Primary coenzyme Q₁₀ (CoQ₁₀) deficiency encompasses a subset of mitochondrial diseases caused by mutations affecting proteins involved in the CoQ₁₀ biosynthetic pathway. One of the most frequent clinical syndromes associated with primary CoQ₁₀ deficiency is the severe infantile multisystemic form, which, until recently, was underdiagnosed. In the last few years, the availability of genetic screening through whole exome sequencing and whole genome sequencing has enabled molecular diagnosis in a growing number of patients with this syndrome and has revealed new disease phenotypes and molecular defects in CoQ₁₀ biosynthetic pathway genes. Early genetic screening can rapidly and non-invasively diagnose primary CoQ₁₀ deficiencies. Early diagnosis is particularly important in cases of CoQ₁₀ deficient steroid-resistant nephrotic syndrome, which frequently improves with treatment. In contrast, the infantile multisystemic forms of CoQ₁₀ deficiency, particularly when manifesting with encephalopathy, present therapeutic challenges, due to poor responses to CoQ₁₀ supplementation. Administration of CoQ₁₀ biosynthetic intermediate compounds is a promising alternative to CoQ₁₀; however, further pre-clinical studies are needed to establish their safety and efficacy, as well as to elucidate the mechanism of actions of the intermediates. Here, we review the molecular defects causes of the multisystemic infantile phenotype of primary CoQ₁₀ deficiency, genotype-phenotype correlations, and recent therapeutic advances.

Electron paramagnetic resonance spectroscopy (EPR) is an analytical technique that, uniquely, can be used to directly interrogate flash-frozen tissue. Quantitative information on the thermodynamic potential of the mitochondrion to synthesize ATP, and the extent of reactive oxygen species-mediated oxidative stress on the mitochondrion and the cell at large, can be obtained. A compromised ability to synthesize ATP and oxidative stress are two of the characteristic sequelae of mitochondrial disease and therapeutic approaches may differ widely depending on which of these is dominant. EPR, therefore, has a role to play in the characterization, diagnosis, and ongoing evaluation of therapies for mitochondrial disease in human patients and model systems. An introduction to mitochondrial disease is followed by a description of EPR, a summary of the EPR signals that can be expected from tissue samples, sample preparation and analytical methods, and a case study in which EPR and complementary techniques were employed on a rat model to study human mitochondrial disease.

Transfer RNA (tRNA) modification and aminoacylation are post-transcriptional processes that play a crucial role in the function of tRNA and thus represent critical steps in gene expression. Knowledge of the exact processes and effects of the defects in various tRNAs remains incomplete, but a rapidly increasing number of publications over the last decade has shown a growing amount of evidence as to the importance of tRNAs for normal human development, including brain formation and the development and maintenance of higher cognitive functions as well. In this review, we present a synopsis of the literature focusing on tRNA-modifying enzymes and aminoacyl-tRNA synthetases (ARSs) that have been found to be involved in the etiology of hereditary forms of intellectual disability. Our overview shows several parallels but also differences in the symptomatic spectrum observed in individuals affected by intellectual disability caused by mutations in tRNA modifier and/or ARS genes. This observation suggests that tRNAs seem to assume diverse roles in a variety of cellular processes possibly even beyond translation and that not only the abundance but also the modification and aminoacylation levels of tRNAs contribute to cell functions in ways that still remain to be understood.

In eukaryotic cells, mitochondria perform the essential function of producing cellular energy in the form of ATP via the oxidative phosphorylation system. This system is composed of 5 multimeric protein complexes of which 13 protein subunits are encoded by the mitochondrial genome: Complex I (7 subunits), Complex III (1 subunit), Complex IV (3 subunits), and Complex V (2 subunits). Effective mitochondrial translation is necessary to produce the protein subunits encoded by the mitochondrial genome (mtDNA). Defects in mitochondrial translation are known to cause a wide variety of clinical disease in humans with high-energy consuming organs generally most prominently affected. Here, we review several classes of disease resulting from defective mitochondrial translation including disorders with mitochondrial tRNA mutations, mitochondrial aminoacyl-tRNA synthetase disorders, mitochondrial rRNA mutations, and mitochondrial ribosomal protein disorders.

Aim: The North American Mitochondrial Disease Consortium (NAMDC) comprises a network of 17 clinical centers with a mission to conduct translational research on mitochondrial diseases. NAMDC is a part of the Rare Disease Clinical Research Network (RDCRN) and is funded by the National Institutes of Health. To foster its mission, NAMDC has implemented a comprehensive Mitochondrial Disease Clinical Registry (hereafter NAMDC Registry), collected biosamples deposited into the NAMDC Biorepository, defined phenotypes and genotypes of specific disorders, collected natural history data, identified outcome measures, characterized safety and long-term toxicity and efficacy of promising therapies, and trained young investigators interested in patient-oriented research in mitochondrial disease.

Methods: Research conducted by NAMDC is built on the foundation of the Clinical Registry. Data within the registry are encrypted and maintained in a centralized database at Columbia University Medical Center. In addition to clinical data, NAMDC has established a mitochondrial disease biorepository, collecting DNA, plasma, cell, and tissue samples. Specimens are assigned coded identifiers in compliance with all relevant regulatory entities and with emerging NIH guidelines for biorepositories. NAMDC funds two pilot projects each year. Pilot grants are small grants typically supporting an early stage concept to obtain preliminary data. Pilot grants must enhance and address major issues in mitochondrial medicine and specific areas of need for the field and for the successful outcome of NAMDC. The grant selection process is facilitated by input from multiple stakeholders including patient organizations and the strategic leadership of NAMDC. To train new mitochondrial disease investigators, NAMDC has established a Fellowship Program which offers a unique training opportunity to senior postdoctoral clinical fellows. The fellowship includes a 6-month period of intensive training in clinical trial methodology through the Clinical Research Enhancement through Supplemental Training program and equivalent programs at the other sites, along with rotations up to 3 months each to two additional consortium sites where a rich and varied training experience is provided. Nine core educational sites participate in this training program, each offering a summer grant program in mitochondrial medicine funded by our NAMDC partner the United Mitochondrial Disease Foundation (www.umdf.org). All clinical research in NAMDC depends on the participation of mitochondrial disease patients. Since individual mitochondrial disorders are often extremely rare, major communication and recruitment efforts are required. Therefore, NAMDC has forged a very close partnership with the premier patient advocacy group for mitochondrial diseases in North America, the United Mitochondrial Disease Foundation (UMDF).

Results: The NAMDC Registry has confirmed the clinical and genetical heterogeneity of mitochondrial diseases due to primary mutations in mitochondrial DNA or nuclear DNA. During the 8 years of this NIH-U54 grant, this consortium, acting in close collaboration with a patient advocacy group, the UMDF, has effectively addressed these complex diseases. NAMDC has expanded a powerful patient registry with more than 1600 patients enrolled to date, a website for education and recruitment of patients (www.namdc.org), a NAMDC biorepository housed at the Mayo Clinic in Rochester, MN, and essential diagnostic guidelines for consensus research. In addition, eight clinical studies have been initiated and the NAMDC fellowship program has been actively training the next generation of mitochondrial disease clinical investigators, of which six have completed the program and remain actively involved in mitochondrial disease research.

Conclusion: The NAMDC Patient Registry and Biorepository is actively facilitating mitochondrial disease research, and accelerating progress in the understanding and treatment of mitochondrial diseases.

Aim: This study aimed to characterize MECP2 gene variants in Indian female patients with classical Rett syndrome (RTT).

Methods: Seventy-two patients fulfilling the revised diagnostic criteria of classical RTT were enrolled and exons 2-4 of MECP2 gene were analyzed by Sanger sequencing followed by quantitative analysis using MLPA. Bioinformatic analysis was done using different software packages to predict the effect of sequence variations on the function of the MeCP2 protein.

Results: A heterogeneous spectrum of MECP2 sequence variants including 13 novel variants was identified with a detection rate of 98.6%. The majority of the variants were distributed in the functional domain of MECP2 with most missense variants clustered in methyl binding domain and truncating variants in interdomain and transcription repression domain of MECP2. Genotype-phenotype correlations revealed that patients carrying early truncating variants presented with a more severe phenotype.

Conclusion: RTT is a childhood neurodevelopmental disorder primarily affecting females. It is caused by mutations in the Methyl-CpG-Binding Protein 2 gene (MECP2 ), an important regulator of gene expression, located at Xq28. Variants in MECP2 can be identified in 95%-97% of individuals with Classical RTT using a combination of molecular techniques. This large cohort study from India showed the highest detection rate of MECP2 variants in classical RTT patients, emphasizing the importance of using diagnostic criteria and having a multidisciplinary team in the assessment of RTT patients, which can further help provide diagnostic testing, genetic counseling, and prenatal testing.

Approximately 2% of the world population is affected by intellectual disability (ID). Huge efforts in sequencing and analysis of individual human genomes have identified several genes and genetic/genomic variants associated with ID. Despite all this knowledge, the relationship between genes, pathophysiology and molecular mechanisms of ID remain highly complex. We summarize the genomic advances related to ID, provide examples on how to discern correlative versus causative roles in genetic variation, understand the physiological consequences of identified variants, and discuss future challenges.