Identification of extremely rare mitochondrial disorders by whole exome sequencing

Molecular pathways in mitochondrial disorders due to a defective mitochondrial protein synthesis

Mitochondria play a central role in cellular metabolism producing the necessary ATP through oxidative phosphorylation. As a remnant of their prokaryotic past, mitochondria contain their own genome, which encodes 13 subunits of the oxidative phosphorylation system, as well as the tRNAs and rRNAs necessary for their translation in the organelle. Mitochondrial protein synthesis depends on the import of a vast array of nuclear-encoded proteins including the mitochondrial ribosome protein components, translation factors, aminoacyl-tRNA synthetases or assembly factors among others. Cryo-EM studies have improved our understanding of the composition of the mitochondrial ribosome and the factors required for mitochondrial protein synthesis and the advances in next-generation sequencing techniques have allowed for the identification of a growing number of genes involved in mitochondrial pathologies with a defective translation. These disorders are often multisystemic, affecting those tissues with a higher energy demand, and often present with neurodegenerative phenotypes. In this article, we review the known proteins required for mitochondrial translation, the disorders that derive from a defective mitochondrial protein synthesis and the animal models that have been established for their study.

A novel biallelic frameshift variant in C2orf69 causing developmental regression, seizures, microcephaly, autistic features, and hypertonia

Combined oxidative phosphorylation deficiency type 53 (COXPD53) is an autosomal recessive neurodevelopmental disorder (NDD) caused by homozygous variants in the gene C2orf69. Here, we report a novel frameshift variant c.187_191dupGCCGA, p.D64Efs*56 identified in an individual with clinical presentation of COXPD53 with developmental regression and autistic features. The variant c.187_191dupGCCGA, p.D64Efs*56 represents the most N-terminal part of C2orf69. Notable clinical features of COXPD53of the proband include developmental delay, developmental regression, seizures, microcephaly, and hypertonia. Structural brain defects of cerebral atrophy, cerebellar atrophy, hypomyelination, and thin corpus callosum were also observed. While we observe strong phenotypic overlap among affected individuals with C2orf69 variants, developmental regression and autistic features have not been previously described in individuals with COXPD53. Together, this case expands the genetic and clinical phenotypic spectrum of C2orf69-associated COXPD53.

AARS2-Related Leukodystrophy: a Case Report and Literature Review

Mutations in the alanyl-transfer RNA synthase 2 (AARS2) represent a heterogenous group of autosomal recessive leukodystrophy characterized by cognitive decline, ataxia, spasticity, and Parkinsonism. AARS2-related leukodystrophy (AARS2-L) is extremely rare. To date, only 45 genetically confirmed cases, explaining the frequent diagnostic delay. Here, we report a 21-year-old male presented with unsteady gait and weakness in the bilateral lower extremities. Examination revealed dysarthria, cerebellar ataxia, paraparesis, and Parkinsonism with generalized hyperreflexia. MRI findings showed extensive white matter lesions in bilateral frontoparietal lobes, immediate periventricular regions, and corpus callosum. Focused exome sequencing revealed compound heterozygous mutations in the AARS2 gene confirming the diagnosis of AARS2-L; two heterogeneous missense mutations (c.452 T > C, p. M151T; c. 2557C > T, p. R853W) appeared together for the first time. We also reviewed phenotypic spectra of AARS2-related leukodystrophies from a total of 45 reported cases.

Novel mitochondrial alanyl-tRNA synthetase 2 (AARS2) heterozygous mutations in a Chinese patient with adult-onset leukoencephalopathy

Background

Missense mutations in the mitochondrial alanyl-tRNA synthetase 2 (AARS2) gene are clinically associated with infantile mitochondrial cardiomyopathy or adult-onset leukoencephalopathy with early ovarian failure. To date, approximately 40 cases have been reported related to AARS2 mutations, while its genetic and phenotypic spectrum remains to be defined.

Case presentation

We identified a 24-year-old Chinese female patient with adult-onset leukoencephalopathy carrying novel compound heterozygous pathogenic mutations in the AARS2 gene (c.718C > T and c.1040 + 1G > A) using a whole-exome sequencing approach.

Conclusions

Our findings further extend the mutational spectrum of AARS2-related leukoencephalopathy and highlight the importance of the whole-exome sequencing in precisely diagnosing adult-onset leukoencephalopathies.

Case Report: Whole Exome Sequencing Identifies Compound Heterozygous Variants in TSFM Gene Causing Juvenile Hypertrophic Cardiomyopathy

We report a case of hypertrophic cardiomyopathy and lactic acidosis in a 3-year-old female. Cardiac and skeletal muscles biopsies exhibited mitochondrial hyperplasia with decreased complex IV activity. Whole exome sequencing identified compound heterozygous variants, p.Arg333Trp and p.Val119Leu, in TSFM, a nuclear gene that encodes a mitochondrial translation elongation factor, resulting in impaired oxidative phosphorylation and juvenile hypertrophic cardiomyopathy.

The emerging neurological spectrum of AARS2-associated disorders

BACKGROUND

The AARS2 gene encodes a mitochondrial alanyl-transfer RNA synthetase. Defects in this gene have been linked with autosomal recessive inheritance of a variety of different clinical phenotypes.

CASE

A 13 year-old boy developed behavioral and psychiatric problems following a mild head injury. At age 21 he developed tremor, parkinsonism, and eye nystagmus. MRI revealed white matter changes consistent with a leukoencephalopathy. Genetic studies revealed two pathogenic mutations in the AARS2 gene (c.647dupG and c.595C > T).

LITERATURE REVIEW

Only 47 cases of AARS2-associated disorders have been reported, with equal numbers of males and females, and age at onset ranging from infancy to 44 years. The most common clinical problems include movement disorders (71%), cognitive impairment (67%), corticospinal signs (64%), behavioral or psychiatric features (46%), and eye signs (34%). Imaging evidence suggestive of leukoencephalopathy is common, but not invariant. Premature ovarian failure is frequent in females, but not universal.

CONCLUSIONS

Defects in the AARS2 gene are a rare cause for a variety of movement disorders, often associated with brain imaging evidence suggestive of leukoencephalopathy.

Clinical Characteristics of Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-Like Episodes

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, a maternally inherited mitochondrial disorder, is characterized by its genetic, biochemical and clinical complexity. The most common mutation associated with MELAS syndrome is the mtDNA A3243G mutation in the MT-TL1 gene encoding the mitochondrial tRNA-leu(UUR), which results in impaired mitochondrial translation and protein synthesis involving the mitochondrial electron transport chain complex subunits, leading to impaired mitochondrial energy production. Angiopathy, either alone or in combination with nitric oxide (NO) deficiency, further contributes to multi-organ involvement in MELAS syndrome. Management for MELAS syndrome is amostly symptomatic multidisciplinary approach. In this article, we review the clinical presentations, pathogenic mechanisms and options for management of MELAS syndrome.

A novel compound heterozygous mutation of the MTO1 gene associated with complex oxidative phosphorylation deficiency type 10

BACKGROUND

The mitochondrial tRNA translation optimization 1 (MTO1) gene, which is closely related to defective mitochondrial oxidative phosphorylation, is an evolutionarily conserved protein expressed in high energy-demanding tissues and is associated with complex oxidative phosphorylation deficiency type 10 (COXPD10) in humans. Related cases and studies are still scarce and have not been reported in the Chinese region.

MATERIALS AND METHODS

Detailed clinical assessment was applied to the patient. Based on next-generation sequencing technology, we performed whole-exome sequencing of the patient and the parents. Sanger sequencing was used for validation. Bioinformatics software and protein simulations were used to predict the pathogenicity of the variants.

RESULTS

The patient was diagnosed with a possible association with mitochondrial disease according to the clinical manifestations and physical examination. A novel frameshift mutation c.344delA (p. Asn115Thrfs*11) and a novel point mutation c.1055C > T (p. Thr352Met) in the MTO1 gene were identified. They were found to cause abnormal changes in amino acids and the protein by biochemical tools, indicating it may be pathogenic.

CONCLUSION

We present two novel and possibly pathogenic variants in the MTO1 gene in a Chinese Han family.

Mitochondrial Protein Translation: Emerging Roles and Clinical Significance in Disease

Mitochondria are one of the most important organelles in cells. Mitochondria are semi-autonomous organelles with their own genetic system, and can independently replicate, transcribe, and translate mitochondrial DNA. Translation initiation, elongation, termination, and recycling of the ribosome are four stages in the process of mitochondrial protein translation. In this process, mitochondrial protein translation factors and translation activators, mitochondrial RNA, and other regulatory factors regulate mitochondrial protein translation. Mitochondrial protein translation abnormalities are associated with a variety of diseases, including cancer, cardiovascular diseases, and nervous system diseases. Mutation or deletion of various mitochondrial protein translation factors and translation activators leads to abnormal mitochondrial protein translation. Mitochondrial tRNAs and mitochondrial ribosomal proteins are essential players during translation and mutations in genes encoding them represent a large fraction of mitochondrial diseases. Moreover, there is crosstalk between mitochondrial protein translation and cytoplasmic translation, and the imbalance between mitochondrial protein translation and cytoplasmic translation can affect some physiological and pathological processes. This review summarizes the regulation of mitochondrial protein translation factors, mitochondrial ribosomal proteins, mitochondrial tRNAs, and mitochondrial aminoacyl-tRNA synthetases (mt-aaRSs) in the mitochondrial protein translation process and its relationship with diseases. The regulation of mitochondrial protein translation and cytoplasmic translation in multiple diseases is also summarized.

Progressive External Ophthalmoplegia in Polish Patients-From Clinical Evaluation to Genetic Confirmation

Mitochondrial encephalomyopathies comprise a group of heterogeneous disorders resulting from impaired oxidative phosphorylation (OxPhos). Among a variety of symptoms progressive external ophthalmoplegia (PEO) seems to be the most common. The aim of this study is to present clinical and genetic characteristics of Polish patients with PEO. Clinical, electrophysiological, neuroradiological, and morphological data of 84 patients were analyzed. Genetic studies of mitochondrial DNA (mtDNA) were performed in all patients. Among nuclear DNA (nDNA) genes POLG was sequenced in 41 patients, TWNK (C10orf2) in 13 patients, and RNASEH1 in 2 patients. Total of 27 patients were included in the chronic progressive external ophthalmoplegia (CPEO) group, 24 in the CPEO+ group. Twenty-six patients had mitochondrial encephalomyopathy (ME), six patients Kearns-Sayre syndrome (KSS), and one patient sensory ataxic neuropathy, dysarthria, ophthalmoparesis (SANDO) syndrome. Genetic analysis of nDNA genes revealed the presence of pathogenic or possibly pathogenic variants in the POLG gene in nine patients, the TWNK gene in five patients and the RNASEH1 gene in two patients. Detailed patients' history and careful assessment of family history are essential in the diagnostic work-up. Genetic studies of both mtDNA and nDNA are necessary for the final diagnosis of progressive external ophthalmoplegia and for genetic counseling.

Proteomics and metabolomics of raw rhubarb and wine-processed rhubarb in the treatment of rats with intracerebral hemorrhage

Background

To explore the mechanisms of raw rhubarb and wine-processed rhubarb treatment in a rat model of intracerebral hemorrhage (ICH).

Methods

After adapting to their environment, 30 male Wistar rats were divided into 5 treatment groups: blank control group (CK) (normal saline), sham operation group (SICH) (normal saline), ICH model group (ICH) (normal saline), ICH + raw rhubarb enema group (RO) (raw rhubarb 3.60 g/kg), and ICH + wine-processed rhubarb enema group (WRO) (wine-processed rhubarb 3.60 g/kg). The rhubarb enema (once a day) was given since 3 days before ICH treatment, and was consistently given until 12 hours, 24 hours, 3 days, and 7 days post operation. Serum oxidative stress and inflammatory markers were detected by ELISA, and differences between raw rhubarb and wine-processed rhubarb treatment in ICH rats were screened by proteomics and metabolomics.

Results

Both rhubarb treatments effectively improved oxidative stress and inflammatory responses. After ICH, the proteins and metabolites in the brain tissue were significantly altered. Compared with raw rhubarb, wine-processed rhubarb had a better protective effect by dysregulating amino acid metabolism in the ICH model.

Conclusions

Our study provides a basis for further research into the mechanisms of rhubarb treatment from different processing technologies, and promotes the application of rhubarb as a therapeutic approach to ICH.