A novel mutation in NDUFB11 unveils a new clinical phenotype associated with lactic acidosis and sideroblastic anemia

Comprehensive Genomic Analysis Identifies a Diverse Landscape of Sideroblastic and Nonsideroblastic Iron-Related Anemias with Novel and Pathogenic Variants in an Iron-Deficient Endemic Setting

Inherited iron metabolism defects are possibly missed or underdiagnosed in iron-deficient endemic settings because of a lack of awareness or a methodical screening approach. Hence, we systematically evaluated anemia cases (2019 to 2021) based on clinical phenotype, normal screening tests (high-performance liquid chromatography, α gene sequencing, erythrocyte sedimentation rate, C-reactive protein, and tissue transglutaminase), and abnormal iron profile by targeted next-generation sequencing (26-gene panel) supplemented with whole-exome sequencing, multiplex ligation probe amplification/mitochondrial DNA sequencing, and chromosomal microarray. Novel variants in ALAS2, STEAP3, and HSPA9 genes were functionally validated. A total of 290 anemia cases were screened, and 41 (14%) enrolled for genomic testing as per inclusion criteria. Comprehensive genomic testing revealed pathogenic variants in 23 of 41 cases (56%). Congenital sideroblastic anemia was the most common diagnosis (14/23; 61%), with pathogenic variations in ALAS2 (n = 6), SLC25A38 (n = 3), HSPA9 (n = 2) and HSCB, SLC19A2, and mitochondrial DNA deletion (n = 1 each). Nonsideroblastic iron defects included STEAP3-related microcytic anemia (2/23; 8.7%) and hypotransferrenemia (1/23; 4.3%). A total of 6 of 22 cases (27%) revealed a non-iron metabolism gene defect on whole-exome sequencing. Eleven novel variants (including variants of uncertain significance) were noted in 13 cases. Genotype-phenotype correlation revealed a significant association of frameshift/nonsense/splice variants with lower presentation age (0.8 months versus 9 years; P < 0.01) compared with missense variants. The systematic evaluation helped uncover an inherited iron defect in 41% (17/41) of cases, suggesting the need for active screening and awareness for these rare diseases in an iron-deficient endemic population.

High expression of NADH Ubiquinone Oxidoreductase Subunit B11 induces catheter-associated venous thrombosis on continuous blood purification

Venous thromboembolism (VTE) is a common vascular disease of venous return disorders, including deep venous thrombosis and pulmonary embolism (PE), with high morbidity and high mortality. However, the relationship between oxidative phosphorylation and NDUFB11 and venous thromboembolism is still unclear. The venous thromboembolism datasets GSE48000 and GSE19151 were downloaded, and the differentially expressed Genes (DEGs) were screened. The protein-protein interaction (PPI) network was constructed. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) were used for functional enrichment analysis. The comparative toxicogenomics database (CTD) was used to identify the diseases most associated with the core genes. TargetScan was used to screen miRNA regulating central DEGs. Western blotting (WB) experiment and real-time quantitative PCR (RT-qPCR) experiment were performed. A total of 500 DEGs were identified. GO analysis showed that the DEGs were mainly enriched in ATP synthesis coupled electron transport, respiratory electron transport chain, cytoplasm, enzyme binding, nonalcoholic fatty liver disease, oxidative phosphorylation, and Alzheimer disease. Enrichment items were similar to GO and KEGG enrichment items of DEGs. The result of CTD showed that 12 genes (RPS24, FAU, RPLP0, RPS15A, RPS29, RPL9, RPL31, RPL27, NDUFB11, RPL34, COX7B, RPS27L) were associated with chemical and drug-induced liver injury, inflammation, kidney disease, and congenital pure red cell aplasia. WB and RT-qPCR results showed that the expression levels of 12 genes in venous thromboembolism were higher than normal whole blood tissue samples. NDUFB11 is highly expressed in catheter-related venous thromboembolism during continuous blood purification, which may lead to the formation of venous thrombosis through oxidative phosphorylation pathway.

AOX delays the onset of the lethal phenotype in a mouse model of Uqcrh (complex III) disease

The alternative oxidase, AOX, provides a by-pass of the cytochrome segment of the mitochondrial respiratory chain when the chain is unavailable. AOX is absent from mammals, but AOX from Ciona intestinalis is benign when expressed in mice. Although non-protonmotive, so does not contribute directly to ATP production, it has been shown to modify and in some cases rescue phenotypes of respiratory-chain disease models. Here we studied the effect of C. intestinalis AOX on mice engineered to express a disease-equivalent mutant of Uqcrh, encoding the hinge subunit of mitochondrial respiratory complex III, which results in a complex metabolic phenotype beginning at 4-5 weeks, rapidly progressing to lethality within a further 6-7 weeks. AOX expression delayed the onset of this phenotype by several weeks, but provided no long-term benefit. We discuss the significance of this finding in light of the known and hypothesized effects of AOX on metabolism, redox homeostasis, oxidative stress and cell signaling. Although not a panacea, the ability of AOX to mitigate disease onset and progression means it could be useful in treatment.

The Mysterious Multitude: Structural Perspective on the Accessory Subunits of Respiratory Complex I

Complex I (CI) is the largest protein complex in the mitochondrial oxidative phosphorylation electron transport chain of the inner mitochondrial membrane and plays a key role in the transport of electrons from reduced substrates to molecular oxygen. CI is composed of 14 core subunits that are conserved across species and an increasing number of accessory subunits from bacteria to mammals. The fact that adding accessory subunits incurs costs of protein production and import suggests that these subunits play important physiological roles. Accordingly, knockout studies have demonstrated that accessory subunits are essential for CI assembly and function. Furthermore, clinical studies have shown that amino acid substitutions in accessory subunits lead to several debilitating and fatal CI deficiencies. Nevertheless, the specific roles of CI’s accessory subunits have remained mysterious. In this review, we explore the possible roles of each of mammalian CI’s 31 accessory subunits by integrating recent high-resolution CI structures with knockout, assembly, and clinical studies. Thus, we develop a framework of experimentally testable hypotheses for the function of the accessory subunits. We believe that this framework will provide inroads towards the complete understanding of mitochondrial CI physiology and help to develop strategies for the treatment of CI deficiencies.

Novel NDUFA12 variants are associated with isolated complex I defect and variable clinical manifestation

Isolated biochemical deficiency of mitochondrial complex I is the most frequent signature among mitochondrial diseases and is associated with a wide variety of clinical symptoms. Leigh syndrome represents the most frequent neuroradiological finding in patients with complex I defect and more than 80 monogenic causes have been involved in the disease. In this report, we describe seven patients from four unrelated families harboring novel NDUFA12 variants, with six of them presenting with Leigh syndrome. Molecular genetic characterization was performed using next-generation sequencing combined with the Sanger method. Biochemical and protein studies were achieved by enzymatic activities, blue native gel electrophoresis, and western blot analysis. All patients displayed novel homozygous mutations in the NDUFA12 gene, leading to the virtual absence of the corresponding protein. Surprisingly, despite the fact that in none of the analyzed patients, NDUFA12 protein was detected, they present a different onset and clinical course of the disease. Our report expands the array of genetic alterations in NDUFA12 and underlines phenotype variability associated with NDUFA12 defect.

Blackout in the powerhouse: clinical phenotypes associated with defects in the assembly of OXPHOS complexes and the mitoribosome

Mitochondria produce the bulk of the energy used by almost all eukaryotic cells through oxidative phosphorylation (OXPHOS) which occurs on the four complexes of the respiratory chain and the F₁–F₀ ATPase. Mitochondrial diseases are a heterogenous group of conditions affecting OXPHOS, either directly through mutation of genes encoding subunits of OXPHOS complexes, or indirectly through mutations in genes encoding proteins supporting this process. These include proteins that promote assembly of the OXPHOS complexes, the post-translational modification of subunits, insertion of cofactors or indeed subunit synthesis. The latter is important for all 13 of the proteins encoded by human mitochondrial DNA, which are synthesised on mitochondrial ribosomes. Together the five OXPHOS complexes and the mitochondrial ribosome are comprised of more than 160 subunits and many more proteins support their biogenesis. Mutations in both nuclear and mitochondrial genes encoding these proteins have been reported to cause mitochondrial disease, many leading to defective complex assembly with the severity of the assembly defect reflecting the severity of the disease. This review aims to act as an interface between the clinical and basic research underpinning our knowledge of OXPHOS complex and ribosome assembly, and the dysfunction of this process in mitochondrial disease.

Linear Skin Defects with Multiple Congenital Anomalies (LSDMCA): An Unconventional Mitochondrial Disorder

Mitochondrial disorders, although heterogeneous, are traditionally described as conditions characterized by encephalomyopathy, hypotonia, and progressive postnatal organ failure. Here, we provide a systematic review of Linear Skin Defects with Multiple Congenital Anomalies (LSDMCA), a rare, unconventional mitochondrial disorder which presents as a developmental disease; its main clinical features include microphthalmia with different degrees of severity, linear skin lesions, and central nervous system malformations. The molecular basis of this disorder has been elusive for several years. Mutations were eventually identified in three X-linked genes, i.e., HCCS, COX7B, and NDUFB11, which are all endowed with defined roles in the mitochondrial respiratory chain. A peculiar feature of this condition is its inheritance pattern: X-linked dominant male-lethal. Only female or XX male individuals can be observed, implying that nullisomy for these genes is incompatible with normal embryonic development in mammals. All three genes undergo X-inactivation that, according to our hypothesis, may contribute to the extreme variable expressivity observed in this condition. We propose that mitochondrial dysfunction should be considered as an underlying cause in developmental disorders. Moreover, LSDMCA should be taken into consideration by clinicians when dealing with patients with microphthalmia with or without associated skin phenotypes.

Regulation and tissue-specific expression of δ-aminolevulinic acid synthases in non-syndromic sideroblastic anemias and porphyrias

Recently, new genes and molecular mechanisms have been identified in patients with porphyrias and sideroblastic anemias (SA). They all modulate either directly or indirectly the δ-aminolevulinic acid synthase (ALAS) activity. ALAS, is encoded by two genes: the erythroid-specific (ALAS2), and the ubiquitously expressed (ALAS1). In the liver, ALAS1 controls the rate-limiting step in the production of heme and hemoproteins that are rapidly turned over in response to metabolic needs. Several heme regulatory targets have been identified as regulators of ALAS1 activity: 1) transcriptional repression via a heme-responsive element, 2) post-transcriptional destabilization of ALAS1 mRNA, 3) post-translational inhibition via a heme regulatory motif, 4) direct inhibition of the activity of the enzyme and 5) breakdown of ALAS1 protein via heme-mediated induction of the protease Lon peptidase 1. In erythroid cells, ALAS2 is a gatekeeper of production of very large amounts of heme necessary for hemoglobin synthesis. The rate of ALAS2 synthesis is transiently increased during the period of active heme synthesis. Its gene expression is determined by trans-activation of nuclear factor GATA1, CACC box and NF-E2-binding sites in the promoter areas. ALAS2 mRNA translation is also regulated by the iron-responsive element (IRE)/iron regulatory proteins (IRP) binding system. In patients, ALAS enzyme activity is affected in most of the mutations causing non-syndromic SA and in several porphyrias. Decreased ALAS2 activity results either directly from loss-of-function ALAS2 mutations as seen in X-linked sideroblastic anemia (XLSA) or from defect in the availability of one of its two mitochondrial substrates: glycine in SLC25A38 mutations and succinyl CoA in GLRX5 mutations. Moreover, ALAS2 gain of function mutations is responsible for X-linked protoporphyria and increased ALAS1 activity lead to acute attacks of hepatic porphyrias. A missense dominant mutation in the Walker A motif of the ATPase binding site in the gene coding for the mitochondrial protein unfoldase CLPX also contributes to increasing ALAS and subsequently protoporphyrinemia. Altogether, these recent data on human ALAS have informed our understanding of porphyrias and sideroblastic anemias pathogeneses and may contribute to new therapeutic strategies.

Molecular pathophysiology and genetic mutations in congenital sideroblastic anemia

Sideroblastic anemia is a heterogeneous congenital and acquired disorder characterized by anemia and the presence of ring sideroblasts in the bone marrow. Congenital sideroblastic anemia (CSA) is a rare disease caused by mutations in genes involved in the heme biosynthesis, iron-sulfur [Fe-S] cluster biosynthesis, and mitochondrial protein synthesis. The most prevalent form of CSA is X-linked sideroblastic anemia, caused by mutations in the erythroid-specific δ-aminolevulinate synthase (ALAS2), which is the first enzyme of the heme biosynthesis pathway in erythroid cells. To date, a remarkable number of genetically undefined CSA cases remain, but a recent application of the next-generation sequencing technology has recognized novel causative genes for CSA. However, in most instances, the detailed molecular mechanisms of how defects of each gene result in the abnormal mitochondrial iron accumulation remain unclear. This review aims to cover the current understanding of the molecular pathophysiology of CSA.

Pathogenic variants in glutamyl-tRNAGln amidotransferase subunits cause a lethal mitochondrial cardiomyopathy disorder

Mitochondrial protein synthesis requires charging a mitochondrial tRNA with its amino acid. Here, the authors describe pathogenic variants in the GatCAB protein complex genes required for the generation of glutaminyl-mt-tRNA^Gln, that impairs mitochondrial translation and presents with cardiomyopathy.

Mitochondrial protein synthesis requires charging mt-tRNAs with their cognate amino acids by mitochondrial aminoacyl-tRNA synthetases, with the exception of glutaminyl mt-tRNA (mt-tRNA^Gln). mt-tRNA^Gln is indirectly charged by a transamidation reaction involving the GatCAB aminoacyl-tRNA amidotransferase complex. Defects involving the mitochondrial protein synthesis machinery cause a broad spectrum of disorders, with often fatal outcome. Here, we describe nine patients from five families with genetic defects in a GatCAB complex subunit, including QRSL1, GATB, and GATC, each showing a lethal metabolic cardiomyopathy syndrome. Functional studies reveal combined respiratory chain enzyme deficiencies and mitochondrial dysfunction. Aminoacylation of mt-tRNA^Gln and mitochondrial protein translation are deficient in patients’ fibroblasts cultured in the absence of glutamine but restore in high glutamine. Lentiviral rescue experiments and modeling in S. cerevisiae homologs confirm pathogenicity. Our study completes a decade of investigations on mitochondrial aminoacylation disorders, starting with DARS2 and ending with the GatCAB complex.

Iron metabolism in erythroid cells and patients with congenital sideroblastic anemia

Sideroblastic anemias are anemic disorders characterized by the presence of ring sideroblasts in a patient's bone marrow. These disorders are typically divided into two types, congenital or acquired sideroblastic anemia. Recently, several genes were reported as responsible for congenital sideroblastic anemia; however, the relationship between the function of the gene products and ring sideroblasts is largely unclear. In this review article, we will focus on the iron metabolism in erythroid cells as well as in patients with congenital sideroblastic anemia.

Microphthalmia, Dermal Aplasia, and Sclerocornea Syndrome: Endoscopic Cyclophotocoagulation in the Management of Congenital Glaucoma

PURPOSE

To report on the use of endoscopic cyclophotocoagulation (ECP) to treat congenital glaucoma in a triple X female with microphthalmia, dermal aplasia, and sclerocornea (MIDAS) syndrome.

OBSERVATIONS

The patient demonstrated linear streaks on the face and neck consistent with dermal aplasia. The corneas were scleralized with ectatic areas of corneal thinning, and the eyes were microphthalmic. Ultrasound biomicroscopy demonstrated congenital aphakia and iris stumps. The patient had elevated intraocular pressure (IOP) that responded to topical glaucoma therapy in the right but not the left eye. Intraoperative endoscopy of the posterior segment revealed multiple hypopigmented chorioretinal lacunae surrounding a pale, cupped optic nerve. ECP of the ciliary processes in the left eye led to marked improvement in IOP.

CONCLUSIONS AND IMPORTANCE

Patients with MIDAS syndrome can develop congenital glaucoma secondary to angle dysgenesis. This is the first case report to demonstrate the safe and effective use of ECP to treat elevated IOP in a patient with MIDAS.

The presence of anaemia negatively influences survival in patients with POLG disease

BACKGROUND

Mitochondria play an important role in iron metabolism and haematopoietic cell homeostasis. Recent studies in mice showed that a mutation in the catalytic subunit of polymerase gamma (POLG) was associated with haematopoietic dysfunction including anaemia. The aim of this study was to analyse the frequency of anaemia in a large cohort of patients with POLG related disease.

METHODS

We conducted a multi-national, retrospective study of 61 patients with confirmed, pathogenic biallelic POLG mutations from six centres, four in Norway and two in the United Kingdom. Clinical, laboratory and genetic data were collected using a structured questionnaire. Anaemia was defined as an abnormally low haemoglobin value adjusted for age and sex. Univariate survival analysis was performed using log-rank test to compare differences in survival time between categories.

RESULTS

Anaemia occurred in 67% (41/61) of patients and in 23% (14/61) it was already present at clinical presentation. The frequency of anaemia in patients with early onset disease including Alpers syndrome and myocerebrohepatopathy spectrum (MCHS) was high (72%) and 35% (8/23) of these had anaemia at presentation. Survival analysis showed that the presence of anaemia was associated with a significantly worse survival (P = 0.004).

CONCLUSION

Our study reveals that anaemia can be a feature of POLG-related disease. Further, we show that its presence is associated with significantly worse prognosis either because anaemia itself is impacting survival or because it reflects the presence of more serious disease. In either case, our data suggests anaemia is a marker for negative prognosis.

An X-chromosome linked mouse model (Ndufa1S55A) for systemic partial Complex I deficiency for studying predisposition to neurodegeneration and other diseases

The respiratory chain Complex I deficiencies are the most common cause of mitochondrial diseases. Complex I biogenesis is controlled by 58 genes and at least 47 of these cause mitochondrial disease in humans. Two of these are X-chromosome linked nuclear (nDNA) genes (NDUFA1 and NDUFB11), and 7 are mitochondrial (mtDNA, MT-ND1-6, -4L) genes, which may be responsible for sex-dependent variation in the presentation of mitochondrial diseases. In this study, we describe an X-chromosome linked mouse model (Ndufa1^S55A) for systemic partial Complex I deficiency. By homologous recombination, a point mutation T > G within 55th codon of the Ndufa1 gene was introduced. The resulting allele Ndufa1^S55A introduced systemic serine-55-alanine (S55A) mutation within the MWFE protein, which is essential for Complex I assembly and stability. The S55A mutation caused systemic partial Complex I deficiency of ∼50% in both sexes. The mutant males (Ndufa1^S55A/Y) displayed reduced respiratory exchange ratio (RER) and produced less body heat. They were also hypoactive and ate less. They showed age-dependent Purkinje neurons degeneration. Metabolic profiling of brain, liver and serum from males showed reduced heme levels in mutants, which correlated with altered expressions of Fech and Hmox1 mRNAs in tissues. This is the first genuine X-chromosome linked mouse model for systemic partial Complex I deficiency, which shows age-dependent neurodegeneration. The effect of Complex I deficiency on survival patterns of males vs. females was different. We believe this model will be very useful for studying sex-dependent predisposition to both spontaneous and stress-induced neurodegeneration, cancer, diabetes and other diseases.

Mutations in the accessory subunit NDUFB10 result in isolated complex I deficiency and illustrate the critical role of intermembrane space import for complex I holoenzyme assembly

An infant presented with fatal infantile lactic acidosis and cardiomyopathy, and was found to have profoundly decreased activity of respiratory chain complex I in muscle, heart and liver. Exome sequencing revealed compound heterozygous mutations in NDUFB10, which encodes an accessory subunit located within the PD part of complex I. One mutation resulted in a premature stop codon and absent protein, while the second mutation replaced the highly conserved cysteine 107 with a serine residue. Protein expression of NDUFB10 was decreased in muscle and heart, and less so in the liver and fibroblasts, resulting in the perturbed assembly of the holoenzyme at the 830 kDa stage. NDUFB10 was identified together with three other complex I subunits as a substrate of the intermembrane space oxidoreductase CHCHD4 (also known as Mia40). We found that during its mitochondrial import and maturation NDUFB10 transiently interacts with CHCHD4 and acquires disulfide bonds. The mutation of cysteine residue 107 in NDUFB10 impaired oxidation and efficient mitochondrial accumulation of the protein and resulted in degradation of non-imported precursors. Our findings indicate that mutations in NDUFB10 are a novel cause of complex I deficiency associated with a late stage assembly defect and emphasize the role of intermembrane space proteins for the efficient assembly of complex I.

A recurring mutation in the respiratory complex 1 protein NDUFB11 is responsible for a novel form of X-linked sideroblastic anemia

The congenital sideroblastic anemias (CSAs) are a heterogeneous group of inherited blood disorders characterized by pathological mitochondrial iron deposition in erythroid precursors. Each known cause has been attributed to a mutation in a protein associated with heme biosynthesis, iron-sulfur cluster biogenesis, mitochondrial translation, or a component of the mitochondrial respiratory chain. Here, we describe a recurring mutation, c.276_278del, p.F93del, in NDUFB11, a mitochondrial respiratory complex I-associated protein encoded on the X chromosome, in 5 males with a variably syndromic, normocytic CSA. The p.F93del mutation results in respiratory insufficiency and loss of complex I stability and activity in patient-derived fibroblasts. Targeted introduction of this allele into K562 erythroleukemia cells results in a proliferation defect with minimal effect on erythroid differentiation potential, suggesting the mechanism of anemia in this disorder.