A novel mutation in TTC19 associated with isolated complex III deficiency, cerebellar hypoplasia, and bilateral basal ganglia lesions

Pathological variants in nuclear genes causing mitochondrial complex III deficiency: An update

Mitochondrial disorders are a group of clinically and biochemically heterogeneous genetic diseases within the group of inborn errors of metabolism. Primary mitochondrial diseases are mainly caused by defects in one or several components of the oxidative phosphorylation system (complexes I-V). Within these disorders, those associated with complex III deficiencies are the least common. However, thanks to a deeper knowledge about complex III biogenesis, improved clinical diagnosis and the implementation of next-generation sequencing techniques, the number of pathological variants identified in nuclear genes causing complex III deficiency has expanded significantly. This updated review summarizes the current knowledge concerning the genetic basis of complex III deficiency, and the main clinical features associated with these conditions.

A TTC19 mutation associated with progressive movement disorders and peripheral neuropathy: Case report and systematic review

BACKGROUND

Mitochondrial complex III (CIII) deficiency is an autosomal recessive disease characterized by symptoms such as ataxia, cognitive dysfunction, and spastic paraplegia. Multiple genes are associated with complex III defects. Among them, the mutation of TTC19 is a rare subtype.

METHODS

We screened a Chinese boy with weakness of limbs and his non-consanguineous parents by whole exome sequencing and targeted sequencing.

RESULTS

We report a Chinese boy diagnosed with mitochondrial complex III defect type 2 carrying a homozygous variant (c.719-732del, p.Leu240Serfs*17) of the TTC19 gene. According to the genotype analysis of his family members, this is an autosomal recessive inheritance. We provide his clinical manifestation.

CONCLUSIONS

A new type of TTC19 mutation (c.719-732del, p.Leu240Serfs*17) was found, which enriched the TTC19 gene mutation spectrum and provided new data for elucidating the pathogenesis of CIII-deficient diseases.

Limitations of Multigene Next-Generation Sequencing Panel for "Cerebral Palsy" Phenotype and Other Complex Movement Disorders

In the past couple of decades, literature in pediatric neurology and clinical genetics has identified hundreds of monogenic disorders that can masquerade as infantile cerebral palsy (CP). Accurate and prompt diagnosis in such cases may be challenging due to several reasons. There are commercial multigene CP panels, but their diagnostic yield is often limited compared with exome sequencing because of diverse etiologies that may mimic CP. We report one such case where a patient with spastic hemiplegia underwent a long diagnostic journey before genetic diagnosis was established with exome sequencing and appropriate management was started. TTC19-related mitochondrial complex III deficiency is an ultrarare disorder of energy metabolism that presents with bilateral lesions in the basal ganglia and a degenerative neuropsychiatric phenotype.

LYRM7-associated mitochondrial complex III deficiency with non-cavitating leukoencephalopathy and stroke-like episodes

Defects of respiratory chain complex III (CIII) result in characteristic but rare mitochondrial disorders associated with distinct neuroradiological findings. The underlying molecular defects affecting mitochondrial CIII assembly factors are few and yet to be identified. LYRM7 assembly factor is required for proper CIII assembly where it acts as a chaperone for the Rieske iron-sulfur (UQCRFS1) protein in the mitochondrial matrix and stabilizing it. We present here the seventeenth individual with LYRM7-associated mitochondrial leukoencephalopathy harboring a previously reported rare pathogenic homozygous LYRM 7 variant, c.2T>C, (p.Met1?). Like previously reported individuals, our 5-year-old male proband presented with recurrent metabolic and lactic acidosis, encephalopathy, and fatigue. Further, he has additional, previously unreported features, including an acute stroke like episode with bilateral central blindness and optic neuropathy, recurrent hyperglycemia and hypertension associated with metabolic crisis. However, he has no signs of psychomotor regression. He has been stable clinically with residual left-sided reduced visual acuity and amblyopia, and no more metabolic crises for 2-year-period while on the mitochondrial cocktail. Although the reported brain MRI findings in other affected individuals are homogenous, it is slightly different in our index, revealing evidence of bilateral almost symmetric multifocal periventricular T2 hyperintensities with hyperintensities of the optic nerves, optic chiasm, and corona radiata but with no cavitation or cystic changes. This report describes new clinical and radiological findings of LYRM7-associated disease. The report also summarizes the clinical and molecular data of previously reported individuals describing the full phenotypic spectrum.

Mitochondrial protein dysfunction in pathogenesis of neurological diseases

Mitochondria are essential organelles for neuronal function and cell survival. Besides the well-known bioenergetics, additional mitochondrial roles in calcium signaling, lipid biogenesis, regulation of reactive oxygen species, and apoptosis are pivotal in diverse cellular processes. The mitochondrial proteome encompasses about 1,500 proteins encoded by both the nuclear DNA and the maternally inherited mitochondrial DNA. Mutations in the nuclear or mitochondrial genome, or combinations of both, can result in mitochondrial protein deficiencies and mitochondrial malfunction. Therefore, mitochondrial quality control by proteins involved in various surveillance mechanisms is critical for neuronal integrity and viability. Abnormal proteins involved in mitochondrial bioenergetics, dynamics, mitophagy, import machinery, ion channels, and mitochondrial DNA maintenance have been linked to the pathogenesis of a number of neurological diseases. The goal of this review is to give an overview of these pathways and to summarize the interconnections between mitochondrial protein dysfunction and neurological diseases.

A Systematic Review of the Impact of Mitochondrial Variations on Male Infertility

According to current estimates, infertility affects one in four couples trying to conceive. Primary or secondary infertility can be due either to both partners or only to the man or the woman. Up to 15% of infertility cases in men can be attributed to genetic factors that can lead to irreversible partial or complete spermatogenic arrest. The increased use of assisted reproductive technology (ART) has provided not only insights into the causes of male infertility but also afforded a diagnostic tool to detect and manage this condition among couples. Genes control a variety of physiological attributes, such as the hypothalamic–pituitary–gonadal axis, development, and germ cell differentiation. In the era of ART, it is important to understand the genetic basis of infertility so as to provide the most tailored therapy and counseling to couples. Genetic factors involved in male infertility can be chromosome abnormalities or single-gene disorders, mitochondrial DNA (mtDNA) mutations, Y-chromosome deletions, multifactorial disorders, imprinting disorders, or endocrine disorders of genetic origin. In this review, we discuss the role of mitochondria and the mitochondrial genome as an indicator of sperm quality and fertility.

Identification of mitochondria-related hub genes in sarcopenia and functional regulation of MFG-E8 on ROS-mediated mitochondrial dysfunction and cell cycle arrest

Sarcopenia has high prevalence in the elderly population, but the genes and pathways related to aging in elderly patients with sarcopenia are poorly understood. Milk fat globule epidermal growth factor 8 (MFG-E8) is a peripheral membrane glycoprotein isolated from the milk fat globule membrane (MFGM). It has been found to exhibit various nutritional effects, including antibacterial, anti-cancer, anti-oxidant, anti-sarcopenia, and improving brain development and cognitive effects. This study aimed to investigate key differentially expressed genes (DEGs) and pathways associated with the progression of sarcopenia using bioinformatics analysis and in vitro myoblast experiment. The gene expression profiles of GSE8479 and GSE9676, which includes 40 young normal samples and 55 elderly samples, were downloaded from the Gene Expression Omnibus Database (GEO). Over 3253 DEGs were identified in the young and elderly samples (adjusted p value <0.05). A total of 213 co-expressed significantly DEGs were identified with Venn diagrams, including 82 up-regulated DEGs and 131 down-regulated DEGs. Based on the analysis of Gene Ontology (GO), protein-protein interaction (PPI) networks and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, 10 hub genes screened by our study have been proved to play a role in regulating the occurrence and development of aging-related sarcopenia mainly via metabolic pathways, Huntington's disease, Parkinson's disease, oxidative phosphorylation and non-alcoholic fatty liver disease pathways. To further verify the protective effect of MFG-E8 on oxidative stress injured myoblasts, the cell cycle distribution, cell viability and reactive oxygen species (ROS) production were measured. The protein and mRNA levels of Akt, extracellular regulated protein kinases (ERK), p21Cip1, p27Kip1, cyclin D1, cyclin E1, cyclin-dependent kinase (CDK) 2 and 4 were quantified using qRT-PCR and western blot analysis. The results indicated that MFG-E8 has potential anti-sarcopenia effects by promoting ERK and Akt activation-mediated cell proliferation and cell cycle progression in myoblasts.

Tackling Dysfunction of Mitochondrial Bioenergetics in the Brain

Oxidative phosphorylation (OxPhos) is the basic function of mitochondria, although the landscape of mitochondrial functions is continuously growing to include more aspects of cellular homeostasis. Thanks to the application of -omics technologies to the study of the OxPhos system, novel features emerge from the cataloging of novel proteins as mitochondrial thus adding details to the mitochondrial proteome and defining novel metabolic cellular interrelations, especially in the human brain. We focussed on the diversity of bioenergetics demand and different aspects of mitochondrial structure, functions, and dysfunction in the brain. Definition such as 'mitoexome', 'mitoproteome' and 'mitointeractome' have entered the field of 'mitochondrial medicine'. In this context, we reviewed several genetic defects that hamper the last step of aerobic metabolism, mostly involving the nervous tissue as one of the most prominent energy-dependent tissues and, as consequence, as a primary target of mitochondrial dysfunction. The dual genetic origin of the OxPhos complexes is one of the reasons for the complexity of the genotype-phenotype correlation when facing human diseases associated with mitochondrial defects. Such complexity clinically manifests with extremely heterogeneous symptoms, ranging from organ-specific to multisystemic dysfunction with different clinical courses. Finally, we briefly discuss the future directions of the multi-omics study of human brain disorders.

Organization of the Respiratory Supercomplexes in Cells with Defective Complex III: Structural Features and Metabolic Consequences

The mitochondrial respiratory chain encompasses four oligomeric enzymatic complexes (complex I, II, III and IV) which, together with the redox carrier ubiquinone and cytochrome c, catalyze electron transport coupled to proton extrusion from the inner membrane. The protonmotive force is utilized by complex V for ATP synthesis in the process of oxidative phosphorylation. Respiratory complexes are known to coexist in the membrane as single functional entities and as supramolecular aggregates or supercomplexes (SCs). Understanding the assembly features of SCs has relevant biomedical implications because defects in a single protein can derange the overall SC organization and compromise the energetic function, causing severe mitochondrial disorders. Here we describe in detail the main types of SCs, all characterized by the presence of complex III. We show that the genetic alterations that hinder the assembly of Complex III, not just the activity, cause a rearrangement of the architecture of the SC that can help to preserve a minimal energetic function. Finally, the major metabolic disturbances associated with severe SCs perturbation due to defective complex III are discussed along with interventions that may circumvent these deficiencies.

Mitochondrial disorders of the OXPHOS system

Mitochondrial disorders are among the most frequent inborn errors of metabolism, their primary cause being the dysfunction of the oxidative phosphorylation system (OXPHOS). OXPHOS is composed of the electron transport chain (ETC), formed by four multimeric enzymes and two mobile electron carriers, plus an ATP synthase [also called complex V (cV)]. The ETC performs the redox reactions involved in cellular respiration while generating the proton motive force used by cV to synthesize ATP. OXPHOS biogenesis involves multiple steps, starting from the expression of genes encoded in physically separated genomes, namely the mitochondrial and nuclear DNA, to the coordinated assembly of components and cofactors building each individual complex and eventually the supercomplexes. The genetic cause underlying around half of the diagnosed mitochondrial disease cases is currently known. Many of these cases result from pathogenic variants in genes encoding structural subunits or additional factors directly involved in the assembly of the ETC complexes. Here, we review the historical and most recent findings concerning the clinical phenotypes and the molecular pathological mechanisms underlying this particular group of disorders.

A Novel TTC19 Mutation in a Patient With Neurological, Psychological, and Gastrointestinal Impairment

Mitochondrial complex III deficiency nuclear type 2 is an autosomal-recessive disorder caused by mutations in TTC19 gene. TTC19 is involved in the preservation of mitochondrial complex III, which is responsible for transfer of electrons from reduced coenzyme Q to cytochrome C and thus, contributes to the formation of electrochemical potential and subsequent ATP generation. Mutations in TTC19 have been found to be associated with a wide range of neurological and psychological manifestations. Herein, we report on a 15-year-old boy born from first-degree cousin parents, who initially presented with psychiatric symptoms. He subsequently developed progressive ataxia, spastic paraparesis with involvement of caudate bodies and lentiform nuclei with cerebellar atrophy. Eventually, the patient developed gastrointestinal involvement. Using whole-exome sequencing (WES), we identified a novel homozygous frameshift mutation in the TTC19 gene in the patient (NM_017775.3, c.581delG: p.Arg194Asnfs^*16). Advanced genetic sequencing technologies developed in recent years have not only facilitated identification of novel disease genes, but also allowed revelations about novel phenotypes associated with mutations in the genes already linked with other clinical features. Our findings expanded the clinical features of TTC19 mutation to potentially include gastrointestinal involvement. Further functional studies are needed to elucidate the underlying pathophysiological mechanisms.

Novel Homozygous Variant in TTC19 Causing Mitochondrial Complex III Deficiency with Recurrent Stroke-Like Episodes: Expanding the Phenotype

A 7-year-old boy with family history of consanguinity presented with developmental delay and recurrent hemiplegia involving both sides of the body, with variable facial and ocular involvement. Brain MRI showed bilateral striatal necrosis with cystic degeneration and lactate peaks on spectroscopy. Biochemical testing demonstrated mildly elevated lactate and pyruvate. Whole-exome sequencing revealed a novel homozygous pathogenic frameshift mutation in gene TTC19, diagnostic of mitochondrial complex III deficiency.

Biogenesis of the bc1 Complex of the Mitochondrial Respiratory Chain

The oxidative phosphorylation system contains four respiratory chain complexes that connect the transport of electrons to oxygen with the establishment of an electrochemical gradient over the inner membrane for ATP synthesis. Due to the dual genetic source of the respiratory chain subunits, its assembly requires a tight coordination between nuclear and mitochondrial gene expression machineries. In addition, dedicated assembly factors support the step-by-step addition of catalytic and accessory subunits as well as the acquisition of redox cofactors. Studies in yeast have revealed the basic principles underlying the assembly pathways. In this review, we summarize work on the biogenesis of the bc₁ complex or complex III, a central component of the mitochondrial energy conversion system.

TTC19 Plays a Husbandry Role on UQCRFS1 Turnover in the Biogenesis of Mitochondrial Respiratory Complex III

Loss-of-function mutations in TTC19 (tetra-tricopeptide repeat domain 19) have been associated with severe neurological phenotypes and mitochondrial respiratory chain complex III deficiency. We previously demonstrated the mitochondrial localization of TTC19 and its link with complex III biogenesis. Here we provide detailed insight into the mechanistic role of TTC19, by investigating a Ttc19?/? mouse model that shows progressive neurological and metabolic decline, decreased complex III activity, and increased production of reactive oxygen species. By using both the Ttc19?/? mouse model and a range of human cell lines, we demonstrate that TTC19 binds to the fully assembled complex III dimer, i.e., after the incorporation of the iron-sulfur Rieske protein (UQCRFS1). The in situ maturation of UQCRFS1 produces N-terminal polypeptides, which remain bound to holocomplex III. We show that, in normal conditions, these UQCRFS1 fragments are rapidly removed, but when TTC19 is absent they accumulate within complex III, causing its structural and functional impairment.

Nuclear gene mutations as the cause of mitochondrial complex III deficiency

Complex III (CIII) deficiency is one of the least common oxidative phosphorylation defects associated to mitochondrial disease. CIII constitutes the center of the mitochondrial respiratory chain, as well as a crossroad for several other metabolic pathways. For more than 10 years, of all the potential candidate genes encoding structural subunits and assembly factors, only three were known to be associated to CIII defects in human pathology. Thus, leaving many of these cases unresolved. These first identified genes were MT-CYB, the only CIII subunit encoded in the mitochondrial DNA; BCS1L, encoding an assembly factor, and UQCRB, a nuclear-encoded structural subunit. Nowadays, thanks to the fast progress that has taken place in the last 3-4 years, pathological changes in seven more genes are known to be associated to these conditions. This review will focus on the strategies that have permitted the latest discovery of mutations in factors that are necessary for a correct CIII assembly and activity, in relation with their function. In addition, new data further establishing the molecular role of LYRM7/MZM1L as a chaperone involved in CIII biogenesis are provided.

Mutations in TTC19: expanding the molecular, clinical and biochemical phenotype

BACKGROUND

TTC19 deficiency is a progressive neurodegenerative disease associated with isolated mitochondrial respiratory chain (MRC) complex III deficiency and loss-of-function mutations in the TT19 gene in the few patients reported so far.

METHODS

We performed exome sequencing and selective mutational analysis of TTC19, respectively, in patients from three unrelated families presenting with initially unspecific clinical signs of muscular hypotonia and global developmental delay followed by regression, ataxia, loss of speech, and rapid neurological deterioration. One patient showed severe lactic acidosis at the neonatal age and during intercurrent illness.

RESULTS

We identified homozygous mutations in all three index cases, in two families novel missense mutations (c.544 T > C/p.Leu185Pro; c.917 T > C/p.Leu324Pro). The younger sister of the severely affected patient 3 showed only mild delay of motor skills and muscular hypotonia so far but is also homozygous for the same mutation. Notably, one patient revealed normal activities of MRC complex III in two independent muscle biopsies. Neuroimaging of the severely affected patients demonstrated lesions in putamen and caudate nuclei, cerebellar atrophy, and the unusual finding of hypertrophic olivary nuclei degeneration. Reviewing the literature revealed striking similarities regarding neuroimaging and clinical course in pediatric patients with TTC19 deficiency: patterns consistent with Leigh or Leigh-like syndrome were found in almost all, hypertrophic olivary nucleus degeneration in all patients reported so far. The clinical course in pediatric patients is characterized by an initially unspecific developmental delay, followed by regression, progressive signs and symptoms of cerebellar, basal ganglia and brainstem affection, especially loss of speech and ataxia. Subsequently, neurological deterioration leading to a vegetative state occurs.

CONCLUSIONS

Our findings add to the phenotypic, genetic, and biochemical spectrum of TTC19 deficiency. However, TTC19 deficient patients do show characteristic clinical and neuroimaging features, which may facilitate diagnosis of this yet rare disorder. Normal MRC complex III activity does not exclude the diagnosis.