Expanding the clinical and genetic heterogeneity of SPAX5

Early Diagnosis of AP5Z1/SPG48 Spastic Paraplegia: Case Report and Review of the Literature

Hereditary spastic paraplegias (HSPs) are a genetically heterogeneous group of neurodegenerative disorders clinically characterized by progressive lower limb spasticity with pyramidal weakness. Around a dozen potential molecular mechanisms are recognized. Childhood HSP is a significant diagnostic challenge in clinical practice. Mutations in AP5Z1, which are associated with spastic paraplegia type 48 (SPG48), are extremely rare and seldom described in children.We report the clinical, radiologic, and molecular studies performed in a child harboring novel biallelic mutations in AP5Z1.The child presented a neurodevelopmental disorder with slight lower limb pyramidal signs. Brain magnetic resonance imaging (MRI) showed minimal white matter changes in the frontal horns of the lateral ventricles and a normally shaped corpus callosum. Western blotting in cultured skin fibroblasts indicated reduced protein expression, which confirmed the genetic diagnosis and framed this as a case of protein reduction in a context of impaired autophagy.Our findings expand the spectrum of phenotypes associated with mutations in AP5Z1, highlighting their clinical and pathophysiologic overlap with lysosomal storage disorders. SPG48 should be considered in the differential diagnosis of neurodevelopmental disorders even when pyramidal signs are minimal and brain MRI not fully informative.

Multifaceted Roles of AFG3L2, a Mitochondrial ATPase in Relation to Neurological Disorders

AFG3L2 is a zinc metalloprotease and an ATPase localized in an inner mitochondrial membrane involved in mitochondrial quality control of several nuclear- and mitochondrial-encoded proteins. Mutations in AFG3L2 lead to diseases like slow progressive ataxia, which is a neurological disorder. This review delineates the cellular functions of AFG3L2 and its dysfunction that leads to major clinical outcomes, which include spinocerebellar ataxia type 28, spastic ataxia type 5, and optic atrophy type 12. It summarizes all relevant AFG3L2 mutations associated with the clinical outcomes to understand the detailed mechanisms attributable to its structure-related multifaceted roles in proteostasis and quality control. We face early diagnostic challenges of ataxia and optic neuropathy due to asymptomatic parents and variable clinical manifestations due to heterozygosity/homozygosity of AFG3L2 mutations. This review intends to promote AFG3L2 as a putative prognostic or diagnostic marker.

SCAR32: Functional characterization and expansion of the clinical-genetic spectrum

OBJECTIVE

Biallelic mutations in PRDX3 have been linked to autosomal recessive spinocerebellar ataxia type 32. In this study, which aims to contribute to the growing body of knowledge on this rare disease, we identified two unrelated patients with mutations in PRDX3. We explored the impact of PRDX3 mutation in patient skin fibroblasts and the role of the gene in neurodevelopment.

METHODS

We performed trio exome sequencing that identified mutations in PRDX3 in two unrelated patients. We also performed functional studies in patient skin fibroblasts and generated a "crispant" zebrafish (Danio rerio) model to investigate the role of the gene during nervous system development.

RESULTS

Our study reports two additional patients. Patient 1 is a 19-year-old male who showed a novel homozygous c.525_535delGTTAGAAGGTT (p. Leu176TrpfsTer11) mutation as the genetic cause of cerebellar ataxia. Patient 2 is a 20-year-old male who was found to present the known c.425C>G/p. Ala142Gly variant in compound heterozygosity with the p. Leu176TrpfsTer11 one. While the fibroblast model failed to recapitulate the pathological features associated with PRDX3 loss of function, our functional characterization of the prdx3 zebrafish model revealed motor defects, increased susceptibility to reactive oxygen species-triggered apoptosis, and an impaired oxygen consumption rate.

CONCLUSIONS

We identified a new variant, thereby expanding the genetic spectrum of PRDX3-related disease. We developed a novel zebrafish model to investigate the consequences of prdx3 depletion on neurodevelopment and thus offered a potential new tool for identifying new treatment opportunities.

Clinical and genetic features of dominant Essential Tremor in Tuscany, Italy: FUS, CAMTA1, ATXN1 and beyond

OBJECTIVE

Essential Tremor (ET) is one of the most common neurological disorders. In most instances ET is inherited as an autosomal dominant trait with age-related penetrance (virtually complete in advanced age); however, ET genetics remains elusive. The current study aims to identify possibly pathogenic genetic variants in a group of well-characterized ET families.

METHODS

34 individuals from 14 families with dominant ET were clinically evaluated and studied by whole exome sequencing studies (after excluding trinucleotide expansion disorders).

RESULTS

Most patients had pure ET. In 4 families, exome studies could identify a genetic variant potentially able to significantly alter the protein structure (CADD >20, REVEL score > 0.25), shared by all the affected individuals (in CAMTA1, FUS, MYH14, SGCE genes). In another family there were two variants in dominant genes (PCDH9 and SQSTM1). Moreover, an interrupted "intermediate" trinucleotide expansion in ATXN1 ("SCA1") was identified in a further family with pure ET.

CONCLUSION

Combining our observations together with earlier reports, we can conclude that ET genes confirmed in at least two families to date include CAMTA1 and FUS (reported here), as well as CACNA1G, NOTCH2NLC and TENM4. Most cases of familial ET, inherited with an autosomal dominant inheritance, may result from "mild" variants of many different genes that, when affected by more harmful genetic variants, lead to more severe neurological syndromes (still autosomal dominant). Thus, ET phenotype may be the "mild", incomplete manifestation of many other dominant neurogenetic diseases. These findings further support evidence of genetic heterogeneity for such disease(s). Author's keywords: cerebellar ataxias, movement disorders, neurogenetics, rare neurological disorders, tremor.

Sustained OMA1-mediated integrated stress response is beneficial for spastic ataxia type 5

AFG3L2 is a mitochondrial protease exerting protein quality control in the inner mitochondrial membrane. Heterozygous AFG3L2 mutations cause spinocerebellar ataxia type 28 (SCA28) or dominant optic atrophy type 12 (DOA12), while biallelic AFG3L2 mutations result in the rare and severe spastic ataxia type 5 (SPAX5). The clinical spectrum of SPAX5 includes childhood-onset cerebellar ataxia, spasticity, dystonia and myoclonic epilepsy. We previously reported that the absence or mutation of AFG3L2 leads to the accumulation of mitochondria-encoded proteins, causing the overactivation of the stress-sensitive protease OMA1, which over-processes OPA1, leading to mitochondrial fragmentation. Recently, OMA1 has been identified as the pivotal player communicating mitochondrial stress to the cytosol via a pathway involving the inner mitochondrial membrane protein DELE1 and the cytosolic kinase HRI, thus eliciting the integrated stress response. In general, the integrated stress response reduces global protein synthesis and drives the expression of cytoprotective genes that allow cells to endure proteotoxic stress. However, the relevance of the OMA1-DELE1-HRI axis in vivo, and especially in a human CNS disease context, has been poorly documented thus far. In this work, we demonstrated that mitochondrial proteotoxicity in the absence/mutation of AFG3L2 activates the OMA1-DELE1-HRI pathway eliciting the integrated stress response. We found enhanced OMA1-dependent processing of DELE1 upon depletion of AFG3L2. Also, in both skin fibroblasts from SPAX5 patients (including a novel case) and in the cerebellum of Afg3l2-/- mice we detected increased phosphorylation of the α-subunit of the eukaryotic translation initiation factor 2 (eIF2α), increased levels of ATF4 and strong upregulation of its downstream targets (Chop, Chac1, Ppp1r15a and Ffg21). Silencing of DELE1 or HRI in SPAX5 fibroblasts (where OMA1 is overactivated at basal state) reduces eIF2α phosphorylation and affects cell growth. In agreement, pharmacological potentiation of integrated stress response via Sephin-1, a drug that selectively inhibits the stress-induced eIF2alpha phosphatase GADD34 (encoded by Ppp1r15a), improved cell growth of SPAX5 fibroblasts and cell survival and dendritic arborization ex vivo in primary Afg3l2-/- Purkinje neurons. Notably, Sephin-1 treatment in vivo extended the lifespan of Afg3l2-/- mice, improved Purkinje neuron morphology, mitochondrial ultrastructure and respiratory capacity. These data indicate that activation of the OMA1-DELE1-HRI pathway is protective in the context of SPAX5. Pharmacological tuning of the integrated stress response may represent a future therapeutic strategy for SPAX5 and other cerebellar ataxias caused by impaired mitochondrial proteostasis.

AFG3L2 Biallelic Mutation: Clinical Heterogeneity in Two Italian Patients

AFG3-like matrix AAA peptidase subunit 2 gene (AFG3L2, OMIM * 604,581) biallelic mutations lead to autosomal recessive spastic ataxia-5 SPAX5, OMIM # 614,487), a rare hereditary form of ataxia. The clinical spectrum includes early-onset cerebellar ataxia, spasticity, and progressive myoclonic epilepsy (PME). In Italy, the epidemiology of the disease is probably underestimated. The advent of next generation sequencing (NGS) technologies has speeded up the diagnosis of hereditary diseases and increased the percentage of diagnosis of rare disorders, such as the rare hereditary ataxia groups. Here, we describe two patients from two different villages in the province of Ferrara, who manifested a different clinical ataxia-plus history, although carrying the same biallelic mutation in AFG3L2 (p.Met625Ile) identified through NGS analysis.

A new genetic cause of spastic ataxia: the p.Glu415Lys variant in TUBA4A

Tubulinopathies encompass neurodevelopmental disorders caused by mutations in genes encoding for different isotypes of α- and β-tubulins, the structural components of microtubules. Less frequently, mutations in tubulins may underlie neurodegenerative disorders. In the present study, we report two families, one with 11 affected individuals and the other with a single patient, carrying a novel, likely pathogenic, variant (p. Glu415Lys) in the TUBA4A gene (NM_006000). The phenotype, not previously described, is that of spastic ataxia. Our findings widen the phenotypic and genetic manifestations of TUBA4A variants and add a new type of spastic ataxia to be taken into consideration in the differential diagnosis.

Novel compound heterozygous mutations in the AFG3L2 gene in a Chinese child with microcephaly, early-onset seizures, and cerebral atrophy

Background

The most common disease caused by biallelic AFG3L2 mutations is spastic ataxia type 5 (SPAX5). Identification of complex phenotypes resulting from biallelic AFG3L2 mutations has been increasing in recent years.

Methods

A retrospective analysis was performed on a child with microcephaly and recurrent seizures. The child underwent physical and neurological examinations, laboratory tests, electroencephalography (EEG), and brain magnetic resonance imaging (MRI). Trio-whole-exome sequencing (trio-WES) was performed to identify possible causative mutations.

Results

We described a child who exhibited early-onset and intractable epilepsy, developmental regression, microcephaly, and premature death. Neuroimaging revealed global cerebral atrophy (GCA) involving the cerebrum, cerebellum, corpus callosum, brainstem, cerebellar vermis, and basal ganglia. On trio-WES, two novel compound heterozygous mutations, c.1834G > T (p.E612*) and c.2176-6T > A in the AFG3L2 gene, were identified in this patient.

Conclusions

Our findings have expanded the mutation spectrum of the AFG3L2 gene and identified a severe neurodegenerative phenotype of global cerebral atrophy caused by biallelic AFG3L2 mutations.

NGS in Hereditary Ataxia: When Rare Becomes Frequent

The term hereditary ataxia (HA) refers to a heterogeneous group of neurological disorders with multiple genetic etiologies and a wide spectrum of ataxia-dominated phenotypes. Massive gene analysis in next-generation sequencing has entered the HA scenario, broadening our genetic and clinical knowledge of these conditions. In this study, we employed a targeted resequencing panel (TRP) in a large and highly heterogeneous cohort of 377 patients with a clinical diagnosis of HA, but no molecular diagnosis on routine genetic tests. We obtained a positive result (genetic diagnosis) in 33.2% of the patients, a rate significantly higher than those reported in similar studies employing TRP (average 19.4%), and in line with those performed using exome sequencing (ES, average 34.6%). Moreover, 15.6% of the patients had an uncertain molecular diagnosis. STUB1, PRKCG, and SPG7 were the most common causative genes. A comparison with published literature data showed that our panel would have identified 97% of the positive cases reported in previous TRP-based studies and 92% of those diagnosed by ES. Proper use of multigene panels, when combined with detailed phenotypic data, seems to be even more efficient than ES in clinical practice.

Episodic ataxia and severe infantile phenotype in spinocerebellar ataxia type 14: expansion of the phenotype and novel mutations

Introduction

Spinocerebellar ataxia type 14 (SCA14) is a dominantly inherited neurological disorder characterized by slowly progressive cerebellar ataxia. SCA14 is caused by mutations in PRKCG, a gene encoding protein kinase C gamma (PKCγ), a master regulator of Purkinje cells development.

Methods

We performed next-generation sequencing targeted resequencing panel encompassing 273 ataxia genes in 358 patients with genetically undiagnosed ataxia.

Results

We identified fourteen patients in ten families harboring nine pathogenic heterozygous variants in PRKCG, seven of which were novel. We encountered four patients with not previously described phenotypes: one with episodic ataxia, one with a spastic paraparesis dominating her clinical manifestations, and two children with an unusually severe phenotype.

Conclusions

Our study broadens the genetic and clinical spectrum of SCA14.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00415-021-10712-5.

New pathogenic variants in COQ4 cause ataxia and neurodevelopmental disorder without detectable CoQ10 deficiency in muscle or skin fibroblasts

COQ4 is a component of an enzyme complex involved in the biosynthesis of coenzyme Q₁₀ (CoQ₁₀), a molecule with primary importance in cell metabolism. Mutations in the COQ4 gene are responsible for mitochondrial diseases showing heterogeneous age at onset, clinical presentations and association with CoQ₁₀ deficiency. We herein expand the phenotypic and genetic spectrum of COQ4-related diseases, by reporting two patients harboring bi-allelic variants but not showing CoQ₁₀ deficiency. One patient was found to harbor compound heterozygous mutations (specifically, c.577C>T/p.Pro193Ser and the previously reported c.718C>T/p.Arg240Cys) associated with progressive spasticity, while the other harbored two novel missense (c.284G>A/p.Gly95Asp and c.305G>A/p.Arg102His) associated with a neurodevelopmental disorder. Both patients presented motor impairment and ataxia. To further understand the role of COQ4, we performed functional studies in patient-derived fibroblasts, yeast and "crispant" zebrafish larvae. Micro-oxygraphy showed impaired oxygen consumption rates in one patient, while yeast complementation assays showed that all the mutations were presumably disease related. Moreover, characterization of the coq4 F0 CRISPR zebrafish line showed motor defects and cell reduction in a specific area of the hindbrain, a region reminiscent of the human cerebellum. Our expanded phenotype associated with COQ4 mutations allowed us to investigate, for the first time, the role of COQ4 in brain development in vivo.

Multimodal evaluation of an Italian family with a hereditary spastic paraplegia and POLR3A mutations

We describe an Italian family with adult‐onset pure hereditary spastic paraplegia due to biallelic variants in POLR3A gene [c.1909 + 22G > A and c.3839dupT (p.M1280fs*20]. MRI showed a mild hyperintensity of superior cerebellar peduncles and cervical spinal cord atrophy. The neurophysiological metrics about intracortical excitability showed higher values of motor thresholds and a significant reduction of short interval intracortical inhibition (SICI) in the patient with a more severe phenotype. Our multimodal evaluation further expands the wide phenotypic spectrum associated with mutations in the POLR3A gene. An extensive genotype–phenotype correlation study is necessary to explain the role of the many new mutations on the function of protein.