CLPB mutations cause 3-methylglutaconic aciduria, progressive brain atrophy, intellectual disability, congenital neutropenia, cataracts, movement disorder

Neutropenia in Childhood-A Narrative Review and Practical Diagnostic Approach

Neutropenia refers to a decrease in the absolute neutrophil count according to age and race norms and poses a common concern in pediatric practice. Neutrophils serve as host defenders and act crucially in acute inflammation procedures. In this narrative review, we systematically present causes of neutropenia in childhood, mainly adopting the pathophysiological classification of Frater, thereby studying (1) neutropenia with reduced bone marrow reserve, (2) secondary neutropenia with reduced bone marrow reserve, and (3) neutropenia with normal bone marrow reserve. Different conditions in each category are thoroughly discussed and practically approached from the clinician's point of view. Secondary mild to moderate neutropenia is usually benign due to childhood viral infections and is expected to resolve in 2-4 weeks. Bacterial and fungal agents are also associated with transient neutropenia, although fever with severe neutropenia constitutes a medical emergency. Drug-induced and immune neutropenias should be suspected following a careful history and a detailed clinical examination. Cytotoxic chemotherapies treating malignancies are responsible for severe neutropenia and neutropenic shock. Rare genetic neutropenias usually manifest with major infections early in life. Our review of taxonomies clinical findings and associates them to specific neutropenia disorders. We consequently propose a practical diagnostic algorithm for managing neutropenic children.

Mitochondrial membrane synthesis, remodelling and cellular trafficking

Mitochondria are dynamic cellular organelles with complex roles in metabolism and signalling. Primary mitochondrial disorders are a group of approximately 400 monogenic disorders arising from pathogenic genetic variants impacting mitochondrial structure, ultrastructure and/or function. Amongst these disorders, defects of complex lipid biosynthesis, especially of the unique mitochondrial membrane lipid cardiolipin, and membrane biology are an emerging group characterised by clinical heterogeneity, but with recurrent features including cardiomyopathy, encephalopathy, neurodegeneration, neuropathy and 3-methylglutaconic aciduria. This review discusses lipid synthesis in the mitochondrial membrane, the mitochondrial contact site and cristae organising system (MICOS), mitochondrial dynamics and trafficking, and the disorders associated with defects of each of these processes. We highlight overlapping functions of proteins involved in lipid biosynthesis and protein import into the mitochondria, pointing to an overarching coordination and synchronisation of mitochondrial functions. This review also focuses on membrane interactions between mitochondria and other organelles, namely the endoplasmic reticulum, peroxisomes, lysosomes and lipid droplets. We signpost disorders of these membrane interactions that may explain the observation of secondary mitochondrial dysfunction in heterogeneous pathological processes. Disruption of these organellar interactions ultimately impairs cellular homeostasis and organismal health, highlighting the central role of mitochondria in human health and disease.

Discerning clinicopathological features of congenital neutropenia syndromes: an approach to diagnostically challenging differential diagnoses

The congenital neutropenia syndromes are rare haematological conditions defined by impaired myeloid precursor differentiation or function. Patients are prone to severe infections with high mortality rates in early life. While some patients benefit from granulocyte colony-stimulating factor treatment, they may still face an increased risk of bone marrow failure, myelodysplastic syndrome and acute leukaemia. Accurate diagnosis is crucial for improved outcomes; however, diagnosis depends on familiarity with a heterogeneous group of rare disorders that remain incompletely characterised. The clinical and pathological overlap between reactive conditions, primary and congenital neutropenias, bone marrow failure, and myelodysplastic syndromes further clouds diagnostic clarity.We review the diagnostically useful clinicopathological and morphological features of reactive causes of neutropenia and the most common primary neutropenia disorders: constitutional/benign ethnic neutropenia, chronic idiopathic neutropenia, cyclic neutropenia, severe congenital neutropenia (due to mutations in ELANE, GFI1, HAX1, G6PC3, VPS45, JAGN1, CSF3R, SRP54, CLPB and WAS), GATA2 deficiency, Warts, hypogammaglobulinaemia, infections and myelokathexis syndrome, Shwachman-Diamond Syndrome, the lysosomal storage disorders with neutropenia: Chediak-Higashi, Hermansky-Pudlak, and Griscelli syndromes, Cohen, and Barth syndromes. We also detail characteristic cytogenetic and molecular factors at diagnosis and in progression to myelodysplastic syndrome/leukaemia.

CLPB disaggregase dysfunction impacts the functional integrity of the proteolytic SPY complex

CLPB is a mitochondrial intermembrane space AAA+ domain-containing disaggregase. CLPB mutations are associated with 3-methylglutaconic aciduria and neutropenia; however, the molecular mechanism underscoring disease and the contribution of CLPB substrates to disease pathology remains unknown. Interactions between CLPB and mitochondrial quality control (QC) factors, including PARL and OPA1, have been reported, hinting at dysregulation of organelle QC in disease. Utilizing proteomic and biochemical approaches, we show a stress-specific aggregation phenotype in a CLPB-null environment and define the CLPB substrate profile. We illustrate an interplay between intermembrane space proteins including CLPB, HAX1, HTRA2, and the inner membrane quality control proteins (STOML2, PARL, YME1L1; SPY complex), with CLPB deficiency impeding SPY complex function by virtue of protein aggregation in the intermembrane space. We conclude that there is an interdependency of mitochondrial QC components at the intermembrane space/inner membrane interface, and perturbations to this network may underscore CLPB disease pathology.

CLPB Deficiency Associated Neonatal Cavitating Leukoencephalopathy: A Potential Pathomechanism Underlying Neurologic Disorder

Caseinolytic peptidase B homolog (CLPB) is a mitochondrial protein which is highly expressed in brain. Its deficiency may be associated with severe neonatal encephalopathy. This report describes a case of fatal neonatal encephalopathy associated with biallelic stop-gain mutation in CLPB (NM_001258392.3:c.1159C>T/p.Arg387*). Neurologic disorder encompasses pre- and post-natal features including polyhydramnios, intrauterine growth restriction, respiratory insufficiency, lethargy, excessive startle reflex, generalized hypertonia, and epileptic seizures. Brain macroscopic examination demonstrates frontal severe periventricular cystic leukoencephalopathy, along with mild ex-vacuo tri-ventricular dilatation. The most striking immunohistopathologic features are striato-thalamic neurodegeneration and deep white matter loss associated with strong reactive astrogliosis. This report supports that CLPB deficiency should be considered among the neurometabolic disorders associated with severe prenatal-onset neurologic impairment that may result from cystic leukoencephalopathy.

HAX1-related congenital neutropenia: Long-term observation in paediatric and adult patients enrolled in the European branch of the Severe Chronic Neutropenia International Registry (SCNIR)

HAX1-related congenital neutropenia (HAX1-CN) is a rare autosomal recessive disorder caused by pathogenic variants in the HAX1 gene. HAX1-CN patients suffer from bone marrow failure as assessed by a maturation arrest of the myelopoiesis revealing persistent severe neutropenia from birth. The disorder is strongly associated with severe bacterial infections and a high risk of developing myelodysplastic syndrome or acute myeloid leukaemia. This study aimed to describe the long-term course of the disease, the treatment, outcome and quality of life in patients with homozygous HAX1 mutations reported to the European branch of the Severe Chronic Neutropenia International Registry. We have analysed a total of 72 patients with different types of homozygous (n = 68), compound heterozygous (n = 3), and digenic (n = 1) HAX1 mutations. The cohort includes 56 paediatric (<18 years) and 16 adult patients. All patients were initially treated with G-CSF with a sufficient increase in absolute neutrophil counts. Twelve patients required haematopoietic stem cell transplantation for leukaemia (n = 8) and non-leukaemic indications (n = 4). While previous genotype-phenotype reports documented a striking correlation between two main transcript variants and clinical neurological phenotypes, our current analysis reveals novel mutation subtypes and clinical overlaps between all genotypes including severe secondary manifestations, e.g., high incidence of secondary ovarian insufficiency.

Dodecamer assembly of a metazoan AAA+ chaperone couples substrate extraction to refolding

Ring-forming AAA+ chaperones solubilize protein aggregates and protect organisms from proteostatic stress. In metazoans, the AAA+ chaperone Skd3 in the mitochondrial intermembrane space (IMS) is critical for human health and efficiently refolds aggregated proteins, but its underlying mechanism is poorly understood. Here, we show that Skd3 harbors both disaggregase and protein refolding activities enabled by distinct assembly states. High-resolution structures of Skd3 hexamers in distinct conformations capture ratchet-like motions that mediate substrate extraction. Unlike previously described disaggregases, Skd3 hexamers further assemble into dodecameric cages in which solubilized substrate proteins can attain near-native states. Skd3 mutants defective in dodecamer assembly retain disaggregase activity but are impaired in client refolding, linking the disaggregase and refolding activities to the hexameric and dodecameric states of Skd3, respectively. We suggest that Skd3 is a combined disaggregase and foldase, and this property is particularly suited to meet the complex proteostatic demands in the mitochondrial IMS.

Structural basis of impaired disaggregase function in the oxidation-sensitive SKD3 mutant causing 3-methylglutaconic aciduria

Mitochondria are critical to cellular and organismal health. To prevent damage, mitochondria have evolved protein quality control machines to survey and maintain the mitochondrial proteome. SKD3, also known as CLPB, is a ring-forming, ATP-fueled protein disaggregase essential for preserving mitochondrial integrity and structure. SKD3 deficiency causes 3-methylglutaconic aciduria type VII (MGCA7) and early death in infants, while mutations in the ATPase domain impair protein disaggregation with the observed loss-of-function correlating with disease severity. How mutations in the non-catalytic N-domain cause disease is unknown. Here, we show that the disease-associated N-domain mutation, Y272C, forms an intramolecular disulfide bond with Cys267 and severely impairs SKD3_Y272C function under oxidizing conditions and in living cells. While Cys267 and Tyr272 are found in all SKD3 isoforms, isoform-1 features an additional α-helix that may compete with substrate-binding as suggested by crystal structure analyses and in silico modeling, underscoring the importance of the N-domain to SKD3 function.

Leigh syndrome

Leigh syndrome, or subacute necrotizing encephalomyelopathy, was initially recognized as a neuropathological entity in 1951. Bilateral symmetrical lesions, typically extending from the basal ganglia and thalamus through brainstem structures to the posterior columns of the spinal cord, are characterized microscopically by capillary proliferation, gliosis, severe neuronal loss, and relative preservation of astrocytes. Leigh syndrome is a pan-ethnic disorder usually with onset in infancy or early childhood, but late-onset forms occur, including in adult life. Over the last six decades it has emerged that this complex neurodegenerative disorder encompasses more than 100 separate monogenic disorders associated with enormous clinical and biochemical heterogeneity. This chapter discusses clinical, biochemical and neuropathological aspects of the disorder, and postulated pathomechanisms. Known genetic causes, including defects of 16 mitochondrial DNA (mtDNA) genes and approaching 100 nuclear genes, are categorized into disorders of subunits and assembly factors of the five oxidative phosphorylation enzymes, disorders of pyruvate metabolism and vitamin and cofactor transport and metabolism, disorders of mtDNA maintenance, and defects of mitochondrial gene expression, protein quality control, lipid remodeling, dynamics, and toxicity. An approach to diagnosis is presented, together with known treatable causes and an overview of current supportive management options and emerging therapies on the horizon.

Comprehensive structural characterization of the human AAA+ disaggregase CLPB in the apo- and substrate-bound states reveals a unique mode of action driven by oligomerization

The human AAA+ ATPase CLPB (SKD3) is a protein disaggregase in the mitochondrial intermembrane space (IMS) and functions to promote the solubilization of various mitochondrial proteins. Loss-of-function CLPB mutations are associated with a few human diseases with neutropenia and neurological disorders. Unlike canonical AAA+ proteins, CLPB contains a unique ankyrin repeat domain (ANK) at its N-terminus. How CLPB functions as a disaggregase and the role of its ANK domain are currently unclear. Herein, we report a comprehensive structural characterization of human CLPB in both the apo- and substrate-bound states. CLPB assembles into homo-tetradecamers in apo-state and is remodeled into homo-dodecamers upon substrate binding. Conserved pore-loops (PLs) on the ATPase domains form a spiral staircase to grip and translocate the substrate in a step-size of 2 amino acid residues. The ANK domain is not only responsible for maintaining the higher-order assembly but also essential for the disaggregase activity. Interactome analysis suggests that the ANK domain may directly interact with a variety of mitochondrial substrates. These results reveal unique properties of CLPB as a general disaggregase in mitochondria and highlight its potential as a target for the treatment of various mitochondria-related diseases.

An approach to recognising and identifying metabolic presentations in the paediatric Irish Traveller population

The Irish Traveller population are an endogamous, traditionally nomadic, Irish population. Irish Travellers practice consanguinity in the majority of marriages, thus resulting in a higher rate of rare autosomal recessive conditions within the population due to homozygous variants. Herein, we outline the clinical phenotypes associated with metabolic conditions seen in this population presenting in the neonatal period, infancy and childhood. Although Irish Travellers are traditionally based in Ireland and the UK, there are populations also living in mainland Europe and the USA. While there is generally an understanding amongst Irish paediatricians of the recessive conditions seen with this population in Ireland, they may be less commonly encountered abroad. It is important to consider a non-genetic aetiology alongside any consideration for a metabolic disorder.

CONCLUSION

This paper acts as a comprehensive review of the metabolic conditions seen and provides a guide for the investigation of an Irish Traveller child with a suspected metabolic condition.

WHAT IS KNOWN

• The Irish Traveller population are an endogenous population. • There are higher rates of inherited metabolic conditions in this population compared to the general population in Ireland.

WHAT IS NEW

• This paper is a comprehensive review of all known inherited metabolic conditions encountered in the Irish Traveller population.

Deciphering the mechanism and function of Hsp100 unfoldases from protein structure

Hsp100 chaperones, also known as Clp proteins, constitute a family of ring-forming ATPases that differ in 3D structure and cellular function from other stress-inducible molecular chaperones. While the vast majority of ATP-dependent molecular chaperones promote the folding of either the nascent chain or a newly imported polypeptide to reach its native conformation, Hsp100 chaperones harness metabolic energy to perform the reverse and facilitate the unfolding of a misfolded polypeptide or protein aggregate. It is now known that inside cells and organelles, different Hsp100 members are involved in rescuing stress-damaged proteins from a previously aggregated state or in recycling polypeptides marked for degradation. Protein degradation is mediated by a barrel-shaped peptidase that physically associates with the Hsp100 hexamer to form a two-component system. Notable examples include the ClpA:ClpP (ClpAP) and ClpX:ClpP (ClpXP) proteases that resemble the ring-forming FtsH and Lon proteases, which unlike ClpAP and ClpXP, feature the ATP-binding and proteolytic domains in a single polypeptide chain. Recent advances in electron cryomicroscopy (cryoEM) together with single-molecule biophysical studies have now provided new mechanistic insight into the structure and function of this remarkable group of macromolecular machines.

Mitochondrial biology and dysfunction in secondary mitochondrial disease

Mitochondrial diseases are a broad, genetically heterogeneous class of metabolic disorders characterized by deficits in oxidative phosphorylation (OXPHOS). Primary mitochondrial disease (PMD) defines pathologies resulting from mutation of mitochondrial DNA (mtDNA) or nuclear genes affecting either mtDNA expression or the biogenesis and function of the respiratory chain. Secondary mitochondrial disease (SMD) arises due to mutation of nuclear-encoded genes independent of, or indirectly influencing OXPHOS assembly and operation. Despite instances of novel SMD increasing year-on-year, PMD is much more widely discussed in the literature. Indeed, since the implementation of next generation sequencing (NGS) techniques in 2010, many novel mitochondrial disease genes have been identified, approximately half of which are linked to SMD. This review will consolidate existing knowledge of SMDs and outline discrete categories within which to better understand the diversity of SMD phenotypes. By providing context to the biochemical and molecular pathways perturbed in SMD, we hope to further demonstrate the intricacies of SMD pathologies outside of their indirect contribution to mitochondrial energy generation.

Premature Ovarian Insufficiency in CLPB Deficiency: Transcriptomic, Proteomic and Phenotypic Insights

CONTEXT

Premature ovarian insufficiency (POI) is a common form of female infertility that usually presents as an isolated condition but can be part of various genetic syndromes. Early diagnosis and treatment of POI can minimize comorbidity and improve health outcomes.

OBJECTIVE

We aimed to determine the genetic cause of syndromic POI, intellectual disability, neutropenia, and cataracts.

METHODS

We performed whole-exome sequencing (WES) followed by functional validation via RT-PCR, RNAseq, and quantitative proteomics, as well as clinical update of previously reported patients with variants in the caseinolytic peptidase B (CLPB) gene.

RESULTS

We identified causative variants in CLPB, encoding a mitochondrial disaggregase. Variants in this gene are known to cause an autosomal recessive syndrome involving 3-methylglutaconic aciduria, neurological dysfunction, cataracts, and neutropenia that is often fatal in childhood; however, there is likely a reporting bias toward severe cases. Using RNAseq and quantitative proteomics we validated causation and gained insight into genotype:phenotype correlation. Clinical follow-up of patients with CLPB deficiency who survived to adulthood identified POI and infertility as a common postpubertal ailment.

CONCLUSION

A novel splicing variant is associated with CLPB deficiency in an individual who survived to adulthood. POI is a common feature of postpubertal female individuals with CLPB deficiency. Patients with CLPB deficiency should be referred to pediatric gynecologists/endocrinologists for prompt POI diagnosis and hormone replacement therapy to minimize associated comorbidities.

Unique structural features govern the activity of a human mitochondrial AAA+ disaggregase, Skd3

The AAA+ protein, Skd3 (human CLPB), solubilizes proteins in the mitochondrial intermembrane space, which is critical for human health. Skd3 variants with defective protein-disaggregase activity cause severe congenital neutropenia (SCN) and 3-methylglutaconic aciduria type 7 (MGCA7). How Skd3 disaggregates proteins remains poorly understood. Here, we report a high-resolution structure of a Skd3-substrate complex. Skd3 adopts a spiral hexameric arrangement that engages substrate via pore-loop interactions in the nucleotide-binding domain (NBD). Substrate-bound Skd3 hexamers stack head-to-head via unique, adaptable ankyrin-repeat domain (ANK)-mediated interactions to form dodecamers. Deleting the ANK linker region reduces dodecamerization and disaggregase activity. We elucidate apomorphic features of the Skd3 NBD and C-terminal domain that regulate disaggregase activity. We also define how Skd3 subunits collaborate to disaggregate proteins. Importantly, SCN-linked subunits sharply inhibit disaggregase activity, whereas MGCA7-linked subunits do not. These advances illuminate Skd3 structure and mechanism, explain SCN and MGCA7 inheritance patterns, and suggest therapeutic strategies.

The Bacterial ClpXP-ClpB Family Is Enriched with RNA-Binding Protein Complexes

In the matrix of bacteria/mitochondria/chloroplasts, Lon acts as the degradation machine for soluble proteins. In stress periods, however, proteostasis and survival depend on the strongly conserved Clp/Hsp100 family. Currently, the targets of ATP-powered unfoldases/disaggregases ClpB and ClpX and of peptidase ClpP heptameric rings are still unclear. Trapping experiments and proteome profiling in multiple organisms triggered confusion, so we analyzed the consistency of ClpP-trap targets in bacteria. We also provide meta-analyses of protein interactions in humans, to elucidate where Clp family members are enriched. Furthermore, meta-analyses of mouse complexomics are provided. Genotype–phenotype correlations confirmed our concept. Trapping, proteome, and complexome data retrieved consistent coaccumulation of CLPXP with GFM1 and TUFM orthologs. CLPX shows broad interaction selectivity encompassing mitochondrial translation elongation, RNA granules, and nucleoids. CLPB preferentially attaches to mitochondrial RNA granules and translation initiation components; CLPP is enriched with them all and associates with release/recycling factors. Mutations in CLPP cause Perrault syndrome, with phenotypes similar to defects in mtDNA/mtRNA. Thus, we propose that CLPB and CLPXP are crucial to counteract misfolded insoluble protein assemblies that contain nucleotides. This insight is relevant to improve ClpP-modulating drugs that block bacterial growth and for the treatment of human infertility, deafness, and neurodegeneration.

How to proceed after "negative" exome: A review on genetic diagnostics, limitations, challenges, and emerging new multiomics techniques

Exome sequencing (ES) in the clinical setting of inborn metabolic diseases (IMDs) has created tremendous improvement in achieving an accurate and timely molecular diagnosis for a greater number of patients, but it still leaves the majority of patients without a diagnosis. In parallel, (personalized) treatment strategies are increasingly available, but this requires the availability of a molecular diagnosis. IMDs comprise an expanding field with the ongoing identification of novel disease genes and the recognition of multiple inheritance patterns, mosaicism, variable penetrance, and expressivity for known disease genes. The analysis of trio ES is preferred over singleton ES as information on the allelic origin (paternal, maternal, "de novo") reduces the number of variants that require interpretation. All ES data and interpretation strategies should be exploited including CNV and mitochondrial DNA analysis. The constant advancements in available techniques and knowledge necessitate the close exchange of clinicians and molecular geneticists about genotypes and phenotypes, as well as knowledge of the challenges and pitfalls of ES to initiate proper further diagnostic steps. Functional analyses (transcriptomics, proteomics, and metabolomics) can be applied to characterize and validate the impact of identified variants, or to guide the genomic search for a diagnosis in unsolved cases. Future diagnostic techniques (genome sequencing [GS], optical genome mapping, long-read sequencing, and epigenetic profiling) will further enhance the diagnostic yield. We provide an overview of the challenges and limitations inherent to ES followed by an outline of solutions and a clinical checklist, focused on establishing a diagnosis to eventually achieve (personalized) treatment.

HAX1-dependent control of mitochondrial proteostasis governs neutrophil granulocyte differentiation

The relevance of molecular mechanisms governing mitochondrial proteostasis to the differentiation and function of hematopoietic and immune cells is largely elusive. Through dissection of the network of proteins related to HCLS1-associated protein X-1, we defined a potentially novel functional CLPB/HAX1/(PRKD2)/HSP27 axis with critical importance for the differentiation of neutrophil granulocytes and, thus, elucidated molecular and metabolic mechanisms underlying congenital neutropenia in patients with HAX1 deficiency as well as bi- and monoallelic mutations in CLPB. As shown by stable isotope labeling by amino acids in cell culture (SILAC) proteomics, CLPB and HAX1 control the balance of mitochondrial protein synthesis and persistence crucial for proper mitochondrial function. Impaired mitochondrial protein dynamics are associated with decreased abundance of the serine-threonine kinase PRKD2 and HSP27 phosphorylated on serines 78 and 82. Cellular defects in HAX1^–/– cells can be functionally reconstituted by HSP27. Thus, mitochondrial proteostasis emerges as a critical molecular and metabolic mechanism governing the differentiation and function of neutrophil granulocytes.

Rare coding variants in DNA damage repair genes associated with timing of natural menopause

The age of menopause is associated with fertility and disease risk, and its genetic control is of great interest. We use whole-exome sequences from 132,370 women in the UK Biobank to test for associations between rare damaging variants and age at natural menopause. Rare damaging variants in five genes are significantly associated with menopause: CHEK2 (p = 3.3 × 10⁻⁵¹), DCLRE1A (p = 8.4 × 10⁻¹³), and HELB (p = 5.7 × 10⁻⁷) with later menopause and TOP3A (p = 7.6 × 10⁻⁸) and CLPB (p = 8.1 × 10⁻⁷) with earlier menopause. Two additional genes are suggestive: RAD54L (p = 2.4 × 10⁻⁶) with later menopause and HROB (p = 2.9 × 10⁻⁶) with earlier menopause. In a follow-up analysis of repeated questionnaires in women who were initially premenopausal, CHEK2, TOP3A, and RAD54L genotypes are associated with subsequent menopause. Consistent with previous genome-wide association studies (GWASs), six of the seven genes are involved in the DNA damage repair pathway. Phenome-wide scans across 398,569 men and women revealed that in addition to known associations with cancers and blood cell counts, rare variants in CHEK2 are also associated with increased risk for uterine fibroids, polycystic ovary syndrome, and prostate hypertrophy; these associations are not shared with higher-penetrance breast cancer genes. Causal mediation analysis suggests that approximately 8% of the breast cancer risk conferred by CHEK2 pathogenic variants after menopause is mediated through delayed menopause.

Mitochondrial Protein Homeostasis and Cardiomyopathy

Human mitochondrial disorders impact tissues with high energetic demands and can be associated with cardiac muscle disease (cardiomyopathy) and early mortality. However, the mechanistic link between mitochondrial disease and the development of cardiomyopathy is frequently unclear. In addition, there is often marked phenotypic heterogeneity between patients, even between those with the same genetic variant, which is also not well understood. Several of the mitochondrial cardiomyopathies are related to defects in the maintenance of mitochondrial protein homeostasis, or proteostasis. This essential process involves the importing, sorting, folding and degradation of preproteins into fully functional mature structures inside mitochondria. Disrupted mitochondrial proteostasis interferes with mitochondrial energetics and ATP production, which can directly impact cardiac function. An inability to maintain proteostasis can result in mitochondrial dysfunction and subsequent mitophagy or even apoptosis. We review the known mitochondrial diseases that have been associated with cardiomyopathy and which arise from mutations in genes that are important for mitochondrial proteostasis. Genes discussed include DnaJ heat shock protein family member C19 (DNAJC19), mitochondrial import inner membrane translocase subunit TIM16 (MAGMAS), translocase of the inner mitochondrial membrane 50 (TIMM50), mitochondrial intermediate peptidase (MIPEP), X-prolyl-aminopeptidase 3 (XPNPEP3), HtraA serine peptidase 2 (HTRA2), caseinolytic mitochondrial peptidase chaperone subunit B (CLPB) and heat shock 60-kD protein 1 (HSPD1). The identification and description of disorders with a shared mechanism of disease may provide further insights into the disease process and assist with the identification of potential therapeutics.

Human mitochondrial AAA+ ATPase SKD3/CLPB assembles into nucleotide-stabilized dodecamers

SKD3, also known as human CLPB, belongs to the AAA+ family of ATPases associated with various activities. Mutations in the SKD3/CLPB gene cause 3-methylglutaconic aciduria type VII and congenital neutropenia. SKD3 is upregulated in acute myeloid leukemia, where it contributes to anti-cancer drug resistance. SKD3 resides in the mitochondrial intermembrane space, where it forms ATP-dependent high-molecular weight complexes, but its biological function and mechanistic links to the clinical phenotypes are currently unknown. Using sedimentation equilibrium and dynamic light scattering, we show that SKD3 is monomeric at low protein concentration in the absence of nucleotides, but it forms oligomers at higher protein concentration or in the presence of adenine nucleotides. The apparent molecular weight of the nucleotide-bound SKD3 is consistent with self-association of 12 monomers. Image-class analysis and averaging from negative-stain electron microscopy (EM) of SKD3 in the ATP-bound state visualized cylinder-shaped particles with an open central channel along the cylinder axis. The dimensions of the EM-visualized particle suggest that the SKD3 dodecamer is formed by association of two hexameric rings. While hexameric structure has been often observed among AAA+ ATPases, a double-hexamer sandwich found for SKD3 appears uncommon within this protein family. A functional significance of the non-canonical structure of SKD3 remains to be determined.

Heterozygous variants of CLPB are a cause of severe congenital neutropenia

Severe congenital neutropenia is an inborn disorder of granulopoiesis. Approximately one third of cases do not have a known genetic cause. Exome sequencing of 104 persons with congenital neutropenia identified heterozygous missense variants of CLPB (caseinolytic peptidase B) in 5 severe congenital neutropenia cases, with 5 more cases identified through additional sequencing efforts or clinical sequencing. CLPB encodes an adenosine triphosphatase that is implicated in protein folding and mitochondrial function. Prior studies showed that biallelic mutations of CLPB are associated with a syndrome of 3-methylglutaconic aciduria, cataracts, neurologic disease, and variable neutropenia. However, 3-methylglutaconic aciduria was not observed and, other than neutropenia, these clinical features were uncommon in our series. Moreover, the CLPB variants are distinct, consisting of heterozygous variants that cluster near the adenosine triphosphate-binding pocket. Both genetic loss of CLPB and expression of CLPB variants result in impaired granulocytic differentiation of human hematopoietic progenitor cells and increased apoptosis. These CLPB variants associate with wild-type CLPB and inhibit its adenosine triphosphatase and disaggregase activity in a dominant-negative fashion. Finally, expression of CLPB variants is associated with impaired mitochondrial function but does not render cells more sensitive to endoplasmic reticulum stress. Together, these data show that heterozygous CLPB variants are a new and relatively common cause of congenital neutropenia and should be considered in the evaluation of patients with congenital neutropenia.

Inborn errors of metabolism associated with 3-methylglutaconic aciduria

A growing number of inborn errors of metabolism (IEM) associated with compromised mitochondrial energy metabolism manifest an unusual phenotypic feature: 3-methylglutaconic (3MGC) aciduria. Two major categories of 3MGC aciduria, primary and secondary, have been described. In primary 3MGC aciduria, IEMs in 3MGC CoA hydratase (AUH) or HMG CoA lyase block leucine catabolism, resulting in a buildup of pathway intermediates, including 3MGC CoA. Subsequent thioester hydrolysis yields 3MGC acid, which is excreted in urine. In secondary 3MGC aciduria, no deficiencies in leucine catabolism enzymes exist and 3MGC CoA is formed de novo from acetyl CoA. In the "acetyl CoA diversion pathway", when IEMs directly, or indirectly, interfere with TCA cycle activity, acetyl CoA accumulates in the matrix space. This leads to condensation of two acetyl CoA to form acetoacetyl CoA, followed by another condensation between acetyl CoA and acetoacetyl CoA to form 3-hydroxy, 3-methylglutaryl (HMG) CoA. Once formed, HMG CoA serves as a substrate for AUH, producing trans-3MGC CoA. Non enzymatic isomerization of trans-3MGC CoA to cis-3MGC CoA precedes intramolecular cyclization to cis-3MGC anhydride plus CoA. Subsequent hydrolysis of cis-3MGC anhydride gives rise to cis-3MGC acid, which is excreted in urine. In reviewing 20 discrete IEMs that manifest secondary 3MGC aciduria, evidence supporting the acetyl CoA diversion pathway was obtained. This biochemical pathway serves as an "overflow valve" in muscle / brain tissue to redirect acetyl CoA to 3MGC CoA when entry to the TCA cycle is impeded.

3-Methylglutaconic aciduria in carriers of primary carnitine deficiency

The etiology of secondary 3-methylglutaconic aciduria (3-MGA-uria) is not well understood although is thought to be a marker of mitochondrial dysfunction. For this reason, suspicion for a secondary 3-MGA-uria often leads to an extensive clinical and laboratory work-up for mitochondrial disease, although in many cases evidence for mitochondrial dysfunction is never found. 3-methylglutaconic aciduria in healthy individuals without known metabolic disease has not been well described. Here, we describe clinical and biochemical features of 23 individuals evaluated at the Greenwood Genetic Center for low plasma free carnitine reported on newborn screening. Of the 23 individuals evaluated, four individuals were diagnosed with primary carnitine deficiency, 16 were identified as carriers for primary carnitine deficiency, and three individuals were determined to be unaffected non-carriers based on molecular and biochemical testing. Elevated 3-MGA (>20 mmol/mol of creatinine) was identified in nine carriers of primary carnitine deficiency, while all unaffected non carriers and all affected individuals with primary carnitine deficiency had a normal 3-MGA level (<20 mmol/mol of creatinine). Average 3-MGA among all carriers was 39.66 mmol/mol of creatinine. Average plasma free carnitine in among all carriers (n = 16) was 13.87 μm/L, and average plasma free carnitine was not significantly different between carriers with and those without elevated 3-MGA (p = 0.66). In summary, we describe elevated 3-MGA as a discriminatory feature in nine healthy carriers of primary carnitine deficiency. Our findings suggest that heterozygosity for pathogenic alterations on SLC22A5 should be considered in the differential for individuals with persistent 3-MGA-uria of unclear etiology.

The AAA+ superfamily: a review of the structural and mechanistic principles of these molecular machines

ATPases associated with diverse cellular activities (AAA+ proteins) are a superfamily of proteins found throughout all domains of life. The hallmark of this family is a conserved AAA+ domain responsible for a diverse range of cellular activities. Typically, AAA+ proteins transduce chemical energy from the hydrolysis of ATP into mechanical energy through conformational change, which can drive a variety of biological processes. AAA+ proteins operate in a variety of cellular contexts with diverse functions including disassembly of SNARE proteins, protein quality control, DNA replication, ribosome assembly, and viral replication. This breadth of function illustrates both the importance of AAA+ proteins in health and disease and emphasizes the importance of understanding conserved mechanisms of chemo-mechanical energy transduction. This review is divided into three major portions. First, the core AAA+ fold is presented. Next, the seven different clades of AAA+ proteins and structural details and reclassification pertaining to proteins in each clade are described. Finally, two well-known AAA+ proteins, NSF and its close relative p97, are reviewed in detail.

Chronic neutropenia: how best to assess severity and approach management?

INTRODUCTION

Neutropenia is a relatively common finding in medical practice and the medical approach requires a gradual and pertinent diagnostic procedure as well as adapted management.

AREAS COVERED

The area of chronic neutropenia remains fragmented between diverse diseases or situations. Here physicians involved in different aspects of chronic neutropenia gather both the data from medical literature till the end of May 2021 and their experience to offer a global approach for the diagnosis of chronic neutropenia as well as their medical care.

EXPERT OPINION

In most cases, the neutropenia is transient, frequently related to a viral infection, and not harmful. However, neutropenia can be chronic (i.e. >3 months) and related to a number of etiologies, some clinically benign, such as so-called 'ethnic' neutropenia. Autoimmune neutropenia is the common form in young children, whereas idiopathic/immune neutropenia is a frequent etiology in young females. Inherited neutropenia (or congenital neutropenia) is exceptional, with approximately 30 new cases per 10⁶ births and 30 known subtypes. Such patients have a high risk of invasive bacterial infections, and oral infections. Supportive therapy, which is primarily based on daily administration of an antibiotic prophylaxis and/or treatment with granulocyte-colony stimulating factor (G-CSF), contributes to avoiding recurrent infections.

Neutropenia and intellectual disability are hallmarks of biallelic and de novo CLPB deficiency

PURPOSE

To investigate monoallelic CLPB variants. Pathogenic variants in many genes cause congenital neutropenia. While most patients exhibit isolated hematological involvement, biallelic CLPB variants underlie a neurological phenotype ranging from nonprogressive intellectual disability to prenatal encephalopathy with progressive brain atrophy, movement disorder, cataracts, 3-methylglutaconic aciduria, and neutropenia. CLPB was recently shown to be a mitochondrial refoldase; however, the exact function remains elusive.

METHODS

We investigated six unrelated probands from four countries in three continents, with neutropenia and a phenotype dominated by epilepsy, developmental issues, and 3-methylglutaconic aciduria with next-generation sequencing.

RESULTS

In each individual, we identified one of four different de novo monoallelic missense variants in CLPB. We show that these variants disturb refoldase and to a lesser extent ATPase activity of CLPB in a dominant-negative manner. Complexome profiling in fibroblasts showed CLPB at very high molecular mass comigrating with the prohibitins. In control fibroblasts, HAX1 migrated predominantly as monomer while in patient samples multiple HAX1 peaks were observed at higher molecular masses comigrating with CLPB thus suggesting a longer-lasting interaction between CLPB and HAX1.

CONCLUSION

Both biallelic as well as specific monoallelic CLPB variants result in a phenotypic spectrum centered around neurodevelopmental delay, seizures, and neutropenia presumably mediated via HAX1.

General Structural and Functional Features of Molecular Chaperones

Molecular chaperones are a group of structurally diverse and highly conserved ubiquitous proteins. They play crucial roles in facilitating the correct folding of proteins in vivo by preventing protein aggregation or facilitating the appropriate folding and assembly of proteins. Heat shock proteins form the major class of molecular chaperones that are responsible for protein folding events in the cell. This is achieved by ATP-dependent (folding machines) or ATP-independent mechanisms (holders). Heat shock proteins are induced by a variety of stresses, besides heat shock. The large and varied heat shock protein class is categorised into several subfamilies based on their sizes in kDa namely, small Hsps (HSPB), J domain proteins (Hsp40/DNAJ), Hsp60 (HSPD/E; Chaperonins), Hsp70 (HSPA), Hsp90 (HSPC), and Hsp100. Heat shock proteins are localised to different compartments in the cell to carry out tasks specific to their environment. Most heat shock proteins form large oligomeric structures, and their functions are usually regulated by a variety of cochaperones and cofactors. Heat shock proteins do not function in isolation but are rather part of the chaperone network in the cell. The general structural and functional features of the major heat shock protein families are discussed, including their roles in human disease. Their function is particularly important in disease due to increased stress in the cell. Vector-borne parasites affecting human health encounter stress during transmission between invertebrate vectors and mammalian hosts. Members of the main classes of heat shock proteins are all represented in Plasmodium falciparum, the causative agent of cerebral malaria, and they play specific functions in differentiation, cytoprotection, signal transduction, and virulence.

Expression and Purification of Recombinant Skd3 (Human ClpB) Protein and Tobacco Etch Virus (TEV) Protease from Escherichia coli

Skd3 (encoded by human CLPB) is a mitochondrial AAA+ protein comprised of an N-terminal ankyrin-repeat domain and a C-terminal HCLR-clade nucleotide-binding domain. The function of Skd3 has long remained unknown due to challenges in purifying the protein to high quality and near homogeneity. Recently we described Skd3 as a human mitochondrial protein disaggregase that solubilizes proteins in the mitochondrial intermembrane space. This protocol overcomes the challenges associated with purifying Skd3 and allows for in depth in vitro study of Skd3 activity. Tobacco etch virus (TEV) protease is required in the purification of Skd3. Thus, we also describe how to purify high quality TEV protease for use in the purification of Skd3, other purification protocols, and in vitro assays requiring TEV protease.

A Next-Generation Sequencing Test for Severe Congenital Neutropenia: Utility in a Broader Clinicopathologic Spectrum of Disease

Severe congenital neutropenia (SCN) is a collection of diverse disorders characterized by chronically low absolute neutrophil count in the peripheral blood, increased susceptibility to infection, and a significant predisposition to the development of myeloid malignancies. SCN can be acquired or inherited. Inherited forms have been linked to variants in a group of diverse genes involved in the neutrophil-differentiation process. Variants that promote resistance to treatment have also been identified. Thus, genetic testing is important for the diagnosis, prognosis, and management of SCN. Herein we describe clinically validated assay developed for assessing patients with suspected SCN. The assay is performed from a whole-exome backbone. Variants are called across all coding exons, and results are filtered to focus on 48 genes that are clinically relevant to SCN. Validation results indicated 100% analytical sensitivity and specificity for the detection of constitutional variants among the 48 reportable genes. To date, 34 individuals have been referred for testing (age range: birth to 67 years). Several pathogenic and likely pathogenic variants have been identified, including one in a patient with late-onset disease. The pattern of cases referred for testing suggests that this assay has clinical utility in a broader spectrum of patients beyond those in the pediatric population who have classic early-onset symptoms characteristic of SCN.

A High-Density Human Mitochondrial Proximity Interaction Network

We used BioID, a proximity-dependent biotinylation assay with 100 mitochondrial baits from all mitochondrial sub-compartments, to create a high-resolution human mitochondrial proximity interaction network. We identified 1,465 proteins, producing 15,626 unique high-confidence proximity interactions. Of these, 528 proteins were previously annotated as mitochondrial, nearly half of the mitochondrial proteome defined by Mitocarta 2.0. Bait-bait analysis showed a clear separation of mitochondrial compartments, and correlation analysis among preys across all baits allowed us to identify functional clusters involved in diverse mitochondrial functions and to assign uncharacterized proteins to specific modules. We demonstrate that this analysis can assign isoforms of the same mitochondrial protein to different mitochondrial sub-compartments and show that some proteins may have multiple cellular locations. Outer membrane baits showed specific proximity interactions with cytosolic proteins and proteins in other organellar membranes, suggesting specialization of proteins responsible for contact site formation between mitochondria and individual organelles.

Human CLPB forms ATP-dependent complexes in the mitochondrial intermembrane space

Human caseinolytic peptidase B protein homolog (CLPB), also known as suppressor of potassium transport defect 3 (SKD3), is a broadly-expressed member of the family of ATPases associated with diverse cellular activities (AAA+). Mutations in the human CLPB gene cause 3-methylglutaconic aciduria type VII. CLPB is upregulated in acute myeloid leukemia (AML), where it contributes to anti-cancer drug resistance. The biological function of CLPB in human cells and mechanistic links to the clinical phenotypes are currently unknown. Herein, subcellular fractionation of human HEK-293 and BT-549 cells showed that a single 57-kDa form of CLPB was present in the mitochondria and not in the cytosolic fraction. Immunofluorescence staining of HEK-293 and BT-549 cells with anti-CLPB antibody co-localized with the mitochondrial staining using a MitoTracker dye. In purified intact mitochondria, CLPB was protected against externally added proteinase K, but it was susceptible to degradation after disruption of the outer membrane, indicating that CLPB resides in the mitochondrial intermembrane space. Overexpressed CLPB, while properly trafficked to the mitochondria, appeared to form large clusters/aggregates that were resistant to extraction with non-ionic detergents and were readily visualized by immunofluorescence microscopy. Importantly, endogenous CLPB formed high molecular weight protein complexes in an ATP-dependent manner that were detected by blue native polyacrylamide gel electrophoresis. These results demonstrate that ATP induces a structural change in CLPB and controls its ability to self-associate or form complexes with other proteins in the intermembrane space of mitochondria.

Expanding the clinical and genetic spectrum of CAD deficiency: an epileptic encephalopathy treatable with uridine supplementation

PURPOSE

Biallelic CAD variants underlie CAD deficiency (or early infantile epileptic encephalopathy-50, [EIEE-50]), an error of pyrimidine de novo biosynthesis amenable to treatment via the uridine salvage pathway. We further define the genotype and phenotype with a focus on treatment.

METHODS

Retrospective case series of 20 patients.

RESULTS

Our study confirms CAD deficiency as a progressive EIEE with recurrent status epilepticus, loss of skills, and dyserythropoietic anemia. We further refine the phenotype by reporting a movement disorder as a frequent feature, and add that milder courses with isolated developmental delay/intellectual disability can occur as well as onset with neonatal seizures. With no biomarker available, the diagnosis relies on genetic testing and functional validation in patient-derived fibroblasts. Underlying pathogenic variants are often rated as variants of unknown significance, which could lead to underrecognition of this treatable disorder. Supplementation with uridine, uridine monophosphate, or uridine triacetate in ten patients was safe and led to significant clinical improvement in most patients.

CONCLUSION

We advise a trial with uridine (monophosphate) in all patients with developmental delay/intellectual disability, epilepsy, and anemia; all patients with status epilepticus; and all patients with neonatal seizures until (genetically) proven otherwise or proven unsuccessful after 6 months. CAD deficiency might represent a condition for genetic newborn screening.

Targeting apoptosis in cancer therapy

For over three decades, a mainstay and goal of clinical oncology has been the development of therapies promoting the effective elimination of cancer cells by apoptosis. This programmed cell death process is mediated by several signalling pathways (referred to as intrinsic and extrinsic) triggered by multiple factors, including cellular stress, DNA damage and immune surveillance. The interaction of apoptosis pathways with other signalling mechanisms can also affect cell death. The clinical translation of effective pro-apoptotic agents involves drug discovery studies (addressing the bioavailability, stability, tumour penetration, toxicity profile in non-malignant tissues, drug interactions and off-target effects) as well as an understanding of tumour biology (including heterogeneity and evolution of resistant clones). While tumour cell death can result in response to therapy, the selection, growth and dissemination of resistant cells can ultimately be fatal. In this Review, we present the main apoptosis pathways and other signalling pathways that interact with them, and discuss actionable molecular targets, therapeutic agents in clinical translation and known mechanisms of resistance to these agents.

Hereditary Predisposition to Hematopoietic Neoplasms: When Bloodline Matters for Blood Cancers

With the advent of precision genomics, hereditary predisposition to hematopoietic neoplasms- collectively known as hereditary predisposition syndromes (HPS)-are being increasingly recognized in clinical practice. Familial clustering was first observed in patients with leukemia, which led to the identification of several germline variants, such as RUNX1, CEBPA, GATA2, ANKRD26, DDX41, and ETV6, among others, now established as HPS, with tendency to develop myeloid neoplasms. However, evidence for hereditary predisposition is also apparent in lymphoid and plasma--cell neoplasms, with recent discoveries of germline variants in genes such as IKZF1, SH2B3, PAX5 (familial acute lymphoblastic leukemia), and KDM1A/LSD1 (familial multiple myeloma). Specific inherited bone marrow failure syndromes-such as GATA2 haploinsufficiency syndromes, short telomere syndromes, Shwachman-Diamond syndrome, Diamond-Blackfan anemia, severe congenital neutropenia, and familial thrombocytopenias-also have an increased predisposition to develop myeloid neoplasms, whereas inherited immune deficiency syndromes, such as ataxia-telangiectasia, Bloom syndrome, Wiskott Aldrich syndrome, and Bruton agammaglobulinemia, are associated with an increased risk for lymphoid neoplasms. Timely recognition of HPS is critical to ensure safe choice of donors and/or conditioning-regimen intensity for allogeneic hematopoietic stem-cell transplantation and to enable direction of appropriate genomics-driven personalized therapies. The purpose of this review is to provide a comprehensive overview of HPS and serve as a useful reference for clinicians to recognize relevant signs and symptoms among patients to enable timely screening and referrals to pursue germline assessment. In addition, we also discuss our institutional approach toward identification of HPS and offer a stepwise diagnostic and management algorithm.

Skd3 (human ClpB) is a potent mitochondrial protein disaggregase that is inactivated by 3-methylglutaconic aciduria-linked mutations

Cells have evolved specialized protein disaggregases to reverse toxic protein aggregation and restore protein functionality. In nonmetazoan eukaryotes, the AAA+ disaggregase Hsp78 resolubilizes and reactivates proteins in mitochondria. Curiously, metazoa lack Hsp78. Hence, whether metazoan mitochondria reactivate aggregated proteins is unknown. Here, we establish that a mitochondrial AAA+ protein, Skd3 (human ClpB), couples ATP hydrolysis to protein disaggregation and reactivation. The Skd3 ankyrin-repeat domain combines with conserved AAA+ elements to enable stand-alone disaggregase activity. A mitochondrial inner-membrane protease, PARL, removes an autoinhibitory peptide from Skd3 to greatly enhance disaggregase activity. Indeed, PARL-activated Skd3 solubilizes α-synuclein fibrils connected to Parkinson's disease. Human cells lacking Skd3 exhibit reduced solubility of various mitochondrial proteins, including anti-apoptotic Hax1. Importantly, Skd3 variants linked to 3-methylglutaconic aciduria, a severe mitochondrial disorder, display diminished disaggregase activity (but not always reduced ATPase activity), which predicts disease severity. Thus, Skd3 is a potent protein disaggregase critical for human health.

Primary Immunodeficiencies With Defects in Innate Immunity: Focus on Orofacial Manifestations

The field of primary immunodeficiencies (PIDs) is rapidly evolving. Indeed, the number of described diseases is constantly increasing thanks to the rapid identification of novel genetic defects by next-generation sequencing. PIDs are now rather referred to as “inborn errors of immunity” due to the association between a wide range of immune dysregulation-related clinical features and the “prototypic” increased infection susceptibility. The phenotypic spectrum of PIDs is therefore very large and includes several orofacial features. However, the latter are often overshadowed by severe systemic manifestations and remain underdiagnosed. Patients with impaired innate immunity are predisposed to a variety of oral manifestations including oral infections (e.g., candidiasis, herpes gingivostomatitis), aphthous ulcers, and severe periodontal diseases. Although less frequently, they can also show orofacial developmental abnormalities. Oral lesions can even represent the main clinical manifestation of some PIDs or be inaugural, being therefore one of the first features indicating the existence of an underlying immune defect. The aim of this review is to describe the orofacial features associated with the different PIDs of innate immunity based on the new 2019 classification from the International Union of Immunological Societies (IUIS) expert committee. This review highlights the important role played by the dentist, in close collaboration with the multidisciplinary medical team, in the management and the diagnostic of these conditions.

Mitochondrial Protein Quality Control Mechanisms

Mitochondria serve as a hub for many cellular processes, including bioenergetics, metabolism, cellular signaling, redox balance, calcium homeostasis, and cell death. The mitochondrial proteome includes over a thousand proteins, encoded by both the mitochondrial and nuclear genomes. The majority (~99%) of proteins are nuclear encoded that are synthesized in the cytosol and subsequently imported into the mitochondria. Within the mitochondria, polypeptides fold and assemble into their native functional form. Mitochondria health and integrity depend on correct protein import, folding, and regulated turnover termed as mitochondrial protein quality control (MPQC). Failure to maintain these processes can cause mitochondrial dysfunction that leads to various pathophysiological outcomes and the commencement of diseases. Here, we summarize the current knowledge about the role of different MPQC regulatory systems such as mitochondrial chaperones, proteases, the ubiquitin-proteasome system, mitochondrial unfolded protein response, mitophagy, and mitochondria-derived vesicles in the maintenance of mitochondrial proteome and health. The proper understanding of mitochondrial protein quality control mechanisms will provide relevant insights to treat multiple human diseases.

BRAIN AND EYE AS POTENTIAL TARGETS FOR IONIZING RADIATION IMPACT. Part І. THE CONSEQUENCES OF IRRADIATION OF THE PARTICIPANTS OF THE LIQUIDATION OF THE CHORNOBYL ACCIDENT

BACKGROUND

Exposure to ionizing radiation could affect the brain and eyes leading to cognitive and vision impairment, behavior disorders and performance decrement during professional irradiation at medical radiology, includinginterventional radiological procedures, long-term space flights, and radiation accidents.

OBJECTIVE

The objective was to analyze the current experimental, epidemiological, and clinical data on the radiation cerebro-ophthalmic effects.

MATERIALS AND METHODS

In our analytical review peer-reviewed publications via the bibliographic and scientometric bases PubMed / MEDLINE, Scopus, Web of Science, and selected papers from the library catalog of NRCRM - theleading institution in the field of studying the medical effects of ionizing radiation - were used.

RESULTS

The probable radiation-induced cerebro-ophthalmic effects in human adults comprise radiation cataracts,radiation glaucoma, radiation-induced optic neuropathy, retinopathies, angiopathies as well as specific neurocognitive deficit in the various neuropsychiatric pathology including cerebrovascular pathology and neurodegenerativediseases. Specific attention is paid to the likely stochastic nature of many of those effects. Those prenatally and inchildhood exposed are a particular target group with a higher risk for possible radiation effects and neurodegenerative diseases.

CONCLUSIONS

The experimental, clinical, epidemiological, anatomical and pathophysiological rationale for visualsystem and central nervous system (CNS) radiosensitivity is given. The necessity for further international studieswith adequate dosimetric support and the follow-up medical and biophysical monitoring of high radiation riskcohorts is justified. The first part of the study currently being published presents the results of the study of theeffects of irradiation in the participants of emergency works at the Chornobyl Nuclear Power Plant (ChNPP).

The basic machineries for mitochondrial protein quality control

Mitochondria play pivotal roles in cellular energy metabolism, the synthesis of essential biomolecules and the regulation of cell death and aging. The proper folding, unfolding and degradation of the many proteins active within mitochondria is surveyed by the mitochondrial quality control machineries. Here, we describe the principal components of the mitochondrial quality control system and recent developments in the elucidation of the molecular mechanisms maintaining a functional mitochondrial proteome.

Mitochondrial dysfunction, AMPK activation and peroxisomal metabolism: A coherent scenario for non-canonical 3-methylglutaconic acidurias

The occurrence of 3-methylglutaconic aciduria (3-MGA) is a well understood phenomenon in leucine oxidation and ketogenesis disorders (primary 3-MGAs). In contrast, its genesis in non-canonical (secondary) 3-MGAs, a growing-up group of disorders encompassing more than a dozen of inherited metabolic diseases, is a mystery still remaining unresolved for three decades. To puzzle out this anthologic problem of metabolism, three clues were considered: (i) the variety of disorders suggests a common cellular target at the cross-road of metabolic and signaling pathways, (ii) the response to leucine loading test only discriminative for primary but not secondary 3-MGAs suggests these latter are disorders of extramitochondrial HMG-CoA metabolism as also attested by their failure to increase 3-hydroxyisovalerate, a mitochondrial metabolite accumulating only in primary 3-MGAs, (iii) the peroxisome is an extramitochondrial site possessing its own pool and displaying metabolism of HMG-CoA, suggesting its possible involvement in producing extramitochondrial 3-methylglutaconate (3-MG). Following these clues provides a unifying common basis to non-canonical 3-MGAs: constitutive mitochondrial dysfunction induces AMPK activation which, by inhibiting early steps in cholesterol and fatty acid syntheses, pipelines cytoplasmic acetyl-CoA to peroxisomes where a rise in HMG-CoA followed by local dehydration and hydrolysis may lead to 3-MGA yield. Additional contributors are considered, notably for 3-MGAs associated with hyperammonemia, and to a lesser extent in CLPB deficiency. Metabolic and signaling itineraries followed by the proposed scenario are essentially sketched, being provided with compelling evidence from the literature coming in their support.

Structural Basis of Mitochondrial Scaffolds by Prohibitin Complexes: Insight into a Role of the Coiled-Coil Region

The coiled-coil motif mediates subunit oligomerization and scaffolding and underlies several fundamental biologic processes. Prohibitins (PHBs), mitochondrial inner membrane proteins involved in mitochondrial homeostasis and signal transduction, are predicted to have a coiled-coil motif, but their structural features are poorly understood. Here we solved the crystal structure of the heptad repeat (HR) region of PHB2 at 1.7-Å resolution, showing that it assembles into a dimeric, antiparallel coiled-coil with a unique negatively charged area essential for the PHB interactome in mitochondria. Disruption of the HR coiled-coil abolishes well-ordered PHB complexes and the mitochondrial tubular networks accompanying PHB-dependent signaling. Using a proximity-dependent biotin identification (BioID) technique in live cells, we mapped a number of mitochondrial intermembrane space proteins whose association with PHB2 relies on the HR coiled-coil region. Elucidation of the PHB complex structure in mitochondria provides insight into essential PHB interactomes required for mitochondrial dynamics as well as signal transduction.

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Heptad repeat (HR) region of PHB2 is essential for PHB complexes in mitochondria

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The HR region of PHB2 assembles into a dimeric, anti-parallel coiled-coil

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Disruption of the PHB2 coiled-coil abolishes mitochondrial dynamics

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The coiled-coil associates with mitochondrial proteins, invoking an immune response

Homozygous loss-of-function variants of TASP1, a gene encoding an activator of the histone methyltransferases KMT2A and KMT2D, cause a syndrome of developmental delay, happy demeanor, distinctive facial features, and congenital anomalies

We report four unrelated children with homozygous loss-of-function variants in TASP1 and an overlapping phenotype comprising developmental delay with hypotonia and microcephaly, feeding difficulties with failure-to-thrive, recurrent respiratory infections, cardiovascular malformations, cryptorchidism, happy demeanor, and distinctive facial features. Two children had a homozygous founder deletion encompassing exons 5-11 of TASP1, the third had a homozygous missense variant, c.701 C>T (p.Thr234Met), affecting the active site of the encoded enzyme, and the fourth had a homozygous nonsense variant, c.199 C>T (p.Arg67*). TASP1 encodes taspase 1 (TASP1), which is responsible for cleaving, thus activating, the lysine methyltransferases KMT2A and KMT2D, which are essential for histone methylation and transcription regulation. The consistency of the phenotype, the critical biological function of TASP1, the deleterious nature of the TASP1 variants, and the overlapping features with Wiedemann-Steiner and Kabuki syndromes respectively caused by pathogenic variants in KMT2A and KMT2D all support that TASP1 is a disease-related gene.

Mutations in TIMM50 cause severe mitochondrial dysfunction by targeting key aspects of mitochondrial physiology

3-Methylglutaconic aciduria (3-MGA-uria) syndromes comprise a heterogeneous group of diseases associated with mitochondrial membrane defects. Whole-exome sequencing identified compound heterozygous mutations in TIMM50 (c.[341 G>A];[805 G>A]) in a boy with West syndrome, optic atrophy, neutropenia, cardiomyopathy, Leigh syndrome, and persistent 3-MGA-uria. A comprehensive analysis of the mitochondrial function was performed in fibroblasts of the patient to elucidate the molecular basis of the disease. TIMM50 protein was severely reduced in the patient fibroblasts, regardless of the normal mRNA levels, suggesting that the mutated residues might be important for TIMM50 protein stability. Severe morphological defects and ultrastructural abnormalities with aberrant mitochondrial cristae organization in muscle and fibroblasts were found. The levels of fully assembled OXPHOS complexes and supercomplexes were strongly reduced in fibroblasts from this patient. High-resolution respirometry demonstrated a significant reduction of the maximum respiratory capacity. A TIMM50-deficient HEK293T cell line that we generated using CRISPR/Cas9 mimicked the respiratory defect observed in the patient fibroblasts; notably, this defect was rescued by transfection with a plasmid encoding the TIMM50 wild-type protein. In summary, we demonstrated that TIMM50 deficiency causes a severe mitochondrial dysfunction by targeting key aspects of mitochondrial physiology, such as the maintenance of proper mitochondrial morphology, OXPHOS assembly, and mitochondrial respiratory capacity.