In-frame de novo mutation in BICD2 in two patients with muscular atrophy and arthrogryposis

Congenital Titinopathy: Comprehensive Characterization of the Most Severe End of the Disease Spectrum

Congenital titinopathy has recently emerged as one of the most common congenital muscle disorders.

OBJECTIVE

To better understand the presentation and clinical needs of the under-characterized extreme end of the congenital titinopathy severity spectrum.

METHODS

We comprehensively analyzed the clinical, imaging, pathology, autopsy, and genetic findings in 15 severely affected individuals from 11 families.

RESULTS

Prenatal features included hypokinesia or akinesia and growth restriction. Six pregnancies were terminated. Nine infants were born at or near term with severe-to-profound weakness and required resuscitation. Seven died following withdrawal of life support. Two surviving children require ongoing respiratory support. Most cohort members had at least 1 disease-causing variant predicted to result in some near-normal-length titin expression. The exceptions, from 2 unrelated families, had homozygous truncating variants predicted to induce complete nonsense mediated decay. However, subsequent analyses suggested that the causative variant in each family had an additional previously unrecognized impact on splicing likely to result in some near-normal-length titin expression. This impact was confirmed by minigene assay for 1 variant.

INTERPRETATION

This study confirms the clinical variability of congenital titinopathy. Severely affected individuals succumb prenatally/during infancy, whereas others survive into adulthood. It is likely that this variability is because of differences in the amount and/or length of expressed titin. If confirmed, analysis of titin expression could facilitate clinical prediction and increasing expression might be an effective treatment strategy. Our findings also further-support the hypothesis that some near-normal-length titin expression is essential to early prenatal survival. Sometimes expression of normal/near-normal-length titin is due to disease-causing variants having an additional impact on splicing. ANN NEUROL 2025;97:611-628.

Clinical and Genetic Profiles of 5q- and Non-5q-Spinal Muscular Atrophy Diseases in Pediatric Patients

BACKGROUND

Spinal muscular atrophy (SMA) is a genetic disease characterized by loss of motor neurons in the spinal cord and lower brainstem. The term "SMA" usually refers to the most common form, 5q-SMA, which is caused by biallelic mutations in SMN1 (located on chromosome 5q13). However, long before the discovery of SMN1, it was known that other forms of SMA existed. Therefore, SMA is currently divided into two groups: 5q-SMA and non-5q-SMA. This is a simple and practical classification, and therapeutic drugs have only been developed for 5q-SMA (nusinersen, onasemnogene abeparvovec, risdiplam) and not for non-5q-SMA disease.

METHODS

We conducted a non-systematic critical review to identify the characteristics of each SMA disease.

RESULTS

Many of the non-5q-SMA diseases have similar symptoms, making DNA analysis of patients essential for accurate diagnosis. Currently, genetic analysis technology using next-generation sequencers is rapidly advancing, opening up the possibility of elucidating the pathology and treating non-5q-SMA.

CONCLUSION

Based on accurate diagnosis and a deeper understanding of the pathology of each disease, treatments for non-5q-SMA diseases may be developed in the near future.

Missense BICD2 variants in fetuses with congenital arthrogryposis and pterygia

Type 2 spinal muscular atrophy with lower extremity dominance (SMALED2) is caused by bicaudal D cargo adaptor 2 (BICD2) variants. However, the SMALED2 genotype and phenotype correlation have not been thoroughly characterized. We identified de novo heterozygous BICD2 missense variants in two fetuses with severe, prenatally diagnosed multiple arthrogryposis congenita. This report provides further insights into the genetics of this rare disease.

Presynaptic perspective: Axonal transport defects in neurodevelopmental disorders

Disruption of synapse assembly and maturation leads to a broad spectrum of neurodevelopmental disorders. Presynaptic proteins are largely synthesized in the soma, where they are packaged into precursor vesicles and transported into distal axons to ensure precise assembly and maintenance of presynapses. Due to their morphological features, neurons face challenges in the delivery of presynaptic cargos to nascent boutons. Thus, targeted axonal transport is vital to build functional synapses. A growing number of mutations in genes encoding the transport machinery have been linked to neurodevelopmental disorders. Emerging lines of evidence have started to uncover presynaptic mechanisms underlying axonal transport defects, thus broadening the view of neurodevelopmental disorders beyond postsynaptic mechanisms. In this review, we discuss presynaptic perspectives of neurodevelopmental disorders by focusing on impaired axonal transport and disturbed assembly and maintenance of presynapses. We also discuss potential strategies for restoring axonal transport as an early therapeutic intervention.

Novel inherited CDX2 variant segregating in a family with diverse congenital malformations of the genitourinary system

Anorectal malformations (ARMs) constitute a group of congenital defects of the gastrointestinal and urogenital systems. They affect males and females, with an estimated worldwide prevalence of 1 in 5000 live births. These malformations are clinically heterogeneous and can be part of a syndromic presentation (syndromic ARM) or as a nonsyndromic entity (nonsyndromic ARM). Despite the well-recognized heritability of nonsyndromic ARM, the genetic etiology in most patients is unknown. In this study, we describe three siblings with diverse congenital anomalies of the genitourinary system, anemia, delayed milestones, and skeletal anomalies. Genome sequencing identified a novel, paternally inherited heterozygous Caudal type Homeobox 2 (CDX2) variant (c.722A > G (p.Glu241Gly)), that was present in all three affected siblings. The variant identified in this family is absent from population databases and predicted to be damaging by most in silico pathogenicity tools. So far, only two other reports implicate variants in CDX2 with ARMs. Remarkably, the individuals described in these studies had similar clinical phenotypes and genetic alterations in CDX2 CDX2 encodes a transcription factor and is considered the master regulator of gastrointestinal development. This variant maps to the homeobox domain of the encoded protein, which is critical for interaction with DNA targets. Our finding provides a potential molecular diagnosis for this family's condition and supports the role of CDX2 in anorectal anomalies. It also highlights the clinical heterogeneity and variable penetrance of ARM predisposition variants, another well-documented phenomenon. Finally, it underscores the diagnostic utility of genomic profiling of ARMs to identify the genetic etiology of these defects.

Role of Nesprin-2 and RanBP2 in BICD2-associated brain developmental disorders

Bicaudal D2 (BICD2) is responsible for recruiting cytoplasmic dynein to diverse forms of subcellular cargo for their intracellular transport. Mutations in the human BICD2 gene have been found to cause an autosomal dominant form of spinal muscular atrophy (SMA-LED2), and brain developmental defects. Whether and how the latter mutations are related to roles we and others have identified for BICD2 in brain development remains little understood. BICD2 interacts with the nucleoporin RanBP2 to recruit dynein to the nuclear envelope (NE) of Radial Glial Progenitor cells (RGPs) to mediate their well-known but mysterious cell-cycle-regulated interkinetic nuclear migration (INM) behavior, and their subsequent differentiation to form cortical neurons. We more recently found that BICD2 also mediates NE dynein recruitment in migrating post-mitotic neurons, though via a different interactor, Nesprin-2. Here, we report that Nesprin-2 and RanBP2 compete for BICD2-binding in vitro. To test the physiological implications of this behavior, we examined the effects of known BICD2 mutations using in vitro biochemical and in vivo electroporation-mediated brain developmental assays. We find a clear relationship between the ability of BICD2 to bind RanBP2 vs. Nesprin-2 in controlling of nuclear migration and neuronal migration behavior. We propose that mutually exclusive RanBP2-BICD2 vs. Nesprin-2-BICD2 interactions at the NE play successive, critical roles in INM behavior in RGPs and in post-mitotic neuronal migration and errors in these processes contribute to specific human brain malformations.

Long-term follow-up of a patient with autosomal dominant lower extremity-predominant spinal muscular atrophy-2 due to a BICD2 variant

INTRODUCTION

Bicaudal D homolog 2 (BICD2) is a causative gene of autosomal-dominant lower extremity-predominant spinal muscular atrophy-2 (SMA-LED2). The severity of SMA-LED2 varies widely, ranging from cases in which patients are able to walk to cases in which severe joint contractures lead to respiratory failure. In this study, we report the long-term course of a case of SMA-LED2 in comparison with previous reports.

CASE REPORT

The patient was a 19-year-old woman. She had knee and hip dislocations with contractures, femoral fracture, and talipes calcaneovalgus since birth, and was diagnosed with arthrogryposis multiplex congenita. Intense respiratory support was not needed during the neonatal period. She had aspiration pneumonia repeatedly, necessitating NICU admission until 8 months of age. She achieved head control at 9 months of age and was able to sit at 2 years of age; however, she could not walk. Tube feeding was required until 3 years of age. At present, she can eat orally, move around with a wheelchair, and write words by herself. She needs non-invasive positive pressure ventilation during sleep because of a restrictive respiratory disorder during adolescence. Exome analysis identified a de novo heterozygous missense variant (c.2320G>A; p.Glu774Lys) in BICD2.

CONCLUSION

Patients with SMA-LED2 may have a relatively better prognosis in terms of social activities in comparison with the dysfunction in the neonatal period. Moreover, it is important to periodically evaluate respiratory function in patients with SMA-LED2 because respiratory dysfunction may occur during adolescence.

Cataract, abnormal electroretinogram and visual evoked potentials in a child with SMA-LED2 - extending the phenotype

This case report describes a girl who presented antenatal arthrogryposis and postnatal hypotonia, generalized and respiratory weakness, joint deformities particularly affecting the lower limbs and poor swallow. By 5 months, cataracts, abnormal electroretinograms, visual evoked potentials and global developmental impairments were recognized. No causative variants were identified on targeted gene panels. After her unexpected death at 11 months, gene-agnostic trio whole exome sequencing revealed a likely pathogenic de novo BICD2 missense variant, NM_001003800.1, c.593T>C, p.(Leu198Pro), confirming the diagnosis of spinal muscular atrophy lower extremity predominant type 2 (SMA-LED2). We propose that cataracts and abnormal electroretinograms are novel features of SMA-LED2.

Case report and review of the literature: immune dysregulation in a large familial cohort due to a novel pathogenic RELA variant

OBJECTIVE

To explore and define the molecular cause(s) of a multi-generational kindred affected by Bechet's-like mucocutaneous ulcerations and immune dysregulation.

METHODS

Whole genome sequencing and confirmatory Sanger sequencing were performed. Components of the NFκB pathway were quantified by immunoblotting, and function was assessed by cytokine production (IL-6, TNF-α, IL-1β) after lipopolysaccharide (LPS) stimulation. Detailed immunophenotyping of T-cell and B-cell subsets was performed in four patients from this cohort.

RESULTS

A novel variant in the RELA gene, p. Tyr349LeufsTer13, was identified. This variant results in premature truncation of the protein before the serine (S) 536 residue, a key phosphorylation site, resulting in enhanced degradation of the p65 protein. Immunoblotting revealed significantly decreased phosphorylated [p]p65 and pIκBα. The decrease in [p]p65 may suggest reduced heterodimer formation between p50/p65 (NFκB1/RelA). Immunophenotyping revealed decreased naïve T cells, increased memory T cells, and expanded senescent T-cell populations in one patient (P1). P1 also had substantially higher IL-6 and TNF-α levels post-stimulation compared with the other three patients.

CONCLUSION

Family members with this novel RELA variant have a clinical phenotype similar to other reported RELA cases with predominant chronic mucocutaneous ulceration; however, the clinical phenotype broadens to include Behçet's syndrome and IBD. Here we describe the clinical, immunological and genetic evaluation of a large kindred to further expand identification of patients with autosomal dominant RELA deficiency, facilitating earlier diagnosis and intervention. The functional impairment of the canonical NFκB pathway suggests that this variant is causal for the clinical phenotype in these patients.

Fetal early motor neuron disruption and prenatal molecular diagnosis in a severe BICD2-opathy

BICD2 (BICD Cargo Adaptor 2, MIM*609797) mutations are associated with severe prenatal-onset forms of spinal muscular atrophy, lower extremity-predominant 2B (SMALED2B MIM 618291) or milder forms with childhood-onset (SMALED2A MIM 615290). Etiopathogenesis is not fully clarified and a wide spectrum of phenotypic presentations is reported, ranging from extreme prenatal forms with adverse outcome, to slow progressive late-onset forms. We report a fetus at 22 gestational weeks with evidence of Arthrogryposis Multiplex Congenita on ultrasound, presenting with fixed extended lower limbs and flexed upper limbs, bilateral clubfoot and absent fetal movements. A trio-based prenatal Exome Sequencing was performed, disclosing a de novo heterozygous pathogenic in frame deletion (NM_015250.3: c.1636_1638delAAT; p.Asn546del) in BICD2. After pregnancy termination, quantitative analysis on NeuN immunostained spinal cord sections of the ventral horns, revealed that neuronal density was markedly reduced compared to the one of an age-matched normal fetus and an age-matched type-I Spinal Muscular Atrophy sample, used as a comparative model. The present case, the first prenatally diagnosed and neuropathologically characterized, showed an early motor neuron loss in SMALED2B, providing further insight into the pathological basis of BICD2-opathies.

A case of severe autosomal dominant spinal muscular atrophy with lower extremity predominance caused by a de novo BICD2 mutation

BACKGROUND

Heterozygous variants in BICD2 cause autosomal dominant spinal muscular atrophy with lower extremity predominance. These variants are also identified in individuals with severe forms of congenital muscle atrophy representing arthrogryposis multiplex.

CASE REPORT

A girl was born with severe muscle weakness and respiratory distress. A fetal ultrasound had detected polyhydramnios and multiple joint contractures in utero. She was born with severe muscle weakness and respiratory distress. Bilateral hip joint dislocation and multiple bone fractures were also present at birth. Although she depends on medical care, including assisted ventilation and tube feeding, she has reached eight years of age. Her motor developmental skills were reduced owing to muscle weakness and deformity of her lower extremities, whereas her cognitive development slowly but steadily grew. Whole exome sequencing revealed a novel de novo missense BICD2 variant (c.1625G > A, p.(Cys542Tyr)), which was evaluated as likely pathogenic.

CONCLUSION

This is the first case report of a severe form of spinal muscular atrophy with lower extremity predominance caused by a de novo BICD2 variant in Japan.

SMALED2 with BICD2 gene mutations: Report of two cases and portrayal of a classical phenotype

The spinal muscular atrophies (SMA) affect lower motor neurons leading to important muscle atrophy and paralysis. Some cases of SMA affect mostly the lower limbs and are called autosomal dominant spinal muscular atrophy, lower extremity predominant (SMALED). So far, two genes have been identified to cause this phenotype, DYNC1H1 (SMALED1) and BICD2 (SMALED2). This pathology is rare, but patients exhibit classical features which should be recognised by physicians. We present two unrelated cases of SMALED2 with previously described c.320C>T BICD2 mutations. Our cases exhibit non-progressive weakness and atrophy of the lower limbs associated with contractures and unique muscle MRI findings suggestive of classical SMALED2. We also performed an extensive review of the literature to present the classical and atypical phenotypes of BICD2. Indeed, some features appear to be highly suggestive of the disease, including upper limb sparing, sparing of the adductors muscles on physical examination and MRI, congenital contractures and normal nerve conductions studies.

Phen2Gene: rapid phenotype-driven gene prioritization for rare diseases

Human Phenotype Ontology (HPO) terms are increasingly used in diagnostic settings to aid in the characterization of patient phenotypes. The HPO annotation database is updated frequently and can provide detailed phenotype knowledge on various human diseases, and many HPO terms are now mapped to candidate causal genes with binary relationships. To further improve the genetic diagnosis of rare diseases, we incorporated these HPO annotations, gene-disease databases and gene-gene databases in a probabilistic model to build a novel HPO-driven gene prioritization tool, Phen2Gene. Phen2Gene accesses a database built upon this information called the HPO2Gene Knowledgebase (H2GKB), which provides weighted and ranked gene lists for every HPO term. Phen2Gene is then able to access the H2GKB for patient-specific lists of HPO terms or PhenoPacket descriptions supported by GA4GH (http://phenopackets.org/), calculate a prioritized gene list based on a probabilistic model and output gene-disease relationships with great accuracy. Phen2Gene outperforms existing gene prioritization tools in speed and acts as a real-time phenotype-driven gene prioritization tool to aid the clinical diagnosis of rare undiagnosed diseases. In addition to a command line tool released under the MIT license (https://github.com/WGLab/Phen2Gene), we also developed a web server and web service (https://phen2gene.wglab.org/) for running the tool via web interface or RESTful API queries. Finally, we have curated a large amount of benchmarking data for phenotype-to-gene tools involving 197 patients across 76 scientific articles and 85 patients' de-identified HPO term data from the Children's Hospital of Philadelphia.

Defects in Axonal Transport in Inherited Neuropathies

Axonal transport is a highly complex process essential for sustaining proper neuronal functioning. Disturbances can result in an altered neuronal homeostasis, aggregation of cargoes, and ultimately a dying-back degeneration of neurons. The impact of dysfunction in axonal transport is shown by genetic defects in key proteins causing a broad spectrum of neurodegenerative diseases, including inherited peripheral neuropathies. In this review, we provide an overview of the cytoskeletal components, molecular motors and adaptor proteins involved in axonal transport mechanisms and their implication in neuronal functioning. In addition, we discuss the involvement of axonal transport dysfunction in neurodegenerative diseases with a particular focus on inherited peripheral neuropathies. Lastly, we address some recent scientific advances most notably in therapeutic strategies employed in the area of axonal transport, patient-derived iPSC models, in vivo animal models, antisense-oligonucleotide treatments, and novel chemical compounds.