Mutations In Fukutin-related Protein Research Articles

Break Down of the Complexity and Inconsistency Between Levels of Matriglycan and Disease Phenotype in FKRP-Related Dystroglycanopathies: A Review and Model of Interpretation.

Dystroglycanopathies are a group of muscle degenerative diseases characterized with significant reduction in matriglycan expression critical in disease pathogenesis. Missense point mutations in the Fukutin-related protein (FKRP) gene cause variable reduction in the synthesis of matriglycan on alpha-dystroglycan (α-DG) and a wide range of disease severity. Data analyses of muscle biopsies from patients fail to show consistent correlation between the levels of matriglycan and clinical phenotypes. By reviewing clinical reports in conjunction with analysis of clinically relevant mouse models, we identify likely causes for the confusion. Nearly all missense FKRP mutations retain variable, but sufficient function for the synthesis of matriglycan during the later stage of muscle development and periods of muscle regeneration. These factors lead to a highly heterogenous pattern of matriglycan expression in diseased muscles, depending on age and stages of muscle regeneration. The limited size in clinical biopsy samples from different parts of even a single muscle tissue at different time points of disease progression may well mis-represent the residual function (base-levels) of the mutated FKRPs and phenotypes. We propose to use a simple Multi Point tool from ImageJ to more accurately measure the signal intensity of matriglycan expression on fiber membrane for assessing mutant FKRP function and therapeutic efficacy. A robust and sensitive immunohistochemical protocol would further improve reliability and comparability for the detection of matriglycan.

Oral ribose supplementation in dystroglycanopathy: A single case study.

Three forms of muscular dystrophy-dystroglycanopathies are linked to the ribitol pathway. These include mutations in the isoprenoid synthase domain-containing protein (ISPD), fukutin-related protein (FKRP), and fukutin (FKTN) genes. The aforementioned enzymes are required for generation of the ribitol phosphate linkage in the O-glycan of alpha-dystroglycan. Mild cases of dystroglycanopathy present with slowly progressive muscle weakness, while in severe cases the eyes and brain are also involved. Previous research showed that ribose increased the intracellular concentrations of cytidine diphosphate-ribitol (CDP-ribitol) and had a therapeutic effect. Here, we report the safety and effects of oral ribose supplementation during 6 months in a patient with limb girdle muscular dystrophy type 2I (LGMD2I) due to a homozygous FKRP mutation. Ribose was well tolerated in doses of 9 g or 18 g/day. Supplementation with 18 g of ribose resulted in a decrease of creatine kinase levels of 70%. Moreover, metabolomics showed a significant increase in CDP-ribitol levels with 18 g of ribose supplementation (p < 0.001). Although objective improvement in clinical and patient-reported outcome measures was not observed, the patient reported subjective improvement of muscle strength, fatigue, and pain. This case study indicates that ribose supplementation in patients with dystroglycanopathy is safe and highlights the importance for future studies regarding its potential effects.

Phenotype Genotype Characterization of FKRP-related Muscular Dystrophy among Indian Patients.

The phenotypic spectrum of Fukutin-related protein (FKRP) mutations is highly variable and comprises of limb girdle muscular dystrophy (LGMD) R9 (previously LGMD 2I) and FKRP related congenital muscular dystrophies. To identify the distinct genotype phenotype pattern in Indian patients with FKRP gene mutations. We retrospectively reviewed the case files of patients with muscular dystrophy having a genetically confirmed FKRP mutation. All patients had undergone genetic testing using next-generation sequencing. Our patients included five males and four females presenting between 1.5 years and seven years of age (median age - 3 years). The initial symptom was a delayed acquisition of gross motor developmental milestones in seven patients and recurrent falls and poor sucking in one patient each. Two patients had a language delay, with both having abnormalities on the brain MRI. Macroglossia, scapular winging, and facial weakness were noted in one, three and four patients respectively. Calf muscle hypertrophy was seen in eight patients and ankle contractures in six. At the last follow-up, three patients had lost ambulation (median age - 7 years; range 6.5-9 years) and three patients had not attained independent ambulation. Creatine kinase levels ranged between 2793 and 32,396 U/L (mean 12,120 U/L). A common mutation - c.1343C>T was noted in 5 patients in our cohort. Additionally, four novel mutations were identified. Overall, six patients had an LGMD R9 phenotype, and three had a congenital muscular dystrophy phenotype. Patients with FKRP mutations can have varied presentations. A Duchenne-like phenotype was the most commonly encountered pattern in our cohort, with c.1343C>T being the most common mutation.

Efficacy and muscle safety assessment of fukutin-related protein gene therapy

Limb-girdle muscular dystrophy type R9 (LGMDR9) is a muscle-wasting disease that begins in the hip and shoulder regions of the body. This disease is caused by mutations in fukutin-related protein (FKRP), a glycosyltransferase critical for maintaining muscle cell integrity. Here we investigated potential gene therapies for LGMDR9 containing an FKRP expression construct with untranslated region (UTR) modifications. Initial studies treated an aged dystrophic mouse model (FKRPP448L) with adeno-associated virus vector serotype 6 (AAV6). Grip strength improved in a dose- and time-dependent manner, injected mice exhibited fewer central nuclei and serum creatine kinase levels were 3- and 5-fold lower compared to those in non-injected FKRPP448L mice. Treatment also partially stabilized the respiratory pattern during exercise and improved treadmill running, partially protecting muscle from exercise-induced damage. Western blotting of C2C12 myotubes using a novel rabbit antibody confirmed heightened translation with the UTR modifications. We further explored the question of FKRP toxicity in wild-type mice using high doses of two additional muscle-tropic capsids: AAV9 and AAVMYO1. No toxic effects were detected with either therapeutic agent. These data further support the feasibility of gene therapy to treat LGMDR9.

Revertant Phenomenon in DMD and LGMD2I and Its Therapeutic Implications: A Review of Study Under Mentorship of Terrence Partridge.

This review recollects my initial research focus on revertant fibers (expressing dystrophin in the background of frame-shifting mutation) in Duchenne muscular dystrophy (DMD) muscles in Professor Terrence Partridge’s Muscle Cell Biology Laboratory in MRC Clinical Research Science Center, Harmmersmith Hospital, London, UK. Our data indicated that revertant fibers are most likely resulted from epigenetic random events which skip exon(s) flanking the mutated exon, leading to the restoration of the reading frame. Some of these events establish themselves as relatively permanent skipping patterns, a mechanism similar to multiple transcript species established in various cell types. With this hypothesis, antisense oligonucleotide-mediated exon skipping is likely to have a great chance to achieve restoration of therapeutic levels of dystrophin in DMD muscles. This leads to our first reports of local and systemic efficacy of antisense oligonucleotide-mediated exon skipping for DMD treatment. The experience under Terry’s mentorship shaped my thinking and led me to explore another revertant feature in the dystroglycanopathy caused by mutations in the Fukutin Related Protein (FKRP) gene which functions as a glycosyltransferase. Mutant FKRPs retain partial function and produce a fraction of normal to no detectable levels of laminin-binding α-dystroglycan (matriglycan) in most of the muscle fibers. Reversion to near normal levels of matriglycan expression in muscles with FKRP mutations depends on muscle regeneration and in muscles of neonate mice, suggesting that changes in metabolism and gene expression could be sufficient to compensate for the reduced function of mutant FKRP genes even those associated with severe congenital muscular dystrophy (CMD). This is now supported by our successful demonstration that supply of FKRP mutant mice with ribitol, a precursor for substrate of FKRP, is sufficient to restore the levels of matriglycan with therapeutic significance. Our data overall suggest that rare events of reversion in muscular dystrophy, and likely other diseases could provide unique insight for mechanisms and therapeutic exploitation.

Phenotype genotype characterization of FKRP mutations in an Indian cohort of limb girdle muscular dystrophy

Mutations in the FKRP (Fukutin Related Protein) gene produces variable phenotypes including Limb Girdle Muscular Dystrophy Type R9 (formerly LGMD2I). It has been rarely reported in South Asian population. We studied a cohort of Indian patients with LGMD2I to elucidate the mutation spectrum and possible genotype–phenotype correlation.

FKRP mutations cause congenital muscular dystrophy 1C and limb-girdle muscular dystrophy 2I in Asian patients

Mutation in the fukutin-related protein (FKRP) gene causes alpha-dystroglycanopathies, a group of autosomal recessive disorders associated with defective glycosylated alpha-dystroglycan (α-DG). The disease phenotype shows a broad spectrum, from the most severe congenital form involving brain and eye anomalies to milder limb-girdle form. FKRP-related alpha-dystroglycanopathies are common in European countries. However, a limited number of patients have been reported in Asian countries. Here, we presented the clinical, pathological, and genetic findings of nine patients with FKRP mutations identified at a single muscle repository center in Japan. Three and six patients were diagnosed with congenital muscular dystrophy type 1C and limb-girdle muscular dystrophy 2I, respectively. None of our Asian patients showed the most severe form of alpha-dystroglycanopathy. While all patients showed a reduction in glycosylated α-DG levels, to variable degrees, these levels did not correlate to clinical severity. Fifteen distinct pathogenic mutations were identified in our cohort, including five novel mutations. Unlike in the populations belonging to European countries, no common mutation was found in our cohort.

A universal gene correction approach for FKRP-associated dystroglycanopathies to enable autologous cell therapy.

SUMMARYMutations in the fukutin-related protein (FKRP) gene result in a broad spectrum of muscular dystrophy (MD) phenotypes, including the severe Walker-Warburg syndrome (WWS). Here, we develop a gene-editing approach that replaces the entire mutant open reading frame with the wild-type sequence to universally correct all FKRP mutations. We apply this approach to correct FKRP mutations in induced pluripotent stem (iPS) cells derived from patients displaying broad clinical severity. Our findings show rescue of functional α-dystroglycan (α-DG) glycosylation in gene-edited WWS iPS cell-derived myotubes. Transplantation of gene-corrected myogenic progenitors in the FKRPP448L-NSG mouse model gives rise to myofiber and satellite cell engraftment and, importantly, restoration of α-DG functional glycosylation in vivo. These findings suggest the potential feasibility of using CRISPR-Cas9 technology in combination with patient-specific iPS cells for the future development of autologous cell transplantation for FKRP-associated MDs.

NAD+ enhances ribitol and ribose rescue of α-dystroglycan functional glycosylation in human FKRP-mutant myotubes.

Mutations in the fukutin-related protein (FKRP) cause Walker-Warburg syndrome (WWS), a severe form of congenital muscular dystrophy. Here, we established a WWS human induced pluripotent stem cell-derived myogenic model that recapitulates hallmarks of WWS pathology. We used this model to investigate the therapeutic effect of metabolites of the pentose phosphate pathway in human WWS. We show that functional recovery of WWS myotubes is promoted not only by ribitol but also by its precursor ribose. Moreover, we found that the combination of each of these metabolites with NAD+ results in a synergistic effect, as demonstrated by rescue of α-dystroglycan glycosylation and laminin binding capacity. Mechanistically, we found that FKRP residual enzymatic capacity, characteristic of many recessive FKRP mutations, is required for rescue as supported by functional and structural mutational analyses. These findings provide the rationale for testing ribose/ribitol in combination with NAD+ to treat WWS and other diseases associated with FKRP mutations.

Fukutin-Related Protein: From Pathology to Treatments.

Fukutin-related protein (FKRP) is a glycosyltransferase involved in the functional glycosylation of α-dystroglycan (DG), a key component in the link between the cytoskeleton and the extracellular matrix (ECM). Mutations in FKRP lead to dystroglycanopathies with broad severity, including limb-girdle and congenital muscular dystrophy. Studies over the past 5 years have elucidated the function of FKRP, which has expanded the number of therapeutic opportunities for patients carrying FKRP mutations. These include small molecules, gene delivery, and cell therapy. Here we summarize recent findings on the function of FKRP and describe available models for studying diseases and testing therapeutics. Lastly, we highlight preclinical studies that hold potential for the treatment of FKRP-associated dystroglycanopathies.

Efficient engraftment of pluripotent stem cell-derived myogenic progenitors in a novel immunodeficient mouse model of limb girdle muscular dystrophy 2I

BackgroundDefects in α-dystroglycan (DG) glycosylation characterize a group of muscular dystrophies known as dystroglycanopathies. One of the key effectors in the α-DG glycosylation pathway is the glycosyltransferase fukutin-related protein (FKRP). Mutations in FKRP lead to a large spectrum of muscular dystrophies, including limb girdle muscular dystrophy 2I (LGMD2I). It remains unknown whether stem cell transplantation can promote muscle regeneration and ameliorate the muscle wasting phenotype associated with FKRP mutations.ResultsHere we transplanted murine and human pluripotent stem cell-derived myogenic progenitors into a novel immunodeficient FKRP-mutant mouse model by intra-muscular injection. Upon both mouse and human cell transplantation, we observe the presence of donor-derived myofibers even in absence of pre-injury, and the rescue of α-DG functional glycosylation, as shown by IIH6 immunoreactivity. The presence of donor-derived cells expressing Pax7 under the basal lamina is indicative of satellite cell engraftment, and therefore, long-term repopulation potential. Functional assays performed in the mouse-to-mouse cohort revealed enhanced specific force in transplanted muscles compared to PBS-injected controls.ConclusionsAltogether, our data demonstrate for the first time the suitability of a cell-based therapeutic approach to improve the muscle phenotype of dystrophic FKRP-mutant mice.

Clinical and electrophysiological evaluation of myasthenic features in an alpha-dystroglycanopathy cohort (FKRP-predominant)

A postsynaptic dysfunction of the neuromuscular junction has been reported in patients with alpha-dystroglycanopathy associated with mutations in guanosine diphosphate (GDP)-mannose pyrophosphorylase B gene (GMPPB), some of whom benefit from symptomatic treatment. In this study, we determine the frequency of myasthenic and fatigue symptoms and neuromuscular junction transmission defects in a fukutin-related protein (FKRP)-predominant alpha-dystroglycanopathy cohort. Thirty-one patients with alpha-dystroglycanopathies due to mutations in FKRP (n = 25), GMPPB (n = 4), POMGNT1 (n = 1), and POMT2 (n = 1) completed a six-question modified questionnaire for myasthenic symptoms and the PROMIS Short Form v1.0-Fatigue 8a survey, and they underwent 3 Hz repetitive nerve stimulation of spinal accessory nerve-trapezius and radial nerve-anconeus pairs. Results showed that fatigue with activity was common; 63% of the cohort reported fatigue with chewing. A defective postsynaptic neuromuscular junction transmission was not identified in any of the patients carrying FKRP mutations but only in one mildly affected patient with GMPPB mutations (c.79 G>C, p.D27H and c.402+1G>A, splice site variant). We conclude that symptoms of fatigue with activity did not predict abnormal neuromuscular junction transmission on electrodiagnostic studies in this cohort and that, unlike GMPPB subgroup, a defective neuromuscular junction transmission does not appear to be present in patients with FKRP-associated muscular dystrophies.

A novel noncoding FKRP mutation in early onset limb-girdle muscular dystrophy.

Limb-girdle muscular dystrophy 2I (LGMD2I, LGMD R9) (OMIM: muscular dystrophy—dystroglycanopathy [limb-girdle], type C, 5) is an autosomal recessive disorder caused by mutations in the fukutin-related protein ( FKRP ) gene.1 The phenotypic spectrum associated with FKRP is heterogeneous; the most severe phenotype, Walker-Warburg-like syndrome, is characterized by congenital hypotonia, progressive muscle weakness and atrophy, ocular and brain malformations, severe motor developmental delay, and profound intellectual disability. Milder phenotypes include the onset of disease from infancy to adulthood and variable clinical course including asymptomatic elevated serum creatinine kinase (CK), early onset rapidly progressive course with loss of ambulation in teenage years, late onset with slow progression, and primary cardiomyopathy with minimal skeletal muscle weakness.1,2 LGMD2I is typically characterized by proximal muscle weakness, calf hypertrophy, hypotonia, elevated CK level, normal cognition, and no structural brain abnormalities. Dilated cardiomyopathy and respiratory muscle weakness can be seen.1 Mutations in the FKRP gene reduce specific O-mannose-linked glycosylation of alpha-dystroglycan, leading to the instability of the linkage between the dystrophin–glycoprotein complex and laminin alpha 2 in the basement membrane of skeletal muscle.3 The most common FKRP mutation seen in LGMD2I is c.826C>A (p.Leu276Ile). Patients who are homozygous for this mutation typically have a milder phenotype, whereas compound heterozygotes have a relatively severe clinical course. In patients heterozygous for c.826C>A, clinical severity may correspond best to the mutation in the second allele.1,4 We describe a 9-year-old boy with LGMD2I who has the c.826C>A mutation in FKRP on 1 allele and a novel variant on the second allele. The authors acknowledge Ms. Terese Nelson who performed the histology and immunostaining and Mr. Joel Carl for assembling the figure

A limb-girdle muscular dystrophy 2I model of muscular dystrophy identifies corrective drug compounds for dystroglycanopathies.

Zebrafish are a powerful tool for studying muscle function owing to their high numbers of offspring, low maintenance costs, evolutionarily conserved muscle functions, and the ability to rapidly take up small molecular compounds during early larval stages. Fukutin-related protein (FKRP) is a putative protein glycosyltransferase that functions in the Golgi apparatus to modify sugar chain molecules of newly translated proteins. Patients with mutations in the FKRP gene can have a wide spectrum of clinical symptoms with varying muscle, eye, and brain pathologies depending on the location of the mutation in the FKRP protein. Patients with a common L276I FKRP mutation have mild adult-onset muscle degeneration known as limb-girdle muscular dystrophy 2I (LGMD2I), whereas patients with more C-terminal pathogenic mutations develop the severe Walker-Warburg syndrome (WWS)/muscle-eye-brain (MEB) disease. We generated fkrp-mutant zebrafish that phenocopy WWS/MEB pathologies including severe muscle breakdowns, head malformations, and early lethality. We have also generated a milder LGMD2I-model zebrafish via overexpression of a heat shock-inducible human FKRP (L276I) transgene that shows milder muscle pathology. Screening of an FDA-approved drug compound library in the LGMD2I zebrafish revealed a strong propensity towards steroids, antibacterials, and calcium regulators in ameliorating FKRP-dependent pathologies. Together, these studies demonstrate the utility of the zebrafish to both study human-specific FKRP mutations and perform compound library screenings for corrective drug compounds to treat muscular dystrophies.

Skeletal muscle contractile properties in a novel murine model for limb girdle muscular dystrophy 2i.

Limb-girdle muscular dystrophy (LGMD) 2i results from mutations in fukutin-related protein and aberrant α-dystroglycan glycosylation. Although this significantly compromises muscle function and ambulation, the comprehensive characteristics of contractile dysfunction are unknown. Therefore, we quantified the in situ contractile properties of the medial gastrocnemius in young adult P448L mice, an affected muscle of a novel model of LGMD2i. Normalized maximal twitch force, tetanic force, and power were significantly smaller in P448L mice, compared with sex-matched, wild-type mice. These differences were consistent with the replacement of contractile fibers by passive tissue. The shape of the active force-length relationships were similar in both groups, regardless of sex, consistent with an intact sarcomeric structure in P448L mice. Passive force-length curves normalized to maximal isometric force were steeper in P448L mice, and passive elements contribute disproportionately more to total contractile force in P448L mice. Sex differences were mostly noted in the force-velocity curves, as normalized values for maximal and optimal velocities were significantly slower in P448L males, compared with wild-type, but not in P448L females. This suggests that the dystrophic phenotype, which may include possible changes in cross-bridge kinetics and fiber-type proportions, progresses more quickly in P448L males. These results together indicate that active force and power generation are compromised in both sexes of P448L mice, while passive forces increase. More importantly, the results identified several functional markers of disease pathophysiology that could aid in developing and assessment of novel therapeutics for LGMD2i and possibly other dystroglycanopathies as well. NEW & NOTEWORTHY Comprehensive assessments of muscle contractile function have, until now, never been performed in an animal model for any dystroglycanopathy. This study suggests that skeletal muscle contractile properties are significantly compromised in a recently developed model for limb-girdle muscular dystrophy 2i, the P448L mouse. It further identifies novel pathological markers of muscle function that are suitable for developing therapeutics and for better understanding of disease pathogenesis.

A novel FKRP-related muscular dystrophy founder mutation in South African Afrikaner patients with a phenotype suggestive of a dystrophinopathy.

Fukutin-related protein (FKRP) muscular dystrophy is an autosomal recessive disorder caused by mutations in the FKRP gene. The condition is often misdiagnosed as a dystrophinopathy. A previously unreported mutation, c.1100T&gt;C in exon 4 of FKRP, had been identified in homozygous form in two white South African (SA) Afrikaner patients clinically diagnosed with a dystrophinopathy. To investigate whether the c.1100T&gt;C mutation and the common European FKRP mutation c.826C&gt;A are present in other patients of Afrikaner origin with suspected dystrophinopathy, and whether a founder haplotype exists. The c.1100T&gt;C mutation was initially tested for using an amplification refractory mutation system technique in 45 white SA Afrikaner patients who had tested negative using multiplex ligation probe amplification screening for exonic deletions/duplications in the dystrophin gene. Sequencing analysis was used to confirm the c.1100T&gt;C mutation and screen for the c.826C&gt;A mutation. Two cohorts (each numbering 100) of Afrikaans and other white controls were screened for the c.1100T&gt;C and c.826C&gt;A mutations, respectively. Of the 45 patients, 8 patients (17.8%) were homozygous for c.1100T&gt;C, 2 (4.4%) were compound heterozygotes for c.1100T&gt;C and c.826C&gt;A, and 1 (2.2%) was heterozygous for c.1100T&gt;C with a second unidentified mutation. The c.1100T&gt;C mutation was found in 1/100 controls, but no heterozygotes for the c.826C&gt;A mutation were identified. Linked marker analysis for c.1100T&gt;C showed a common haplotype, suggesting a probable founder mutation in the SA Afrikaner population. FKRP mutations may be relatively common in Afrikaners, and screening should be considered in patients who have a suggestive phenotype and test negative for a dystrophinopathy. This test will be useful for offering diagnostic, carrier and prenatal testing for affected individuals and their families. As FKRP muscular dystrophy is autosomal recessive in inheritance, the implications of a positive diagnosis in a family differ significantly from those of an X-linked dystrophinopathy.

FKRP mutations, including a founder mutation, cause phenotype variability in Chinese patients with dystroglycanopathies.

Mutations in the fukutin-related protein (FKRP) gene have been associated with dystroglycanopathies, which are common in Europe but rare in Asia. Our study aimed to retrospectively analyze and characterize the clinical, myopathological and genetic features of 12 Chinese patients with FKRP mutations. Three patients were diagnosed with congenital muscular dystrophy type 1C (MDC1C) and nine patients were diagnosed with limb girdle muscular dystrophy type 2I (LGMD2I). Three muscle biopsy specimens had dystrophic changes and reduced glycosylated α-dystroglycan staining, and two showed reduced expression of laminin α2. Two known and 13 novel mutations were identified in our single center cohort. Interestingly, the c.545A>G mutation was found in eight of the nine LGMD2I patients as a founder mutation and this founder mutation in Chinese patients differs from the one seen in European patients. Moreover, patients homozygous for the c.545A>G mutation were clinically asymptomatic, a less severe phenotype than in compound heterozygous patients with the c.545A>G mutation. The 13 novel mutations of FKRP significantly expanded the mutation spectrum of MDC1C and LGMD2I, and the different founder mutations indicate the ethnic difference in FKRP mutations.

Rhabdomyolysis featuring muscular dystrophies

BackgroundRhabdomyolysis is a potentially life threatening condition of various etiology. The association between rhabdomyolysis and muscular dystrophies is under-recognized in clinical practice. ObjectiveTo identify muscular dystrophies presenting with rhabdomyolysis at onset or as predominant feature. MethodsWe retrospectively reviewed clinical and laboratory data of patients with a genetically confirmed muscular dystrophy in whom rhabdomyolysis was the presenting or main clinical manifestation. ResultsThirteen unrelated patients (males=6; females=7) were identified. Median age at time of rhabdomyolysis was 18years (range, 2–47) and median duration between the first episode of rhabdomyolysis and molecular diagnosis was 2years. Fukutin-related protein (FKRP) muscular dystrophy (n=6) was the most common diagnosis, followed by anoctaminopathy-5 (n=3), calpainopathy-3 (n=2) and dystrophinopathy (n=2). Four patients experienced recurrent rhabdomyolysis. Eight patients were asymptomatic and 3 reported myalgia and exercise intolerance prior to the rhabdomyolysis. Exercise (n=6) and fever (n=4) were common triggers; rhabdomyolysis was unprovoked in 3 patients. Twelve patients required hospitalization. Baseline CK levels were elevated in all patients (median 1200IU/L; range, 600–3600). ConclusionMuscular dystrophies can present with rhabdomyolysis; FKRP mutations are particularly frequent in causing such complication. A persistently elevated CK level in patients with rhabdomyolysis warrants consideration for underlying muscular dystrophy.

Restoration of Functional Glycosylation of α-Dystroglycan in FKRP Mutant Mice Is Associated with Muscle Regeneration

Mutations in fukutin-related protein (FKRP) gene are characterized with lack of functionally glycosylated α-dystroglycan (F-α-DG). Surprisingly, a few muscle fibers express strong F-α-DG. Herein, we investigated the restoration of F-α-DG in the FKRP mutant muscles and showed that the restoration of glycosylation is associated with muscle regeneration and dependent on the expression of both like-glycosyltransferase (LARGE) and partially functional FKRP. F-α-DG in the regenerating fibers reaches up to normal levels and lasts for >4 weeks, but no up-regulation of the LARGE and FKRP is detected during the regeneration process. The FKRP protein with P448L mutation is sufficient for functional glycosylation of α-DG in regenerating fibers, but not in mature fibers. Thus, factors other than FKRP enable regenerating fibers to produce functional α-DG, compensating for the defect in FKRP function. Identification of factors other than LARGE and FKRP could generate new approaches for restoration of F-α-DG in mature muscle fibers with defects in FKRP functions.

Muscle and Heart Function Restoration in a Limb Girdle Muscular Dystrophy 2I (LGMD2I) Mouse Model by Systemic FKRP Gene Delivery

Mutations in fukutin-related protein (FKRP) gene cause a wide spectrum of disease phenotypes including the mild limb-girdle muscular dystrophy 2I (LGMD2I), the severe Walker-Warburg syndrome, and muscle-eye-brain disease. FKRP deficiency results in α-dystroglycan (α-DG) hypoglycosylation in the muscle and heart, which is a biochemical hallmark of dystroglycanopathies. To study gene replacement therapy, we generated and characterized a new mouse model of LGMD2I harboring the human mutation leucine 276 to isoleucine (L276I) in the mouse alleles. The homozygous knock-in mice (L276I(KI)) mimic the classic late onset phenotype of LGMD2I in both skeletal and cardiac muscles. Systemic delivery of human FKRP gene by AAV9 vector in the L276I(KI) mice, at either neonatal age or at the age of 9 months, rendered body wide FKRP expression and restored glycosylation of α-DG in both skeletal and cardiac muscles. FKRP gene therapy ameliorated dystrophic pathology and cardiomyopathy such as muscle degeneration, fibrosis, and myofiber membrane leakage, resulting in restoration of muscle and heart contractile functions. Thus, these results demonstrated that the treatment based on FKRP gene replacement was effective.