The m.7510T>C mutation: Hearing impairment and a complex neurologic phenotype

Syrian child carrying multiple pathogenic variants in MBOAT7 and MT-TS1 genes: a case report on neurodevelopmental phenotypes and mitochondrial inheritance

Introduction

The authors identify two patterns of inheritance in a Syrian child from consanguineous parents. The membrane-bound O-acyltranferase domain-containing7 (MBOAT7) gene encodes Lysophosphatidylinositol acyltranferase (LPIAT1), which is responsible for the neurodevelopment of the brain cortex. Patients with MBOAT7 variants exhibit pathogenic nervous manifestations such as global developmental delays affecting speech and motor function, intellectual disability (ID), poor coordination, and seizures, with or without MRI abnormalities. MT_TS1, the mitochondrial tRNASer(UCN) gene, is a hotspot for pathogenic mutations causing variable mitochondrial phenotypes, including hearing impairment (HI), ataxia and cognitive impairment.

Clinical presentation

The authors present a case of a 4-year-old child with motor and speech delay, truncal hypotonia, visual tic, poor coordination, autistic features and generalized seizures at 7 months of age. After normal results from lab tests and MRI imaging, along with the family's history of neurological disorders, genetic analysis was necessary to diagnose and assess the possibility of genetic counselling. Next-generation sequencing (NGS) showed two variable variants in the MBOAT7 and MT-TS1 genes. The first mutation is a homozygous variant of uncertain significance in the MBOAT7 gene, associated with the autosomal recessive Mental retardation type 57. The second variant is a heteroplasmic pathogenic variant in the MT-TS1 gene, indicative of mitochondrial disorders.

Conclusion

The presence of the MBOAT7 and MT-TS1 gene variants in the same child is noteworthy. The authors must keep genetic mutations of MBOAT7 and MT-TS1 gene in mind as a differential diagnosis for intellectual disability, seizures and autistic features in children, especially in consanguineous families.

Mitochondrial tRNASer(UCN) mutations associated non-syndromic sensorineural hearing loss in Chinese families

Mitochondrial transfer RNA mutation is one of the most important causes of hereditary hearing loss in humans. Mitochondrial transfer RNASer (UCN) gene is another hot spot for mutations associated with non-syndromic hearing loss, besides the 12S ribosomal RNA gene. In this study, we assessed the clinical phenotype and the molecular characteristics of two Chinese families with non-syndromic hearing loss. Mutational analysis revealed that 7445A > G and 7510T > C mutations in the mitochondrial transfer RNASer (UCN) gene were the molecular etiology of Family 1 and Family 2, respectively. However, the clinical and genetic characteristics of the two families carrying the above mutations in the transfer RNASer (UCN) gene exhibited a variable expression of hearing loss and an incomplete penetrance. Sequencing analysis of the complete mitochondrial genome showed the presence of transfer RNATrp 5568A > G and NADH-ubiquinone oxidoreductase chain 4 11696G > A mutations in Family 1. The mitochondrial haplotype analysis showed that the two families belonged to Asian D4 and M80'D haplotypes, respectively, and no pathogenic variations were found in the nuclear genes. To our knowledge, our study is the first to report 7445A > G and 7510T > C mutations in the mitochondrial transfer RNASer (UCN) gene, in multi-generation non-syndromic hearing loss pedigrees from China. Our study suggests that 5568A > G and 11696G > A mutations may enhance the penetrance of hearing loss in Chinese Family 1, while mitochondrial haplotypes and known nuclear genes may not be modifiers for the phenotypic expression of 7445A > G and 7510T > C mutations in these Chinese families.

A mitochondrial myopathy-associated tRNASer(UCN) 7453G>A mutation alters tRNA metabolism and mitochondrial function

BACKGROUND

Mitochondrial disorders are a group of heterogeneous diseases characterized by biochemical disturbances in oxidative phosphorylation (OXPHOS). Mutations in mitochondrial transfer RNA (mt-tRNA) genes are the most frequently in mitochondrial disease. However, few studies have detailed the molecular mechanisms behind these mutations.

METHODS

We performed clinical evaluation, genetic analysis, muscle histochemistry, and molecular and biochemical investigations in muscle tissue and proband-derived cybrid cell lines.

RESULTS

We found a mitochondrial tRNA^Ser(UCN) mutation (m.7453G>A) in a 15-year-old patient with severe mitochondrial myopathy. We demonstrated that this mutation caused impairment of mitochondrial translation, respiratory deficiency, overproduction of reactive oxygen species (ROS), and decreased mitochondrial membrane potential (MMP), which ultimately led to severe mitochondrial myopathy.

CONCLUSION

Our findings offer valuable new insights into the tRNA^Ser(UCN) m.7453G>A mutation for both the pathogenic mechanism and functional consequences.

Ancient mitochondrial DNA pathogenic variants putatively associated with mitochondrial disease

Mitochondrial DNA variants associated with diseases are widely studied in contemporary populations, but their prevalence has not yet been investigated in ancient populations. The publicly available AmtDB database contains 1443 ancient mtDNA Eurasian genomes from different periods. The objective of this study was to use this data to establish the presence of pathogenic mtDNA variants putatively associated with mitochondrial diseases in ancient populations. The clinical significance, pathogenicity prediction and contemporary frequency of mtDNA variants were determined using online platforms. The analyzed ancient mtDNAs contain six variants designated as being "confirmed pathogenic" in modern patients. The oldest of these, m.7510T>C in the MT-TS1 gene, was found in a sample from the Neolithic period, dated 5800-5400 BCE. All six have well established clinical association, and their pathogenic effect is corroborated by very low population frequencies in contemporary populations. Analysis of the geographic location of the ancient samples, contemporary epidemiological trends and probable haplogroup association indicate diverse spatiotemporal dynamics of these variants. The dynamics in the prevalence and distribution is conceivably result of de novo mutations or human migrations and subsequent evolutionary processes. In addition, ten variants designated as possibly or likely pathogenic were found, but the clinical effect of these is not yet well established and further research is warranted. All detected mutations putatively associated with mitochondrial disease in ancient mtDNA samples are in tRNA coding genes. Most of these mutations are in a mt-tRNA type (Model 2) that is characterized by loss of D-loop/T-loop interaction. Exposing pathogenic variants in ancient human populations expands our understanding of their origin and prevalence dynamics.

The m.7510T>C mutation: Hearing impairment and a complex neurologic phenotype

OBJECTIVES

Mutations in mitochondrial DNA cause a variety of clinical phenotypes ranging from a mild hearing impairment (HI) to severe encephalomyopathy. The MT-TS1 gene is a hotspot for mutations causing HI. The m.7510T>C mutation in MT-TS1 has been previously associated with non-syndromic HI in four families from different ethnic backgrounds.

MATERIALS AND METHODS

We describe the clinical, genetic, and histopathological findings in a Finnish family with the heteroplasmic m.7510T>C mutation in mitochondrial DNA.

RESULTS

The family proband presented with a progressive mitochondrial disease phenotype including migraine, epilepsy, mild ataxia, and cognitive impairment in addition to HI. One young adult presented with HI only. Other family members had a mild phenotype comprising ataxia and tremor in addition to HI. Mutation heteroplasmy was 90% in the blood of maternal grandmother and ≥99% in the muscle and blood of the three other family members. Muscle histology was consistent with mitochondrial myopathy in three family members. The mitochondrial haplogroup of the family was a different branch of the haplogroup H than in the previous reports of this mutation.

CONCLUSION

Our results suggest that, in addition to sensorineural HI, the m.7510T>C mutation is associated with a spectrum of mitochondrial disease clinical features including migraine, epilepsy, cognitive impairment, ataxia, and tremor, and with evidence of mitochondrial myopathy.