Monoallelic KIF1A-related disorders: a multicenter cross sectional study and systematic literature review

BACKGROUND

Monoallelic variants in the KIF1A gene are associated with a large set of clinical phenotypes including neurodevelopmental and neurodegenerative disorders, underpinned by a broad spectrum of central and peripheral nervous system involvement.

METHODS

In a multicenter study conducted in patients presenting spastic gait or complex neurodevelopmental disorders, we analyzed the clinical, genetic and neuroradiological features of 28 index cases harboring heterozygous variants in KIF1A. We conducted a literature systematic review with the aim to comparing our findings with previously reported KIF1A-related phenotypes.

RESULTS

Among 28 patients, we identified nine novel monoallelic variants, and one a copy number variation encompassing KIF1A. Mutations arose de novo in most patients and were prevalently located in the motor domain. Most patients presented features of a continuum ataxia-spasticity spectrum with only five cases showing a prevalently pure spastic phenotype and six presenting congenital ataxias. Seventeen mutations occurred in the motor domain of the Kinesin-1A protein, but location of mutation did not correlate with neurological and imaging presentations. When tested in 15 patients, muscle biopsy showed oxidative metabolism alterations (6 cases), impaired respiratory chain complexes II + III activity (3/6) and low CoQ10 levels (6/9). Ubiquinol supplementation (1gr/die) was used in 6 patients with subjective benefit.

CONCLUSIONS

This study broadened our clinical, genetic, and neuroimaging knowledge of KIF1A-related disorders. Although highly heterogeneous, it seems that manifestations of ataxia-spasticity spectrum disorders seem to occur in most patients. Some patients also present secondary impairment of oxidative metabolism; in this subset, ubiquinol supplementation therapy might be appropriate.

Bi-allelic variants in MDH2: Expanding the clinical phenotype

Bi-allelic alterations in the MDH2 gene have recently been reported in three unrelated toddlers with early-onset severe encephalopathy. Here, we describe a new case of a child carrying novel variants in MDH2. This child presented with early-onset encephalocardiopathy requiring heart transplant and showed cerebellar ataxia and drug-responsive epilepsy; his family history was significant for multiple cancers, a feature often associated with monoallelic variants in MDH2. Functional studies in cultured skin fibroblasts from the proband showed reduced protein levels and impaired enzyme activity, further corroborating the genetic results. The relatively mild neurological presentation and severe cardiac manifestations requiring heart transplant distinguish this case from previous reports. This patient thus expands the spectrum of clinical features associated with MDH2 variants.

Expanding the clinical and genetic spectrum of pathogenic variants in STIM1

INTRODUCTION/AIMS

Stromal interaction molecule 1 (STIM1) is a reticular Ca²⁺ sensor composed of a luminal and a cytosolic domain. Autosomal dominant mutations in STIM1 cause tubular aggregate myopathy and Stormorken syndrome or its variant York platelet syndrome. In this study we aimed to expand the features related to new variants in STIM1.

METHODS

We performed a cross-sectional study of individuals harboring monoallelic STIM1 variants recruited at five tertiary centers involved in a study of inherited myopathies analyzed with a multigene-targeted panel.

RESULTS

We identified seven individuals (age range, 26-57 years) harboring variants in STIM1, including five novel changes: three located in the EF-hand domain, one in the sterile α motif (SAM) domain, and one in the cytoplasmatic region of the protein. Functional evaluation of the pathogenic variants using a heterologous expression system and measuring store-operated calcium entry demonstrated their causative role and suggested a link of new variants with the clinical phenotype. Muscle contractures, found in three individuals, showed variability in body distribution and in the number of joints involved. Three patients showed cardiac and respiratory involvement. Short stature, hyposplenism, sensorineural hearing loss, hypothyroidism, and Gilbert syndrome were variably observed among the patients. Laboratory tests revealed hyperCKemia in six patients, thrombocytopenia in two patients, and hypocalcemia in one patient. Muscle biopsy showed the presence of tubular aggregates in three patients, type I fiber atrophy in one patient, and nonspecific myopathic changes in two patients.

DISCUSSION

Our clinical, histological, and molecular data expand the genetic and clinical spectrum of STIM1-related diseases.

Loss of ap4s1 in zebrafish leads to neurodevelopmental defects resembling spastic paraplegia 52

Autosomal recessive spastic paraplegia 52 is caused by biallelic mutations in AP4S1 which encodes a subunit of the adaptor protein complex 4 (AP‐4). Using next‐generation sequencing, we identified three novel unrelated SPG52 patients from a cohort of patients with cerebral palsy. The discovered variants in AP4S1 lead to reduced AP‐4 complex formation in patient‐derived fibroblasts. To further understand the role of AP4S1 in neuronal development and homeostasis, we engineered the first zebrafish model of AP‐4 deficiency using morpholino‐mediated knockdown of ap4s1. In this model, we discovered several phenotypes mimicking SPG52, including altered CNS development, locomotor deficits, and abnormal neuronal excitability.

Next-generation sequencing approach to hyperCKemia: A 2-year cohort study

Objective

Next-generation sequencing (NGS) was applied in molecularly undiagnosed asymptomatic or paucisymptomatic hyperCKemia to investigate whether this technique might allow detection of the genetic basis of the condition.

Methods

Sixty-six patients with undiagnosed asymptomatic or paucisymptomatic hyperCKemia, referred to tertiary neuromuscular centers over an approximately 2-year period, were analyzed using a customized, targeted sequencing panel able to investigate the coding exons and flanking intronic regions of 78 genes associated with limb-girdle muscular dystrophies, rhabdomyolysis, and metabolic and distal myopathies.

Results

A molecular diagnosis was reached in 33 cases, corresponding to a positive diagnostic yield of 50%. Variants of unknown significance were found in 17 patients (26%), whereas 16 cases (24%) remained molecularly undefined. The major features of the diagnosed cases were mild proximal muscle weakness (found in 27%) and myalgia (in 24%). Fourteen patients with a molecular diagnosis and mild myopathic features on muscle biopsy remained asymptomatic at a 24-month follow-up.

Conclusions

This study of patients with undiagnosed hyperCKemia, highlighting the advantages of NGS used as a first-tier diagnostic approach in genetically heterogeneous conditions, illustrates the ongoing evolution of molecular diagnosis in the field of clinical neurology. Isolated hyperCKemia can be the sole feature alerting to a progressive muscular disorder requiring careful surveillance.

Spinocerebellar ataxia 48 presenting with ataxia associated with cognitive, psychiatric, and extrapyramidal features: A report of two Italian families

INTRODUCTION

Spinocerebellar ataxia 48 has recently been described as an adult onset ataxia associated with a cerebellar cognitive affective syndrome, caused by a heterozygous mutation in the STUB1 gene.

METHODS

We characterized the clinical and neuroimaging phenotype of eight patients from two autosomal dominant ataxia multigenerational Italian families, in whom we conducted whole exome sequencing, targeted multigene sequencing, and Sanger sequencing studies.

RESULTS

We describe a complex syndrome characterized by ataxia and cognitive-psychiatric disorder in all cases, variably associated with chorea, parkinsonism, dystonia, urinary symptoms, and epilepsy. MRI showed a significant cerebellar atrophy, coupled to a T2-weighted hyperintensity affecting the dentate nuclei and extending to the middle cerebellar peduncles, whereas FDG-PET studies revealed glucose hypometabolism in cerebellum, striatum, and cerebral cortex. We identified two different novel STUB1 mutations segregating in the two families. One of the two mutations, p.(Gly33Ser), occurs in the TRP domain, whereas p.(Pro228Ser) is located in the ubiquitin ligase region.

DISCUSSION

We emphasize the similarity of the described clinical picture with that of SCAR16, an autosomal recessive ataxia caused by biallelic mutations in the same gene, and of spinocerebellar ataxia type 17, which is considered the main Huntington's disease-like syndrome. The pathogenesis of the disease and the relationship between SCA48 and SCAR16 remain to be clarified.

Complex multisystem phenotype associated with the mitochondrial DNA m.5522G>A mutation

Mitochondrial tRNAs are responsible for more than half of pathogenic point mutations in the mitochondrial genome (mtDNA). Different mutations give rise to widely differing phenotypes, ranging from isolated organ-specific diseases to multisystem conditions. Herein, we report a 40-year-old woman presenting with a complex multisystem phenotype including sensorineural hearing loss, retinopathy, severe dilated cardiomyopathy, non-insulin dependent diabetes mellitus, and renal failure. Sequence analysis of mtDNA identified the m.5522G>A mutation in MT-TW, the gene encoding mitochondrial tRNA for tryptophan. The heteroplasmic variant, thus far described once, was almost exclusively confined to skeletal muscle tissue, as shown by massive parallel sequencing and corroborated by an ad hoc designed PCR-based strategy. This patient, presenting a severe, multisystem involvement apparently sparing the brain, contributes to the genetic heterogeneity of mitochondrial diseases caused by mutations in mitochondrial tRNAs.

Autophagic vacuolar myopathy caused by a CLN3 mutation. A case report

We present a 29-year-old man with visual failure since childhood, muscle weakness, subtle heart muscle hypertrophy, and seizures who was initially considered to be affected by a mitochondrial encephalomyopathy because of the multiple unspecific involvement of brain, muscle and retinal tissues. Only the muscle biopsy findings correctly guided the genetic investigations and the identification of an autophagic vacuolar myopathy due to a homozygous mutation in CLN3. We believe that information in autophagic muscle disorders should further alert clinicians to consider CLN3 in individuals with vacuolar myopathy, especially if they have visual and cardiac involvement.

Congenital myopathies: clinical phenotypes and new diagnostic tools

Congenital myopathies are a group of genetic muscle disorders characterized clinically by hypotonia and weakness, usually from birth, and a static or slowly progressive clinical course. Historically, congenital myopathies have been classified on the basis of major morphological features seen on muscle biopsy. However, different genes have now been identified as associated with the various phenotypic and histological expressions of these disorders, and in recent years, because of their unexpectedly wide genetic and clinical heterogeneity, next-generation sequencing has increasingly been used for their diagnosis. We reviewed clinical and genetic forms of congenital myopathy and defined possible strategies to improve cost-effectiveness in histological and imaging diagnosis.

Dolichol-phosphate mannose synthase depletion in zebrafish leads to dystrophic muscle with hypoglycosylated α-dystroglycan

Defective dolichol-phosphate mannose synthase (DPMS) complex is a rare cause of congenital muscular dystrophy associated with hypoglycosylation of alpha-dystroglycan (α-DG) in skeletal muscle. We used the zebrafish (Danio rerio) to model muscle abnormalities due to defects in the subunits of DPMS. The three zebrafish ortholog subunits (encoded by the dpm1, dpm2 and dpm3 genes, respectively) showed high similarity to the human proteins, and their expression displayed localization in the midbrain/hindbrain area and somites. Antisense morpholino oligonucleotides targeting each subunit were used to transiently deplete the dpm genes. The resulting morphant embryos showed early death, muscle disorganization, low DPMS complex activity, and increased levels of apoptotic nuclei, together with hypoglycosylated α-DG in muscle fibers, thus recapitulating most of the characteristics seen in patients with mutations in DPMS. Our results in zebrafish suggest that DPMS plays a role in stabilizing muscle structures and in apoptotic cell death.

Elevated serum creatine kinase and small cerebellum prompt diagnosis of congenital muscular dystrophy due to FKRP mutations

Fukutin-related protein (FKRP) is a putative glycosyltransferase that mediate O-linked glycosylation of the α-dystroglycan. Mutations in the FKRP gene cause a spectrum of diseases ranging from a limb girdle muscular dystrophy 2I (LGMD2I), to severe Walker-Warburg or muscle-eye-brain forms and a congenital muscular dystrophy (with or without mental retardation) termed MDC1C. This article reports on a Moroccan infant who presented at birth with moderate floppiness, high serum creatine kinase (CK) levels, and brain ultrasonograph suggestive of widening of the posterior fossa. Muscle biopsy displayed moderate dystrophic pattern with complete absence of α-distroglycan and genetic studies identified a homozygous missense variant in FKRP. Mutations in FKRP should be looked for in forms of neonatal-onset hyperCKaemia with floppiness and small cerebellum.