Whole exome sequencing in foetal akinesia expands the genotype-phenotype spectrum of GBE1 glycogen storage disease mutations

Severe neuromuscular forms of glycogen storage disease type IV: Histological, clinical, biochemical, and molecular findings in a large French case series

Glycogen storage disease type IV (GSD IV), also called Andersen disease, or amylopectinosis, is a highly heterogeneous autosomal recessive disorder caused by a glycogen branching enzyme (GBE, 1,4-alpha-glucan branching enzyme) deficiency secondary to pathogenic variants on GBE1 gene. The incidence is evaluated to 1:600 000 to 1:800 000 of live births. GBE deficiency leads to an excessive deposition of structurally abnormal, amylopectin-like glycogen in affected tissues (liver, skeletal muscle, heart, nervous system, etc.). Diagnosis is often guided by histological findings and confirmed by GBE activity deficiency and molecular studies. Severe neuromuscular forms of GSD IV are very rare and of disastrous prognosis. Identification and characterization of these forms are important for genetic counseling for further pregnancies. Here we describe clinical, histological, enzymatic, and molecular findings of 10 cases from 8 families, the largest case series reported so far, of severe neuromuscular forms of GSD IV along with a literature review. Main antenatal features are: fetal akinesia deformation sequence or arthrogryposis/joint contractures often associated with muscle atrophy, decreased fetal movement, cystic hygroma, and/or hydrops fetalis. If pregnancy is carried to term, the main clinical features observed at birth are severe hypotonia and/or muscle atrophy, with the need for mechanical ventilation, cardiomyopathy, retrognathism, and arthrogryposis. All our patients were stillborn or died within 1 month of life. In addition, we identified five novel GBE1 variants.

Glycogen storage disease type IV without detectable polyglucosan bodies: importance of broad gene panels

Glycogen storage disease type IV (GSD IV) is caused by mutations in the glycogen branching enzyme 1 (GBE1) gene and is characterized by accumulation of polyglucosan bodies in liver, muscle and other tissues. We report three cases with neuromuscular forms of GSD IV, none of whom had polyglucosan bodies on muscle biopsy. The first case had no neonatal problems and presented with delayed walking. The other cases presented at birth: one with arthrogryposis, hypotonia, and respiratory distress, the other with talipes and feeding problems. All developed a similar pattern of axial weakness, proximal upper limb weakness and scapular winging, and much milder proximal lower limb weakness. Our cases expand the phenotypic spectrum of neuromuscular GSD IV, highlight that congenital myopathy and limb girdle weakness can be caused by mutations in GBE1, and emphasize that GSD IV should be considered even in the absence of characteristic polyglucosan bodies on muscle biopsy.

An Unusual Case of Neonatal Hypotonia and Femur Fracture: Neuromuscular Variant of Glycogen Storage Disease Type IV

Glycogen storage disease type IV (GSD IV) (OMIM #232500) is an autosomal recessive disorder caused by deficiency of the glycogen-branching enzyme. Here, we report a patient presenting with prematurity and severe hypotonia resulting from a complicated pregnancy with polyhydramnios. During her stay in the neonatal unit, the infant remained dependent on a ventilator, and her movements were mostly absent, except for occasional small movements of her fingers. A spontaneous fracture of femur shaft occurred in the postnatal fourth week. Whole-exome sequencing of DNA from the patient revealed a homozygous missense variant in the GBE1 gene (c.1693C>T, p.Arg565Trp). The variation detected in the index case was also confirmed by Sanger sequencing in the patient and respective parents. This study showed that the neuromuscular subtypes of GSD-IV should be considered as a possible differential diagnosis in severe neonatal hypotonia cases.

Deficit of homozygosity among 1.52 million individuals and genetic causes of recessive lethality

Genotypes causing pregnancy loss and perinatal mortality are depleted among living individuals and are therefore difficult to find. To explore genetic causes of recessive lethality, we searched for sequence variants with deficit of homozygosity among 1.52 million individuals from six European populations. In this study, we identified 25 genes harboring protein-altering sequence variants with a strong deficit of homozygosity (10% or less of predicted homozygotes). Sequence variants in 12 of the genes cause Mendelian disease under a recessive mode of inheritance, two under a dominant mode, but variants in the remaining 11 have not been reported to cause disease. Sequence variants with a strong deficit of homozygosity are over-represented among genes essential for growth of human cell lines and genes orthologous to mouse genes known to affect viability. The function of these genes gives insight into the genetics of intrauterine lethality. We also identified 1077 genes with homozygous predicted loss-of-function genotypes not previously described, bringing the total set of genes completely knocked out in humans to 4785.

Fetal arthrogryposis-what do we tell the prospective parents?

Arthrogryposis, also termed arthrogryposis multiplex congenita, is a descriptive term for conditions with multiple congenital contractures (MCC). The etiology is extremely heterogeneous. More than 400 specific disorders have been identified so far, which may lead to or are associated with MCC and/or fetal hypo- and akinesia as a clinical sign. With improved sensitivity of prenatal ultrasound and expanding prenatal diagnostic options, clinicians are tasked with providing early detection in order to counsel the prospective parents regarding further prenatal diagnostic as well as management options. We summarize the most important knowledge to raise awareness for early detection in pregnancy. We review essential points for counseling when MCC is detected in order to provide answers to common questions, which, however, cannot replace interdisciplinary expert opinion in the individual case.

Whole-exome sequencing reveals a role of HTRA1 and EGFL8 in brain white matter hyperintensities

White matter hyperintensities (WMH) are among the most common radiological abnormalities in the ageing population and an established risk factor for stroke and dementia. While common variant association studies have revealed multiple genetic loci with an influence on their volume, the contribution of rare variants to the WMH burden in the general population remains largely unexplored. We conducted a comprehensive analysis of this burden in the UK Biobank using publicly available whole-exome sequencing data (n up to 17 830) and found a splice-site variant in GBE1, encoding 1,4-alpha-glucan branching enzyme 1, to be associated with lower white matter burden on an exome-wide level [c.691+2T>C, β = -0.74, standard error (SE) = 0.13, P = 9.7 × 10-9]. Applying whole-exome gene-based burden tests, we found damaging missense and loss-of-function variants in HTRA1 (frequency of 1 in 275 in the UK Biobank population) to associate with an increased WMH volume (P = 5.5 × 10-6, false discovery rate = 0.04). HTRA1 encodes a secreted serine protease implicated in familial forms of small vessel disease. Domain-specific burden tests revealed that the association with WMH volume was restricted to rare variants in the protease domain (amino acids 204-364; β = 0.79, SE = 0.14, P = 9.4 × 10-8). The frequency of such variants in the UK Biobank population was 1 in 450. The WMH volume was brought forward by ∼11 years in carriers of a rare protease domain variant. A comparison with the effect size of established risk factors for WMH burden revealed that the presence of a rare variant in the HTRA1 protease domain corresponded to a larger effect than meeting the criteria for hypertension (β = 0.26, SE = 0.02, P = 2.9 × 10-59) or being in the upper 99.8% percentile of the distribution of a polygenic risk score based on common genetic variants (β = 0.44, SE = 0.14, P = 0.002). In biochemical experiments, most (6/9) of the identified protease domain variants resulted in markedly reduced protease activity. We further found EGFL8, which showed suggestive evidence for association with WMH volume (P = 1.5 × 10-4, false discovery rate = 0.22) in gene burden tests, to be a direct substrate of HTRA1 and to be preferentially expressed in cerebral arterioles and arteries. In a phenome-wide association study mapping ICD-10 diagnoses to 741 standardized Phecodes, rare variants in the HTRA1 protease domain were associated with multiple neurological and non-neurological conditions including migraine with aura (odds ratio = 12.24, 95%CI: 2.54-35.25; P = 8.3 × 10-5]. Collectively, these findings highlight an important role of rare genetic variation and the HTRA1 protease in determining WMH burden in the general population.

The Clinical and Genotypic Spectrum of Scoliosis in Multiple Pterygium Syndrome: A Case Series on 12 Children

Background: Multiple pterygium syndrome (MPS) is a genetically heterogeneous rare form of arthrogryposis multiplex congenita characterized by joint contractures and webbing or pterygia, as well as distinctive facial features related to diminished fetal movement. It is divided into prenatally lethal (LMPS, MIM253290) and nonlethal (Escobar variant MPS, MIM 265000) types. Developmental spine deformities are common, may present early and progress rapidly, requiring regular fo llow-up and orthopedic management. Methods: Retrospective chart review and prospective data collection were conducted at three hospital centers. Molecular diagnosis was confirmed with whole exome or whole genome sequencing. Results: This case series describes the clinical features and scoliosis treatment on 12 patients from 11 unrelated families. A molecular diagnosis was confirmed in seven; two with MYH3 variants and five with CHRNG. Scoliosis was present in all but our youngest patient. The remaining 11 patients spanned the spectrum between mild (curve ≤ 25°) and malignant scoliosis (≥50° curve before 4 years of age); the two patients with MYH3 mutations presented with malignant scoliosis. Bracing and serial spine casting appear to be beneficial for a few years; non-fusion spinal instrumentation may be needed to modulate more severe curves during growth and spontaneous spine fusions may occur in those cases. Conclusions: Molecular diagnosis and careful monitoring of the spine is needed in children with MPS.

GMPPB-congenital disorders of glycosylation associate with decreased enzymatic activity of GMPPB

The congenital disorders of glycosylation (CDG) are a family of metabolic diseases in which glycosylation of proteins or lipids is deficient. GDP-mannose pyrophosphorylase B (GMPPB) mutations lead to CDG, characterized by neurological and muscular defects. However, the genotype-phenotype correlation remains elusive, limiting our understanding of the underlying mechanism and development of therapeutic strategy. Here, we report a case of an individual presenting congenital muscular dystrophy with cerebellar involvement, who presents two heterozygous GMPPB mutations (V111G and G214S). The V111G mutation significantly decreases GMPPB’s enzymatic activity. By measuring enzymatic activities of 17 reported GMPPB mutants identified in patients diagnosed with GMPPB-CDG, we discover that all tested GMPPB variants exhibit significantly decreased enzymatic activity. Using a zebrafish model, we find that Gmppb is required for neuronal and muscle development, and further demonstrate that enzymatic activity of GMPPB mutants correlates with muscular and neuronal phenotypes in zebrafish. Taken together, our findings discover the importance of GMPPB enzymatic activity for the pathogenesis of GMPPB-CDG, and shed light for the development of additional indicators and therapeutic strategy.

Phenotypic spectrum and genomics of undiagnosed arthrogryposis multiplex congenita

BACKGROUND

Arthrogryposis multiplex congenita (AMC) is characterised by congenital joint contractures in two or more body areas. AMC exhibits wide phenotypic and genetic heterogeneity. Our goals were to improve the genetic diagnosis rates of AMC, to evaluate the added value of whole exome sequencing (WES) compared with targeted exome sequencing (TES) and to identify new genes in 315 unrelated undiagnosed AMC families.

METHODS

Several genomic approaches were used including genetic mapping of disease loci in multiplex or consanguineous families, TES then WES. Sanger sequencing was performed to identify or validate variants.

RESULTS

We achieved disease gene identification in 52.7% of AMC index patients including nine recently identified genes (CNTNAP1, MAGEL2, ADGRG6, ADCY6, GLDN, LGI4, LMOD3, UNC50 and SCN1A). Moreover, we identified pathogenic variants in ASXL3 and STAC3 expanding the phenotypes associated with these genes. The most frequent cause of AMC was a primary involvement of skeletal muscle (40%) followed by brain (22%). The most frequent mode of inheritance is autosomal recessive (66.3% of patients). In sporadic patients born to non-consanguineous parents (n=60), de novo dominant autosomal or X linked variants were observed in 30 of them (50%).

CONCLUSION

New genes recently identified in AMC represent 21% of causing genes in our cohort. A high proportion of de novo variants were observed indicating that this mechanism plays a prominent part in this developmental disease. Our data showed the added value of WES when compared with TES due to the larger clinical spectrum of some disease genes than initially described and the identification of novel genes.

Adult polyglucosan body disease: an acute presentation leading to unmasking of this rare disorder

Introduction: Adult polyglucosan body disease (APBD) is an autosomal recessive leukodystrophy caused by abnormal intracellular accumulation of glycogen byproducts. This disorder is linked to a deficiency in glycogen branching enzyme-1 (GBE-1). Neurologic manifestations include upper and lower motor neuron signs, dementia, and peripheral neuropathy. APBD is typically a progressive disease. In this report, we discuss a novel case of APBD in a patient who had a sudden onset of spastic quadriparesis preceded by gradual difficulty with gait. Genetic and postmortem analysis confirmed the diagnosis of APBD.Case report: A 65-year-old man was evaluated for a new-onset of spastic quadriparesis, right-gaze preference, and left-sided beat nystagmus. Magnetic resonance imaging (MRI) of the brain revealed areas of white matter hyperintensities most prominent in the brainstem and periventricular regions. MRI of the cervical spine showed marked cord atrophy. Laboratory workup and cerebrospinal fluid analysis were unremarkable. Genetic testing supported the diagnosis of APBD due to GBE-1 deficiency. Postmortem analysis showed multiple white matter abnormalities suggestive of a leukodystrophy syndrome, and histopathologic testing revealed abnormal accumulation of polyglucosan bodies in samples from the patient's central nervous system supporting the diagnosis of APBD.Conclusion: APBD is a rare disorder that can affect the nervous system. The diagnosis can be confirmed with a combination of genetic testing and pathologic analysis of affected brain tissue.

A Pooling Genome-Wide Association Study Identifies Susceptibility Loci and Signaling Pathways of Immune Thrombocytopenia in Chinese Han Population

Immune thrombocytopenia (ITP) is an acquired bleeding disease due to immune-mediated destruction of antilogous platelets and ineffective thrombopoiesis. Although the etiology of ITP remains unknown, genetic variants are thought to predispose individuals to the disease. Several candidate gene analyses have identified several loci that increased ITP susceptibility, but no systematic genetic analysis on a genome-wide scope. To extend the genetic evidence and to identify novel candidates of ITP, we performed a pooling genome-wide association study (GWAS) by IlluminaHumanOmniZhongHua-8 combining pathway analysis in 200 ITP cases and 200 controls from Chinese Han population (CHP). The results revealed that 4 novel loci (rs117503120, rs5998634, rs4483616, and rs16866133) were strongly associated with ITP (P < 1.0 × 10⁻⁷). Expect for rs4483616, other three loci were validated by the TaqMan probe genotyping assay (P < 0.05) in another cohort including 250 ITP cases and 250 controls. And rs5998634 T allele was more sensitive to glucocorticoids for ITP patients (χ² = 7.30, P < 0.05). Moreover, we identified three overrepresented signaling pathways including the neuroactive ligand-receptor interaction, pathways in cancer, and the JAK-STAT pathway, which involved in the etiology of ITP. In conclusion, our results revealed four novel loci and three pathways related to ITP and provided new clues to explore the pathogenesis of ITP.

Fetal arthrogryposis: Challenges and perspectives for prenatal detection and management

Antenatal identification of fetuses with multiple congenital contractures or arthrogryposis multiplex congenita (AMC) may be challenging. The first clinical sign is often reduced fetal movement and/or contractures, as seen on prenatal ultrasounds. This can be apparent at any point, from early to late pregnancy, may range from mild to severe involvement, with or without associated other structural anomalies. Possible etiologies and their prognosis need to be interpreted with respect to developmental timing. The etiology of AMC is highly heterogeneous and making the specific diagnosis will guide prognosis, counseling and prenatal and perinatal management. Current ultrasound practice identifies only approximately 25% of individuals with arthrogryposis prenatally before 24 weeks of pregnancy in a general obstetrics care population. There are currently no studies and guidelines that address the question of when and how to assess for fetal contractures and movements during pregnancy. The failure to identify fetuses with arthrogryposis before 24 weeks of pregnancy means that physicians and families are denied reproductive options and interventions that may improve outcome. We review current practice and recommend adjusting the current prenatal imaging and genetic diagnostic strategies to achieve early prenatal detection and etiologic diagnosis. We suggest exploring options for in utero therapy to increase fetal movement for ongoing pregnancies.

Novel pathogenic variants in GBE1 causing fetal akinesia deformation sequence and severe neuromuscular form of glycogen storage disease type IV

Glycogen storage disease IV (GSD IV), caused by a defect in GBE1, is a clinically heterogeneous disorder. A classical hepatic form and a neuromuscular form have been described. The severe neuromuscular form presents as a fetal akinesia deformation sequence or a congenital subtype. We ascertained three unrelated families with fetuses/neonates who presented with fetal akinesia deformation sequence to our clinic for genetic counseling. We performed a detailed clinical evaluation, exome sequencing, and histopathology examination of two fetuses and two neonates from three unrelated families presenting with these perinatally lethal neuromuscular forms of GSD IV. Exome sequencing in the affected fetuses/neonates identified four novel pathogenic variants (c.1459G>T, c.144-1G>A, c.1680C>G, and c.1843G>C) in GBE1 (NM_000158). Histopathology examination of tissues from the affected fetuses/neonate was consistent with the diagnosis. Here, we add three more families with the severe perinatally lethal neuromuscular forms of GSD IV to the GBE1 mutation spectrum.

Glycogen Storage Disease Type IV: A Rare Cause for Neuromuscular Disorders or Often Missed?

Advancements in genetic testing now allow early identification of previously unresolved neuromuscular phenotypes. To illustrate this, we here present diagnoses of glycogen storage disease IV (GSD IV) in two patients with hypotonia and delayed development of gross motor skills. Patient 1 was diagnosed with congenital myopathy based on a muscle biopsy at the age of 6 years. The genetic cause of his disorder (two compound heterozygous missense mutations in GBE1 (c.[760A>G] p.[Thr254Ala] and c.[1063C>T] p.[Arg355Cys])), however, was only identified at the age of 17, after panel sequencing of 314 genes associated with neuromuscular disorders. Thanks to the availability of next-generation sequencing, patient 2 was diagnosed before the age of 2 with two compound heterozygous mutations in GBE1 (c.[691+2T>C] (splice donor variant) and the same c.[760A>G] p.[Thr254Ala] mutation as patient 1). GSD IV is an autosomal recessive metabolic disorder with a broad and expanding clinical spectrum, which hampers targeted diagnostics. The current cases illustrate the value of novel genetic testing for rare genetic disorders with neuromuscular phenotypes, especially in case of clinical heterogeneity. We argue that genetic testing by gene panels or whole exome sequencing should be considered early in the diagnostic procedure of unresolved neuromuscular disorders.

Pena-Shokeir syndrome: current management strategies and palliative care

Pena-Shokeir syndrome (PSS) type 1, also known as fetal akinesia deformation sequence, is a rare genetic syndrome that almost always results in intrauterine or early neonatal death. It is characterized by markedly decreased fetal movements, intrauterine growth restriction, joint contractures, short umbilical cord, and features of pulmonary hypoplasia. Antenatal diagnosis can be difficult. Ultrasound features are varied and may overlap with those of Trisomy 18. The poor prognosis of PSS is due to pulmonary hypoplasia, which is an important feature that distinguishes PSS from arthrogryposis multiplex congenital without pulmonary hypoplasia, which has a better prognosis. If diagnosed in the antenatal period, a late termination of pregnancy can be considered following ethical discussion (if the law allows). In most cases, a diagnosis is only made in the neonatal period. Parents of a baby affected with PSS require detailed counseling that includes information on the imprecise recurrence risks and a plan for subsequent pregnancies.

Analysis of GBE1 mutations via protein expression studies in glycogen storage disease type IV: A report on a non-progressive form with a literature review

Background

Glycogen storage disease type IV (GSD IV), caused by GBE1 mutations, has a quite wide phenotypic variation. While the classic hepatic form and the perinatal/neonatal neuromuscular forms result in early mortality, milder manifestations include non-progressive form (NP-GSD IV) and adult polyglucosan body disease (APBD). Thus far, only one clinical case of a patient with compound heterozygous mutations has been reported for the molecular analysis of NP-GSD IV. This study aimed to elucidate the molecular basis in a NP-GSD IV patient via protein expression analysis and to obtain a clearer genotype-phenotype relationship in GSD IV.

Case presentation

A Japanese boy presented hepatosplenomegaly at 2 years of age. Developmental delay, neurological symptoms, and cardiac dysfunction were not apparent. Observation of hepatocytes with periodic acid-Schiff-positive materials resistant to diastase, coupled with resolution of hepatosplenomegaly at 8 years of age, yielded a diagnosis of NP-GSD IV. Glycogen branching enzyme activity was decreased in erythrocytes. At 13 years of age, he developed epilepsy, which was successfully controlled by carbamazepine.

Molecular analysis

In this study, we identified compound heterozygous GBE1 mutations (p.Gln46Pro and p.Glu609Lys). The branching activities of the mutant proteins expressed using E. coli were examined in a reaction with starch. The result showed that both mutants had approximately 50% activity of the wild type protein.

Conclusion

This is the second clinical report of a NP-GSD IV patient with a definite molecular elucidation. Based on the clinical and genotypic overlapping between NP-GSD IV and APBD, we suggest both are in a continuum.

New era in genetics of early-onset muscle disease: Breakthroughs and challenges

Early-onset muscle disease includes three major entities that present generally at or before birth: congenital myopathies, congenital muscular dystrophies and congenital myasthenic syndromes. Almost exclusively there is weakness and hypotonia, although cases manifesting hypertonia are increasingly being recognised. These diseases display a wide phenotypic and genetic heterogeneity, with the uptake of next generation sequencing resulting in an unparalleled extension of the phenotype-genotype correlations and "diagnosis by sequencing" due to unbiased sequencing. Perhaps now more than ever, detailed clinical evaluations are necessary to guide the genetic diagnosis; with arrival at a molecular diagnosis frequently occurring following dialogue between the molecular geneticist, the referring clinician and the pathologist. There is an ever-increasing blurring of the boundaries between the congenital myopathies, dystrophies and myasthenic syndromes. In addition, many novel disease genes have been described and new insights have been gained into skeletal muscle development and function. Despite the advances made, a significant percentage of patients remain without a molecular diagnosis, suggesting that there are many more human disease genes and mechanisms to identify. It is now technically- and clinically-feasible to perform next generation sequencing for severe diseases on a population-wide scale, such that preconception-carrier screening can occur. Newborn screening for selected early-onset muscle diseases is also technically and ethically-achievable, with benefits to the patient and family from early management of these diseases and should also be implemented. The need for world-wide Reference Centres to meticulously curate polymorphisms and mutations within a particular gene is becoming increasingly apparent, particularly for interpretation of variants in the large genes which cause early-onset myopathies: NEB, RYR1 and TTN. Functional validation of candidate disease variants is crucial for accurate interpretation of next generation sequencing and appropriate genetic counseling. Many published "pathogenic" variants are too frequent in control populations and are thus likely rare polymorphisms. Mechanisms need to be put in place to systematically update the classification of variants such that accurate interpretation of variants occurs. In this review, we highlight the recent advances made and the challenges ahead for the molecular diagnosis of early-onset muscle diseases.

A novel neuromuscular form of glycogen storage disease type IV with arthrogryposis, spinal stiffness and rare polyglucosan bodies in muscle

Glycogen storage disease type IV (GSD IV) is an autosomal recessive disorder causing polyglucosan storage in various tissues. Neuromuscular forms present with fetal akinesia deformation sequence, lethal myopathy, or mild hypotonia and weakness. A 3-year-old boy presented with arthrogryposis, motor developmental delay, weakness, and rigid spine. Whole body MRI revealed fibroadipose muscle replacement but sparing of the sartorius, gracilis, adductor longus and vastus intermedialis muscles. Polyglucosan bodies were identified in muscle, and GBE1 gene analysis revealed two pathogenic variants. We describe a novel neuromuscular GSD IV phenotype and confirm the importance of muscle morphological studies in early onset neuromuscular disorders.

Next generation sequencing in a large cohort of patients presenting with neuromuscular disease before or at birth

BACKGROUND

Fetal akinesia/hypokinesia, arthrogryposis and severe congenital myopathies are heterogeneous conditions usually presenting before or at birth. Although numerous causative genes have been identified for each of these disease groups, in many cases a specific genetic diagnosis remains elusive. Due to the emergence of next generation sequencing, virtually the entire coding region of an individual's DNA can now be analysed through "whole" exome sequencing, enabling almost all known and novel disease genes to be investigated for disorders such as these.

METHODS

Genomic DNA samples from 45 patients with fetal akinesia/hypokinesia, arthrogryposis or severe congenital myopathies from 38 unrelated families were subjected to next generation sequencing. Clinical features and diagnoses for each patient were supplied by referring clinicians. Genomic DNA was used for either whole exome sequencing or a custom-designed neuromuscular sub-exomic supercapture array containing 277 genes responsible for various neuromuscular diseases. Candidate disease-causing variants were investigated and confirmed using Sanger sequencing. Some of the cases within this cohort study have been published previously as separate studies.

RESULTS

A conclusive genetic diagnosis was achieved for 18 of the 38 families. Within this cohort, mutations were found in eight previously known neuromuscular disease genes (CHRND, CHNRG, ECEL1, GBE1, MTM1, MYH3, NEB and RYR1) and four novel neuromuscular disease genes were identified and have been published as separate reports (GPR126, KLHL40, KLHL41 and SPEG). In addition, novel mutations were identified in CHRND, KLHL40, NEB and RYR1. Autosomal dominant, autosomal recessive, X-linked, and de novo modes of inheritance were observed.

CONCLUSIONS

By using next generation sequencing on a cohort of 38 unrelated families with fetal akinesia/hypokinesia, arthrogryposis, or severe congenital myopathy we therefore obtained a genetic diagnosis for 47% of families. This study highlights the power and capacity of next generation sequencing (i) to determine the aetiology of genetically heterogeneous neuromuscular diseases, (ii) to identify novel disease genes in small pedigrees or isolated cases and (iii) to refine the interplay between genetic diagnosis and clinical evaluation and management.

Exome sequencing positively identified relevant alterations in more than half of cases with an indication of prenatal ultrasound anomalies

OBJECTIVE

Exome sequencing is a successful option for diagnosing individuals with previously uncharacterized genetic conditions, however little has been reported regarding its utility in a prenatal setting. The goal of this study is to describe the results from a cohort of fetuses for which exome sequencing was performed.

METHODS

We performed a retrospective analysis of the first seven cases referred to our laboratory for exome sequencing following fetal demise or termination of pregnancy. All seven pregnancies had multiple congenital anomalies identified by level II ultrasound. Exome sequencing was performed on trios using cultured amniocytes or products of conception from the affected fetuses.

RESULTS

Relevant alterations were identified in more than half of the cases (4/7). Three of the four were categorized as 'positive' results, and one of the four was categorized as a 'likely positive' result. The provided diagnoses included osteogenesis imperfecta II (COL1A2), glycogen storage disease IV (GBE1), oral-facial-digital syndrome 1 (OFD1), and RAPSN-associated fetal akinesia deformation sequence.

CONCLUSION

This data suggests that exome sequencing is likely to be a valuable diagnostic testing option for pregnancies with multiple congenital anomalies detected by prenatal ultrasound; however, additional studies with larger cohorts of affected pregnancies are necessary to confirm these findings.

A germline MTOR mutation in Aboriginal Australian siblings with intellectual disability, dysmorphism, macrocephaly, and small thoraces

We report on three Aboriginal Australian siblings with a unique phenotype which overlaps with known megalencephaly syndromes and RASopathies, including Costello syndrome. A gain-of-function mutation in MTOR was identified and represents the first reported human condition due to a germline, familial MTOR mutation. We describe the findings in this family to highlight that (i) the path to determination of pathogenicity was confounded by the lack of genomic reference data for Australian Aboriginals and that (ii) the disease biology, functional analyses in this family, and studies on the tuberous sclerosis complex support consideration of an mTOR inhibitor as a therapeutic agent.

Expanding the phenotype of GMPPB mutations

Dystroglycanopathies are a heterogeneous group of diseases with a broad phenotypic spectrum ranging from severe disorders with congenital muscle weakness, eye and brain structural abnormalities and intellectual delay to adult-onset limb-girdle muscular dystrophies without mental retardation. Most frequently the disease onset is congenital or during childhood. The exception is FKRP mutations, in which adult onset is a common presentation. Here we report eight patients from five non-consanguineous families where next generation sequencing identified mutations in the GMPPB gene. Six patients presented as an adult or adolescent-onset limb-girdle muscular dystrophy, one presented with isolated episodes of rhabdomyolysis, and one as a congenital muscular dystrophy. This report expands the phenotypic spectrum of GMPPB mutations to include limb-girdle muscular dystrophies with adult onset with or without intellectual disability, or isolated rhabdomyolysis.

Adult onset distal and proximal myopathy with complete ophthalmoplegia associated with a novel de novo p.(Leu1877Pro) mutation in MYH2

An MYH2 mutation p.(Glu706Lys) was originally described in a family with autosomal dominant inheritance, where the affected family members presented with multiple congenital contractures and ophthalmoplegia, progressing to a proximal myopathy in adulthood. Another patient with a dominant mutation p.(Leu1870Pro) was described, presenting as a congenital myopathy with ophthalmoplegia. Here, we present a patient with symptoms beginning at age 16 years, of prominent distal but also proximal weakness, bulbar involvement and ophthalmoplegia. Initially, clinically classified as oculopharyngodistal myopathy, the patient was found to carry a novel, de novo MYH2 mutation c.5630T>C p.(Leu1877Pro). This expands the phenotype of dominant MYH2 myopathies with the clinical phenotype overlapping the oculopharyngodistal myopathy spectrum.

Exome sequencing for gene discovery in lethal fetal disorders--harnessing the value of extreme phenotypes

Massively parallel sequencing has revolutionized our understanding of Mendelian disorders, and many novel genes have been discovered to cause disease phenotypes when mutant. At the same time, next-generation sequencing approaches have enabled non-invasive prenatal testing of free fetal DNA in maternal blood. However, little attention has been paid to using whole exome and genome sequencing strategies for gene identification in fetal disorders that are lethal in utero, because they can appear to be sporadic and Mendelian inheritance may be missed. We present challenges and advantages of applying next-generation sequencing approaches to gene discovery in fetal malformation phenotypes and review recent successful discovery approaches. We discuss the implication and significance of recessive inheritance and cross-species phenotyping in fetal lethal conditions. Whole exome sequencing can be used in individual families with undiagnosed lethal congenital anomaly syndromes to discover causal mutations, provided that prior to data analysis, the fetal phenotype can be correlated to a particular developmental pathway in embryogenesis. Cross-species phenotyping allows providing further evidence for causality of discovered variants in genes involved in those extremely rare phenotypes and will increase our knowledge about normal and abnormal human developmental processes. Ultimately, families will benefit from the option of early prenatal diagnosis.

New insights in the field of muscle glycogenoses

PURPOSE OF REVIEW

This review highlights recent contributions regarding clinical heterogeneity, pathogenic mechanisms, therapeutic trials, and animal models of the muscle glycogenoses.

RECENT FINDINGS

Most recent publications have dealt with the clinical effects of enzyme replacement therapy (ERT) in glycogenosis type II (Pompe disease), including the cognitive development of children with the infantile form who have reached school age. Standardized exercise testing has shown the similarity between McArdle disease and one of the most recently described muscle glycogenoses, phosphoglucomutase deficiency. Cycle ergometry in patients with glycogenosis type III (debrancher deficiency) without overt weakness has documented exercise intolerance relieved by glucose infusion, consistent with the glycogenolytic block. A mouse model of McArdle disease faithfully recapitulates most features of the human disease and will prove valuable for a better understanding of pathogenesis and therapeutic modalities. Polyglucosan body myopathy with cardiomyopathy has been associated with mutations in RBCK1, a ubiquitin ligase, which have also been reported in children with early-onset immune disorder. The role of polyglucosan storage in muscle and in both central and peripheral nervous systems has been confirmed in the infantile and late-onset forms of glycogenosis type IV (brancher enzyme deficiency). Additional novel findings include the involvement of the heart in one patient with phosphofructokinase (PFK) deficiency and the presence of tubular aggregates in a manifesting heterozygote with phosphoglycerate mutase deficiency.

SUMMARY

Important recent developments in the field of muscle glycogenoses include a new disease entity, a new animal model of McArdle disease, and better knowledge of the pathogenesis in some glycogenoses and of the long-term effects of enzyme replacement therapy in Pompe disease.

Branching enzyme deficiency: expanding the clinical spectrum

IMPORTANCE

The neuromuscular presentation of glycogen branching enzyme deficiency includes a severe infantile form and a late-onset variant known as adult polyglucosan body disease. Herein, we describe 2 patients with adult acute onset of fluctuating neurological signs and brain magnetic resonance imaging lesions simulating multiple sclerosis. A better definition of this new clinical entity is needed to facilitate diagnosis.

OBJECTIVES

To describe the clinical presentation and progression of a new intermediate variant of glycogen branching enzyme deficiency and to discuss genotype-phenotype correlations.

DESIGN, SETTING, AND PARTICIPANTS

Clinical, biochemical, morphological, and molecular study of 2 patients followed up for 6 years and 8 years at academic medical centers. The participants were 2 patients of non-Ashkenazi descent with adult acute onset of neurological signs initially diagnosed as multiple sclerosis.

MAIN OUTCOMES AND MEASURES

Clinical course, muscle and nerve morphology, longitudinal study of brain magnetic resonance imaging, and glycogen branching enzyme activity and GBE1 molecular analysis.

RESULTS

Molecular analysis showed that one patient was homozygous (c.1544G>A) and the other patient was compound heterozygous (c.1544G>A and c.1961-1962delCA) for GBE1 mutations. Residual glycogen branching enzyme activity was 16% and 30% of normal in leukocytes. Both patients manifested acute episodes of transient neurological symptoms, and neurological impairment was mild at age 45 years and 53 years. Brain magnetic resonance imaging revealed nonprogressive white matter lesions and spinocerebellar atrophy similar to typical adult polyglucosan body disease.

CONCLUSIONS AND RELEVANCE

GBE1 mutations can cause an early adult-onset relapsing-remitting form of polyglucosan body disease distinct from adult polyglucosan body disease in several ways, including younger age at onset, history of infantile liver involvement, and subacute and remitting course simulating multiple sclerosis. This should orient neurologists toward the correct diagnosis.

Mutations in KLHL40 are a frequent cause of severe autosomal-recessive nemaline myopathy

Nemaline myopathy (NEM) is a common congenital myopathy. At the very severe end of the NEM clinical spectrum are genetically unresolved cases of autosomal-recessive fetal akinesia sequence. We studied a multinational cohort of 143 severe-NEM-affected families lacking genetic diagnosis. We performed whole-exome sequencing of six families and targeted gene sequencing of additional families. We identified 19 mutations in KLHL40 (kelch-like family member 40) in 28 apparently unrelated NEM kindreds of various ethnicities. Accounting for up to 28% of the tested individuals in the Japanese cohort, KLHL40 mutations were found to be the most common cause of this severe form of NEM. Clinical features of affected individuals were severe and distinctive and included fetal akinesia or hypokinesia and contractures, fractures, respiratory failure, and swallowing difficulties at birth. Molecular modeling suggested that the missense substitutions would destabilize the protein. Protein studies showed that KLHL40 is a striated-muscle-specific protein that is absent in KLHL40-associated NEM skeletal muscle. In zebrafish, klhl40a and klhl40b expression is largely confined to the myotome and skeletal muscle, and knockdown of these isoforms results in disruption of muscle structure and loss of movement. We identified KLHL40 mutations as a frequent cause of severe autosomal-recessive NEM and showed that it plays a key role in muscle development and function. Screening of KLHL40 should be a priority in individuals who are affected by autosomal-recessive NEM and who present with prenatal symptoms and/or contractures and in all Japanese individuals with severe NEM.