining for alpha, beta, gamma, and delta subunits and mutational analysis using denaturing high pressure liquid chromatography and direct sequencing. RESULTS: Mutations in SG accounted for 26.4% of the cohort of limb girdle muscular dystrophy. The mean age of these 18 patients was 22.5 years. Generally, proximal weakness affected the flexor and adductor compartments of the lower and upper limbs. The clinical profile of various mutations was indistinguishable from each other. Gamma SG mutations were most common, seen in 8 patients, followed by delta SG mutation in 5 patients and alpha mutation in 4 patients, while only 1 patient had mutation in the beta sarcoglycan gene. The most prevalent mutation in the gamma SG gene was 525del T. This is of interest as the mutation has been known to exist only in specific populations. CONCLUSION: This study, the first mutational analysis of Indian patients with sarcoglycanopathies suggests gamma SG mutations were the most common and the most prevalent mutation in the gamma SG gene was 525del T.

Analysis of Dp71 contribution in the severity of mental retardation through comparison of Duchenne and Becker patients differing by mutation consequences on Dp71 expression.

The presence of variable degrees of cognitive impairment, extending from severe mental retardation to specific deficits, in patients with dystrophinopathies is a well-recognized problem. However, molecular basis underlying mental retardation and its severity remain poorly understood and still a matter of debate. Here, we report one of the largest study based on the comparison of clinical, cognitive, molecular and expression data in a large cohort of 81 patients affected with Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) bearing mutations predicted to affect either all dystrophin products, including Dp71 or all dystrophin products, except Dp71. In addition to the consistent data defining molecular basis underlying mental retardation in DMD, we show that BMD patients with MR have mutations that significantly affect Dp71 expression or with mutations located in exons 75 and 76. We also show that mutations upstream to exon 62, with DMD phenotype, predicted to lead to a loss-of-function of all dystrophin products, except Dp71 isoform, are associated, predominantly, with normal or borderline cognitive performances. Altogether, these reliable phenotype-genotype correlations in combination with Dp71 mRNA and protein expression studies, strongly indicate that loss-of-function of all dystrophin products is systematically associated with severe form of MR, and Dp71 deficit is a factor that contributes in the severity of MR and may account for a shift of 2 SD downward of the intelligence quotient.