Spastic ataxia associated with colour vision deficiency due to DDHD2 mutations

Biallelic DDHD2 mutations in patients with adult-onset complex hereditary spastic paraplegia

OBJECTIVE

Hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative disorders characterized by slowly progressive lower limb spasticity and weakness. HSP type 54 (SPG54) is autosomal recessively inherited and caused by mutations in the DDHD2 gene. This study investigated the clinical characteristics and molecular features of DDHD2 mutations in a cohort of Taiwanese patients with HSP.

METHODS

Mutational analysis of DDHD2 was performed for 242 unrelated Taiwanese patients with HSP. The clinical, neuroimaging, and genetic features of the patients with biallelic DDHD2 mutations were characterized. A cell-based study was performed to assess the effects of the DDHD2 mutations on protein expression.

RESULTS

SPG54 was diagnosed in three patients. Among them, two patients carried compound heterozygous DDHD2 mutations, p.[R112Q];[Y606*] and p.[R112Q];[p.D660H], and the other one was homozygous for the DDHD2 p.R112Q mutation. DDHD2 p.Y606* is a novel mutation, whereas DDHD2 p.D660H and p.R112Q have been reported in the literature. All three patients manifested adult onset complex HSP with additional cerebellar ataxia, polyneuropathy, or cognitive impairment. Brain proton magnetic resonance spectroscopy revealed an abnormal lipid peak in thalamus of all three patients. In vitro studies demonstrated that all the three DDHD2 mutations were associated with a considerably lower DDHD2 protein level.

INTERPRETATION

SPG54 was detected in approximately 1.2% (3 of 242) of the Taiwanese HSP cohort. This study expands the known mutational spectrum of DDHD2, provides molecular evidence of the pathogenicity of the DDHD2 mutations, and underlines the importance of considering SPG54 as a potential diagnosis of adult-onset HSP.

Clinical Manifestations Associated with the Domain-Containing Protein 2 Gene Mutation in an Iranian Family with Spastic Paraplegia 54

INTRODUCTION

Spastic paraplegia type 54 (SPG54) is an autosomal recessive disorder caused by bi-allelic mutations in the DDHD-domain-containing protein 2 (DDHD2) gene. Worldwide, over 24 SPG54 families and 24 pathogenic variants have been reported. Our study aimed to describe the clinical and molecular findings of a pediatric patient from a consanguineous Iranian family with significant motor development delay, walking problems, paraplegia, and optic atrophy.

METHODS

The patient was a 7-year-old boy with severe neurodevelopmental and psychomotor problems. Neurological examinations, laboratory tests, electroencephalography, computed tomography scan, and brain magnetic resonance scan (MRI) were carried out for clinical evaluation. Whole-exome sequencing and in silico analysis were undertaken to identify the genetic cause of the disorder.

RESULTS

The neurological examination showed developmental delay, spasticity in the lower extremities, ataxia, foot contractures, and deep tendon reflexes in the extremities. The computed tomography scan was normal, but MRI revealed corpus callosum thinning with atrophic changes in the white matter. The genetic study reported a homozygous variant (c.856 C>T, p.Gln286Ter) in the DDHD2 gene. The homozygous state was confirmed by direct sequencing in the proband and his 5-year-old brother. This variant was not reported as a pathogenic variant in the literature or genetic databases and was predicted to affect the function of the DDHD2 protein.

CONCLUSION

The clinical symptoms in our cases were similar to the previously reported phenotype of SPG54. Our results deepen the molecular and clinical spectrum of SPG54 to facilitate future diagnoses.

Ataxia and spastic paraplegia in mitochondrial disease

Degenerative ataxias and hereditary spastic paraplegias (HSPs) form a continuous, often overlapping disease spectrum sharing not only phenotypic features and underlying genes, but also cellular pathways and disease mechanisms. Mitochondrial metabolism presents a major molecular theme underlying both multiple ataxias and HSPs, thus indicating a heightened vulnerability of Purkinje cells, spinocerebellar tracts, and motor neurons to mitochondrial dysfunction, which is of particular interest for translational approaches. Mitochondrial dysfunction might be the primary (upstream) or secondary (downstream) result of a genetic defect, with underlying genetic defects in nuclear-encoded genes being much more frequent than in mtDNA genes in both, ataxias and HSPs. Here, we outline the substantial number of ataxias, spastic ataxias and HSPs caused by mutated genes implicated in (primary or secondary) mitochondrial dysfunction, highlighting several key "mitochondrial" ataxias and HSPs which are of particular interest for their frequency, pathogenesis and translational opportunities. We then showcase prototypic mitochondrial mechanisms by which disruption of these ataxia and HSP genes contributes to Purkinje cells or corticospinal neuron dysfunction, thus elucidating hypotheses on Purkinje cells and corticospinal neuron vulnerability to mitochondrial dysfunction.

Longitudinal Assessment Using Optical Coherence Tomography in Patients with Friedreich's Ataxia

Ocular abnormalities occur frequently in Friedreich's ataxia (FRDA), although visual symptoms are not always reported. We evaluated a cohort of patients with FRDA to characterise the clinical phenotype and optic nerve findings as detected with optical coherence tomography (OCT). A total of 48 patients from 42 unrelated families were recruited. Mean age at onset was 13.8 years (range 4-40), mean disease duration 19.5 years (range 5-43), mean disease severity as quantified with the Scale for the Assessment and Rating of Ataxia 22/40 (range 4.5-38). All patients displayed variable ataxia and two-thirds had ocular abnormalities. Statistically significant thinning of average retinal nerve fibre layer (RNFL) and thinning in all but the temporal quadrant compared to controls was demonstrated on OCT. Significant RNFL and macular thinning was documented over time in 20 individuals. Disease severity and visual acuity were correlated with RNFL and macular thickness, but no association was found with disease duration. Our results highlight that FDRA is associated with subclinical optic neuropathy. This is the largest longitudinal study of OCT findings in FRDA to date, demonstrating progressive RNFL thickness decline, suggesting that RNFL thickness as measured by OCT has the potential to become a quantifiable biomarker for the evaluation of disease progression in FRDA.

Coinheritance of novel mutations in NAGLU causing mucopolysaccharidosis type IIIB and in DDHD2 causing spastic paraplegia54 in a Turkish family

Mucopolysaccharidosis type IIIB (MPSIIIB) is one of the lysosomal storage diseases, clinically related to developmental delay in the early phase and loss of skills in the late phases of the disease. The disease is caused by homozygous mutations in the NAGLU gene. Spastic paraplegia54 (SPG54) is a neurodegenerative disorder caused by homozygous mutations in the DDHD2 gene. Clinical features are progressive spasticity and weakness in the lower limbs and corpus callosum agenesis. We report on two siblings in a consanguineous family, presenting both the clinical and molecular diagnoses of MPSIIIB and SPG54 with novel mutations by using whole exome sequencing (WES). This interesting finding shows that we should be aware of the importance of using WES for diagnosing rare diseases in consanguineous families.