TREX1 Deficiency Induces ER Stress-Mediated Neuronal Cell Death by Disrupting Ca2+ Homeostasis

TREX1 is an exonuclease that degrades extranuclear DNA species in mammalian cells. Herein, we show a novel mechanism by which TREX1 interacts with the BiP/GRP78 and TREX1 deficiency triggers ER stress through the accumulation of single-stranded DNA and activates unfolded protein response (UPR) signaling via the disruption of the TREX1-BiP/GRP78 interaction. In TREX1 knockdown cells, the activation of ER stress signaling disrupted ER Ca2+ homeostasis via the ERO1α-IP3R1-CaMKII pathway, leading to neuronal cell death. Moreover, TREX1 knockdown dysregulated the Golgi-microtubule network through Golgi fragmentation and decreased Ac-α-tubulin levels, contributing to neuronal injury. These alterations were also observed in neuronal cells harboring a TREX1 mutation (V91M) that has been identified in hereditary spastic paraplegia (HSP) patients in Korea. Notably, this mutation leads to defects in the TREX1-BiP/GRP78 interaction and mislocalization of TREX1 from the ER and possible disruption of the Golgi-microtubule network. In summary, the current study reveals TREX1 as a novel regulator of the BiP/GRP78 interaction and shows that TREX1 deficiency promotes ER stress-mediated neuronal cell death, which indicates that TREX1 may hold promise as a therapeutic target for neurodegenerative diseases such as HSP.

Multifaceted and Age-Dependent Phenotypes Associated With Biallelic PNPLA6 Gene Variants: Eight Novel Cases and Review of the Literature

A wide spectrum of neurodegenerative diseases has been associated with pathogenic variants in the PNPLA6 (patatin-like phospholipase domain-containing protein 6) gene, including spastic paraplegia type 39, Gordon—Holmes, Boucher—Neuhauser, Oliver—Mc Farlane, and Laurence—Moon syndromes. These syndromes present variable and overlapping clinical symptoms, encompassing cerebellar ataxia, hypogonadotropic hypogonadism, chorioretinal dystrophy, spastic paraplegia, muscle wasting, peripheral neuropathy, and cognitive impairment. In the present study, we performed a wide genetic screening in 292 patients presenting with ataxia or spastic paraplegia using a probe-based customized gene panel, covering >200 genes associated with spinocerebellar diseases. We identified six novel and four recurrent PNPLA6 gene variants in eight patients (2.7%). Six patients presented an infantile or juvenile onset (age <18), and two patients had an adult onset. Cerebellar ataxia was observed in seven patients and spastic paraplegia in one patient. Progression of cerebellar symptoms was slow in all patients, who retained ambulation even after a mean disease duration of 15 years. Brain MRI showed cerebellar atrophy in 6/8 patients, more pronounced in superior and dorsal vermis lobules (I to VII). Additional clinical features included hypogonadotropic hypogonadism (5/8), growth hormone deficiency (2/8), peripheral axonal neuropathy (4/8), cognitive impairment (3/8), chorioretinal dystrophy (2/8), and bilateral vestibular areflexia with a reduced visual vestibule-ocular reflex (1/8). In accordance with previous studies, chorioretinal dystrophy was the most frequent presenting symptom in early onset patients, hypogonadotropic hypogonadism in juvenile onset cases, and cerebellar ataxia in adult patients. One patient had an initial clinical presentation compatible with Cerebellar Ataxia with Neuropathy and Vestibular Areflexia Syndrome (CANVAS), but no pathological expansions in the RFC1 gene. In conclusion, patients with PNPLA6 variants present a variable age of onset spanning from infancy to adulthood, and each clinical symptom has an age-dependent manifestation thus requiring a multi-systemic diagnostic approach. The description of patients presenting very late-onset cerebellar ataxia suggests that PNPLA6 genetic screening should also be considered in the diagnostic workout of adult cerebellar ataxia.

Novel Splice-Site and Missense Variant of PNPLA6 in an Austrian Family Causing Spastic Paraplegia-39 with Cerebellar Oculomotor Disorder

Background The term hereditary spastic paraplegia comprises an ever-expanding array of neurological disorders with distinct aetiologies. Spastic paraplegia 39 is one of the many variants with additional features of other organs and neurological systems. We describe a large kindred with two hitherto undescribed mutations of PNPLA6 and a novel clinical phenotype as, in addition to spastic paraplegia, affected subjects suffered from a prominent cerebellar oculomotor dysfunction. Methods Three of five genetically tested family members of a large kindred were affected by spastic gait and cerebellar oculomotor dysfunction. Clinical, imaging, laboratory and electrophysiological data were analyzed. Genetic analysis was done using next-generation sequencing. Segregation analyses were performed by Sanger sequencing. To assess the pathogenicity of genetic variants on the encoded protein, in silico assessments were carried out. Results Two hitherto unknown sequence variants in the PNPLA6 gene, a splice-site variant c.1635+3G>T and a missense variant c.3401A>T, p.(Asp1134Val), were detected. Compound-heterozygous siblings presented with lower limb spasticity and a marked cerebellar oculomotor disorder accompanied by moderate hypogonadotropic hypogonadism in the female. A paternal uncle being homozygous for the splice-site variant of PNPLA6 presented with increased lower limb reflexes, an unstable gait and cerebellar oculomotor dysfunction. Treatment with 4-aminopyridin, a potassium channel blocker, lead to meaningful improvement of clinical symptoms. Conclusion PNPLA6 gene variants are associated with a broad phenotypic spectrum to which we add cerebellar oculomotor dysfunction. In our kindred, the full clinical manifestation only occurred in compound-heterozygous subjects indicating that biallelic pathogenic variants lead to more serious and earlier onset of symptoms. Our findings emphasize the role of PNPLA6 in different neurodegenerative disorders.

Easily Misdiagnosed cblC Deficiency in Adolescents: the Clinical and Metabolic Studies

Purpose: Adolescents are easily attacked by potential inherited metabolic disorders. cblC deficiency is the most common type of methylmalonic aciduria in China. The late-onset patients present with varied non-specific symptoms and usually being misdiagnosed. The purpose of this study is to investigate the clinical features of patients with adolescence-onset cblC deficiency and explore the prevention and control strategies. Methods: Fifty-seven patients (34 males and 23 females) with adolescence-onset cblC deficiency were admitted in our clinic from 2002 to September 2021. The diagnosis was confirmed by metabolic and genetic tests. The clinical and biochemical features, disease triggers, outcome and genotypes-phenotypes correlation were examined.Results: The onset ages ranged from 10 to 25 years old (median age was 12 years). 16 cases (28.0%) presented with symptoms after infection or sports training. 46 patients (80.7%) had neuropsychiatric diseases. 14 patients (24.6%) displayed cardiovascular diseases. Five cases (8.9%) showed pulmonary hypertension. Renal damage was observed in seven cases (12.3%). 23 mutations were identified from the MMACHC gene of 57 patients. 37 patients demonstrated c.482G>A (64.9%) and 16 cases had c.609G>A (26.3%). Among 13 patients that exhibited spastic paraplegia as a main manifestation, 10 patients had c.482G>A (76.9%). Five patients presented with psychotic disorders and spastic paraplegia with c.482G>A. All patients improved after metabolic treatment with cobalamin, L-carnitine, and betaine. 30 school-aged patients returned to school. Two patients were married and had healthy babies.Conclusion: Patients with adolescence-onset cblC deficiency presented with varied neuropsychiatric symptoms or multiple organ damage. Metabolic studies and individualized treatment are keys to improve the outcome of the patients.

An autopsied case report of spastic paraplegia with thin corpus callosum carrying a novel mutation in the SPG11 gene: widespread degeneration with eosinophilic inclusions

Background The detailed neuropathological features of patients with autosomal recessive hereditary spastic paraplegia with a thin corpus callosum (TCC) and SPG11 mutations are poorly understood, as only a few autopsies have been reported. Herein, we describe the clinicopathological findings of a patient with this disease who received long-term care at our medical facility. Case presentation A Japanese man exhibited a mild developmental delay in early childhood and intellectual disability, followed by the appearance of a spastic gait by age 13. At the age of 25 years, he became bedridden and needed a ventilator. Genetic analysis revealed a homozygous splice site variant in the SPG11 gene (c. 4162–2A > G) after the provision of genetic counselling and acquisition of informed consent from his parents. He died of pneumonia at the age of 44. His brain weighed 967 g and was characterized by a TCC, and his spinal cord was flattened. Microscopically, degeneration was observed in the posterior spinocerebellar tract, the gracile fasciculus, and the posterior column in addition to the corticospinal tract. Marked neuronal loss and gliosis were observed in the anterior horn, Clarke’s column, and hypoglossal and facial nuclei. Various types of neurons, in addition to motor neurons, showed coarse eosinophilic granules that were immunoreactive for p62. The loss of pigmented neurons with gliosis was apparent in both the substantia nigra and locus coeruleus. Lateral geniculate body degeneration was a characteristic feature of this patient. Furthermore, peripheral Lewy body-related α-synucleinopathy and scattered α-synuclein–immunoreactive neurites in the locus coeruleus and reticular formation of the brainstem were observed. Conclusions In patients with hereditary spastic paraplegia with SPG11 mutations, a variety of clinical phenotypes develop due to widespread lesions containing p62-immunoreactive neuronal cytoplasmic inclusions. We herein report the lateral geniculate body as another degenerative site related to SPG11-related pathologies that should be studied in future investigations.