Biallelic null variants in PNPLA8 cause microcephaly by reducing the number of basal radial glia.

Patatin-like phospholipase domain-containing lipase 8 (PNPLA8), one of the calcium-independent phospholipase A2 enzymes, is involved in various physiological processes through the maintenance of membrane phospholipids. Biallelic variants in PNPLA8 have been associated with a range of paediatric neurodegenerative disorders. However, the phenotypic spectrum, genotype-phenotype correlations and the underlying mechanisms are poorly understood. Here, we newly identified 14 individuals from 12 unrelated families with biallelic ultra-rare variants in PNPLA8 presenting with a wide phenotypic spectrum of clinical features. Analysis of the clinical features of current and previously reported individuals (25 affected individuals across 20 families) showed that PNPLA8-related neurological diseases manifest as a continuum ranging from variable developmental and/or degenerative epileptic-dyskinetic encephalopathy to childhood-onset neurodegeneration. We found that complete loss of PNPLA8 was associated with the more profound end of the spectrum, with congenital microcephaly. Using cerebral organoids generated from human induced pluripotent stem cells, we found that loss of PNPLA8 led to developmental defects by reducing the number of basal radial glial cells and upper-layer neurons. Spatial transcriptomics revealed that loss of PNPLA8 altered the fate specification of apical radial glial cells, as reflected by the enrichment of gene sets related to the cell cycle, basal radial glial cells and neural differentiation. Neural progenitor cells lacking PNPLA8 showed a reduced amount of lysophosphatidic acid, lysophosphatidylethanolamine and phosphatidic acid. The reduced number of basal radial glial cells in patient-derived cerebral organoids was rescued, in part, by the addition of lysophosphatidic acid. Our data suggest that PNPLA8 is crucial to meet phospholipid synthetic needs and to produce abundant basal radial glial cells in human brain development.

Leigh syndrome with developmental regression and ataxia due to a novel splicing variant in the PMPCB gene.

Only five children with pathogenic PMPCB gene variants have been described and all carried missense variants. Clinical features included a Leigh-like syndrome of developmental regression, basal ganglia lesions and ataxia with or without dystonia and epilepsy. Three of the five died in childhood and none was older than age six when described. We report the first splice site variant in the PMPCB gene in a 39-year old individual who experienced developmental regression and ataxia following otitis media in childhood. A minigene assay confirms this variant results in aberrant splicing and skipping of exon 12.

Inhibition of human cytomegalovirus replication by interferon alpha can involve multiple anti-viral factors.

The shortcomings of current direct-acting anti-viral therapy against human cytomegalovirus (HCMV) has led to interest in host-directed therapy. Here we re-examine the use of interferon proteins to inhibit HCMV replication utilizing both high and low passage strains of HCMV. Pre-treatment of cells with interferon alpha (IFNα) was required for robust and prolonged inhibition of both low and high passage HCMV strains, with no obvious toxicity, and was associated with an increased anti-viral state in HCMV-infected cells. Pre-treatment of cells with IFNα led to poor expression of HCMV immediate-early proteins from both high and low passage strains, which was associated with the presence of the anti-viral factor SUMO-PML. Inhibition of HCMV replication in the presence of IFNα involving ZAP proteins was HCMV strain-dependent, wherein a high passage HCMV strain was obviously restricted by ZAP and a low passage strain was not. This suggested that strain-specific combinations of anti-viral factors were involved in inhibition of HCMV replication in the presence of IFNα. Overall, this work further supports the development of strategies involving IFNα that may be useful to inhibit HCMV replication and highlights the complexity of the anti-viral response to HCMV in the presence of IFNα.

Biallelic variants in OGDH encoding oxoglutarate dehydrogenase lead to a neurodevelopmental disorder characterized by global developmental delay, movement disorder, and metabolic abnormalities.

PURPOSE: This study aimed to establish the genetic cause of a novel autosomal recessive neurodevelopmental disorder characterized by global developmental delay, movement disorder, and metabolic abnormalities. METHODS: We performed a detailed clinical characterization of 4 unrelated individuals from consanguineous families with a neurodevelopmental disorder. We used exome sequencing or targeted-exome sequencing, cosegregation, in silico protein modeling, and functional analyses of variants in HEK293 cells and Drosophila melanogaster, as well as in proband-derived fibroblast cells. RESULTS: In the 4 individuals, we identified 3 novel homozygous variants in oxoglutarate dehydrogenase (OGDH) (NM_002541.3), which encodes a subunit of the tricarboxylic acid cycle enzyme α-ketoglutarate dehydrogenase. In silico homology modeling predicts that c.566C>T:p.(Pro189Leu) and c.890C>A:p.(Ser297Tyr) variants interfere with the structure and function of OGDH. Fibroblasts from individual 1 showed that the p.(Ser297Tyr) variant led to a higher degradation rate of the OGDH protein. OGDH protein with p.(Pro189Leu) or p.(Ser297Tyr) variants in HEK293 cells showed significantly lower levels than the wild-type protein. Furthermore, we showed that expression of Drosophila Ogdh (dOgdh) carrying variants homologous to p.(Pro189Leu) or p.(Ser297Tyr), failed to rescue developmental lethality caused by loss of dOgdh. SpliceAI, a variant splice predictor, predicted that the c.935G>A:p.(Arg312Lys)/p.(Phe264_Arg312del) variant impacts splicing, which was confirmed through a mini-gene assay in HEK293 cells. CONCLUSION: We established that biallelic variants in OGDH cause a neurodevelopmental disorder with metabolic and movement abnormalities.

Brain monoamine vesicular transport disease caused by homozygous SLC18A2 variants: A study in 42 affected individuals.

PURPOSE: Brain monoamine vesicular transport disease is an infantile-onset movement disorder that mimics cerebral palsy. In 2013, the homozygous SLC18A2 variant, p.Pro387Leu, was first reported as a cause of this rare disorder, and dopamine agonists were efficient for treating affected individuals from a single large family. To date, only 6 variants have been reported. In this study, we evaluated genotype-phenotype correlations in individuals with biallelic SLC18A2 variants. METHODS: A total of 42 affected individuals with homozygous SLC18A2 variant alleles were identified. We evaluated genotype-phenotype correlations and the missense variants in the affected individuals based on the structural modeling of rat VMAT2 encoded by Slc18a2, with cytoplasm- and lumen-facing conformations. A Caenorhabditis elegans model was created for functional studies. RESULTS: A total of 19 homozygous SLC18A2 variants, including 3 recurrent variants, were identified using exome sequencing. The affected individuals typically showed global developmental delay, hypotonia, dystonia, oculogyric crisis, and autonomic nervous system involvement (temperature dysregulation/sweating, hypersalivation, and gastrointestinal dysmotility). Among the 58 affected individuals described to date, 16 (28%) died before the age of 13 years. Of the 17 patients with p.Pro237His, 9 died, whereas all 14 patients with p.Pro387Leu survived. Although a dopamine agonist mildly improved the disease symptoms in 18 of 21 patients (86%), some affected individuals with p.Ile43Phe and p.Pro387Leu showed milder phenotypes and presented prolonged survival even without treatment. The C. elegans model showed behavioral abnormalities. CONCLUSION: These data expand the phenotypic and genotypic spectra of SLC18A2-related disorders.

HIDEA syndrome is caused by biallelic, pathogenic, rare or founder P4HTM variants impacting the active site or the overall stability of the P4H-TM protein.

HIDEA syndrome is caused by biallelic pathogenic variants in P4HTM. The phenotype is characterized by muscular and central hypotonia, hypoventilation including obstructive and central sleep apneas, intellectual disability, dysautonomia, epilepsy, eye abnormalities, and an increased tendency to develop respiratory distress during pneumonia. Here, we report six new patients with HIDEA syndrome caused by five different biallelic P4HTM variants, including three novel variants. We describe two Finnish enriched pathogenic P4HTM variants and demonstrate that these variants are embedded within founder haplotypes. We review the clinical data from all previously published patients with HIDEA and characterize all reported P4HTM pathogenic variants associated with HIDEA in silico. All known pathogenic variants in P4HTM result in either premature stop codons, an intragenic deletion, or amino acid changes that impact the active site or the overall stability of P4H-TM protein. In all cases, normal P4H-TM enzyme function is expected to be lost or severely decreased. This report expands knowledge of the genotypic and phenotypic spectrum of the disease.

MED27 Variants Cause Developmental Delay, Dystonia, and Cerebellar Hypoplasia.

The Mediator multiprotein complex functions as a regulator of RNA polymerase II-catalyzed gene transcription. In this study, exome sequencing detected biallelic putative disease-causing variants in MED27, encoding Mediator complex subunit 27, in 16 patients from 11 families with a novel neurodevelopmental syndrome. Patient phenotypes are highly homogeneous, including global developmental delay, intellectual disability, axial hypotonia with distal spasticity, dystonic movements, and cerebellar hypoplasia. Seizures and cataracts were noted in severely affected individuals. Identification of multiple patients with biallelic MED27 variants supports the critical role of MED27 in normal human neural development, particularly for the cerebellum. ANN NEUROL 2021;89:828-833.

Mutations in GPAA1, Encoding a GPI Transamidase Complex Protein, Cause Developmental Delay, Epilepsy, Cerebellar Atrophy, and Osteopenia.

Approximately one in every 200 mammalian proteins is anchored to the cell membrane through a glycosylphosphatidylinositol (GPI) anchor. These proteins play important roles notably in neurological development and function. To date, more than 20 genes have been implicated in the biogenesis of GPI-anchored proteins. GPAA1 (glycosylphosphatidylinositol anchor attachment 1) is an essential component of the transamidase complex along with PIGK, PIGS, PIGT, and PIGU (phosphatidylinositol-glycan biosynthesis classes K, S, T, and U, respectively). This complex orchestrates the attachment of the GPI anchor to the C terminus of precursor proteins in the endoplasmic reticulum. Here, we report bi-allelic mutations in GPAA1 in ten individuals from five families. Using whole-exome sequencing, we identified two frameshift mutations (c.981_993del [p.Gln327Hisfs∗102] and c.920delG [p.Gly307Alafs∗11]), one intronic splicing mutation (c.1164+5C>T), and six missense mutations (c.152C>T [p.Ser51Leu], c.160_161delinsAA [p.Ala54Asn], c.527G>C [p.Trp176Ser], c.869T>C [p.Leu290Pro], c.872T>C [p.Leu291Pro], and c.1165G>C [p.Ala389Pro]). Most individuals presented with global developmental delay, hypotonia, early-onset seizures, cerebellar atrophy, and osteopenia. The splicing mutation was found to decrease GPAA1 mRNA. Moreover, flow-cytometry analysis of five available individual samples showed that several GPI-anchored proteins had decreased cell-surface abundance in leukocytes (FLAER, CD16, and CD59) or fibroblasts (CD73 and CD109). Transduction of fibroblasts with a lentivirus encoding the wild-type protein partially rescued the deficiency of GPI-anchored proteins. These findings highlight the role of the transamidase complex in the development and function of the cerebellum and the skeletal system.

Defective mitochondrial RNA processing due to PNPT1 variants causes Leigh syndrome.

Leigh syndrome is a severe infantile encephalopathy with an exceptionally variable genetic background. We studied the exome of a child manifesting with Leigh syndrome at one month of age and progressing to death by the age of 2.4 years, and identified novel compound heterozygous variants in PNPT1, encoding the polynucleotide phosphorylase (PNPase). Expression of the wild type PNPT1 in the subject's myoblasts functionally complemented the defects, and the pathogenicity was further supported by structural predictions and protein and RNA analyses. PNPase is a key enzyme in mitochondrial RNA metabolism, with suggested roles in mitochondrial RNA import and degradation. The variants were predicted to locate in the PNPase active site and disturb the RNA processing activity of the enzyme. The PNPase trimer formation was not affected, but specific RNA processing intermediates derived from mitochondrial transcripts of the ND6 subunit of Complex I, as well as small mRNA fragments, accumulated in the subject's myoblasts. Mitochondrial RNA processing mediated by the degradosome consisting of hSUV3 and PNPase is poorly characterized, and controversy on the role and location of PNPase within human mitochondria exists. Our evidence indicates that PNPase activity is essential for the correct maturation of the ND6 transcripts, and likely for the efficient removal of degradation intermediates. Loss of its activity will result in combined respiratory chain deficiency, and a classic respiratory chain-deficiency-associated disease, Leigh syndrome, indicating an essential role for the enzyme for normal function of the mitochondrial respiratory chain.

Absence of the Autophagy Adaptor SQSTM1/p62 Causes Childhood-Onset Neurodegeneration with Ataxia, Dystonia, and Gaze Palsy.

SQSTM1 (sequestosome 1; also known as p62) encodes a multidomain scaffolding protein involved in various key cellular processes, including the removal of damaged mitochondria by its function as a selective autophagy receptor. Heterozygous variants in SQSTM1 have been associated with Paget disease of the bone and might contribute to neurodegeneration in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Using exome sequencing, we identified three different biallelic loss-of-function variants in SQSTM1 in nine affected individuals from four families with a childhood- or adolescence-onset neurodegenerative disorder characterized by gait abnormalities, ataxia, dysarthria, dystonia, vertical gaze palsy, and cognitive decline. We confirmed absence of the SQSTM1/p62 protein in affected individuals' fibroblasts and found evidence of a defect in the early response to mitochondrial depolarization and autophagosome formation. Our findings expand the SQSTM1-associated phenotypic spectrum and lend further support to the concept of disturbed selective autophagy pathways in neurodegenerative diseases.

Defective mitochondrial ATPase due to rare mtDNA m.8969G>A mutation-causing lactic acidosis, intellectual disability, and poor growth.

Mutations in mitochondrial ATP synthase 6 (MT-ATP6) are a frequent cause of NARP (neurogenic muscle weakness, ataxia, and retinitis pigmentosa) or Leigh syndromes, especially a point mutation at nucleotide position 8993. M.8969G>A is a rare MT-ATP6 mutation, previously reported only in three individuals, causing multisystem disorders with mitochondrial myopathy, lactic acidosis, and sideroblastic anemia or IgA nephropathy. We present two siblings with the m.8969G>A mutation and a novel, substantially milder phenotype with lactic acidosis, poor growth, and intellectual disability. Our findings expand the phenotypic spectrum and show that mtDNA mutations should be taken account also with milder, stable phenotypes.

Mitochondrial phenylalanyl-tRNA synthetase mutations underlie fatal infantile Alpers encephalopathy.

Next-generation sequencing has turned out to be a powerful tool to uncover genetic basis of childhood mitochondrial disorders. We utilized whole-exome analysis and discovered novel compound heterozygous mutations in FARS2 (mitochondrial phenylalanyl transfer RNA synthetase), encoding the mitochondrial phenylalanyl transfer RNA (tRNA) synthetase (mtPheRS) in two patients with fatal epileptic mitochondrial encephalopathy. The mutations affected highly conserved amino acids, p.I329T and p.D391V. Recently, a homozygous FARS2 variant p.Y144C was reported in a Saudi girl with mitochondrial encephalopathy, but the pathogenic role of the variant remained open. Clinical features, including postnatal onset, catastrophic epilepsy, lactic acidemia, early lethality and neuroimaging findings of the patients with FARS2 variants, resembled each other closely, and neuropathology was consistent with Alpers syndrome. Our structural analysis of mtPheRS predicted that p.I329T weakened ATP binding in the aminoacylation domain, and in vitro studies with recombinant mutant protein showed decreased affinity of this variant to ATP. Furthermore, p.D391V and p.Y144C were predicted to disrupt synthetase function by interrupting the rotation of the tRNA anticodon stem-binding domain from a closed to an open form. In vitro characterization indicated reduced affinity of p.D391V mutant protein to phenylalanine, whereas p.Y144C disrupted tRNA binding. The stability of p.I329T and p.D391V mutants in a refolding assay was impaired. Our results imply that the three FARS2 mutations directly impair aminoacylation function and stability of mtPheRS, leading to a decrease in overall tRNA charging capacity. This study establishes a new genetic cause of infantile mitochondrial Alpers encephalopathy and reports a new mitochondrial aminoacyl-tRNA synthetase as a cause of mitochondrial disease.

Whole-exome sequencing identifies a mutation in the mitochondrial ribosome protein MRPL44 to underlie mitochondrial infantile cardiomyopathy.

BACKGROUND: The genetic complexity of infantile cardiomyopathies is remarkable, and the importance of mitochondrial translation defects as a causative factor is only starting to be recognised. We investigated the genetic basis for infantile onset recessive hypertrophic cardiomyopathy in two siblings. METHODS AND RESULTS: Analysis of respiratory chain enzymes revealed a combined deficiency of complexes I and IV in the heart and skeletal muscle. Exome sequencing uncovered a homozygous mutation (L156R) in MRPL44 of both siblings. MRPL44 encodes a protein in the large subunit of the mitochondrial ribosome and is suggested to locate in close proximity to the tunnel exit of the yeast mitochondrial ribosome. We found severely reduced MRPL44 levels in the patient's heart, skeletal muscle and fibroblasts suggesting that the missense mutation affected the protein stability. In patient fibroblasts, decreased MRPL44 affected assembly of the large ribosomal subunit and stability of 16S rRNA leading to complex IV deficiency. Despite this assembly defect, de novo mitochondrial translation was only mildly affected in fibroblasts suggesting that MRPL44 may have a function in the assembly/stability of nascent mitochondrial polypeptides exiting the ribosome. Retroviral expression of wild-type MRPL44 in patient fibroblasts rescued the large ribosome assembly defect and COX deficiency. CONCLUSIONS: These findings indicate that mitochondrial ribosomal subunit defects can generate tissue-specific manifestations, such as cardiomyopathy.

Supernumerary proteins of the human mitochondrial ribosomal small subunit are integral for assembly and translation.

Mitochondrial ribosomes (mitoribosomes) have undergone substantial evolutionary structural remodeling accompanied by loss of ribosomal RNA, while acquiring unique protein subunits located on the periphery. We generated CRISPR-mediated knockouts of all 14 unique (mitochondria-specific/supernumerary) human mitoribosomal proteins (snMRPs) in the small subunit to study the effect on mitoribosome assembly and protein synthesis, each leading to a unique mitoribosome assembly defect with variable impact on mitochondrial protein synthesis. Surprisingly, the stability of mS37 was reduced in all our snMRP knockouts of the small and large ribosomal subunits and patient-derived lines with mitoribosome assembly defects. A redox-regulated CX9C motif in mS37 was essential for protein stability, suggesting a potential mechanism to regulate mitochondrial protein synthesis. Together, our findings support a modular assembly of the human mitochondrial small ribosomal subunit mediated by essential supernumerary subunits and identify a redox regulatory role involving mS37 in mitochondrial protein synthesis in health and disease.

Recessive TMOD1 mutation causes childhood cardiomyopathy.

Familial cardiomyopathy in pediatric stages is a poorly understood presentation of heart disease in children that is attributed to pathogenic mutations. Through exome sequencing, we report a homozygous variant in tropomodulin 1 (TMOD1; c.565C>T, p.R189W) in three individuals from two unrelated families with childhood-onset dilated and restrictive cardiomyopathy. To decipher the mechanism of pathogenicity of the R189W mutation in TMOD1, we utilized a wide array of methods, including protein analyses, biochemistry and cultured cardiomyocytes. Structural modeling revealed potential defects in the local folding of TMOD1R189W and its affinity for actin. Cardiomyocytes expressing GFP-TMOD1R189W demonstrated longer thin filaments than GFP-TMOD1wt-expressing cells, resulting in compromised filament length regulation. Furthermore, TMOD1R189W showed weakened activity in capping actin filament pointed ends, providing direct evidence for the variant's effect on actin filament length regulation. Our data indicate that the p.R189W variant in TMOD1 has altered biochemical properties and reveals a unique mechanism for childhood-onset cardiomyopathy.

Ketogenic diet slows down mitochondrial myopathy progression in mice.

Mitochondrial dysfunction is a major cause of neurodegenerative and neuromuscular diseases of adult age and of multisystem disorders of childhood. However, no effective treatment exists for these progressive disorders. Cell culture studies suggested that ketogenic diet (KD), with low glucose and high fat content, could select against cells or mitochondria with mutant mitochondrial DNA (mtDNA), but proper patient trials are still lacking. We studied here the transgenic Deletor mouse, a disease model for progressive late-onset mitochondrial myopathy, accumulating mtDNA deletions during aging and manifesting subtle progressive respiratory chain (RC) deficiency. We found that these mice have widespread lipidomic and metabolite changes, including abnormal plasma phospholipid and free amino acid levels and ketone body production. We treated these mice with pre-symptomatic long-term and post-symptomatic shorter term KD. The effects of the diet for disease progression were followed by morphological, metabolomic and lipidomic tools. We show here that the diet decreased the amount of cytochrome c oxidase negative muscle fibers, a key feature in mitochondrial RC deficiencies, and prevented completely the formation of the mitochondrial ultrastructural abnormalities in the muscle. Furthermore, most of the metabolic and lipidomic changes were cured by the diet to wild-type levels. The diet did not, however, significantly affect the mtDNA quality or quantity, but rather induced mitochondrial biogenesis and restored liver lipid levels. Our results show that mitochondrial myopathy induces widespread metabolic changes, and that KD can slow down progression of the disease in mice. These results suggest that KD may be useful for mitochondrial late-onset myopathies.

Mitochondrial myopathy induces a starvation-like response.

Mitochondrial respiratory chain (RC) deficiency is among the most common causes of inherited metabolic disease, but its physiological consequences are poorly characterized. We studied the skeletal muscle gene expression profiles of mice with late-onset mitochondrial myopathy. These animals express a dominant patient mutation in the mitochondrial replicative helicase Twinkle, leading to accumulation of multiple mtDNA deletions and progressive subtle RC deficiency in the skeletal muscle. The global gene expression pattern of the mouse skeletal muscle showed induction of pathways involved in amino acid starvation response and activation of Akt signaling. Furthermore, the muscle showed induction of a fasting-related hormone, fibroblast growth factor 21 (Fgf21). This secreted regulator of lipid metabolism was also elevated in the mouse serum, and the animals showed widespread changes in their lipid metabolism: small adipocyte size, low fat content in the liver and resistance to high-fat diet. We propose that RC deficiency induces a mitochondrial stress response, with local and global changes mimicking starvation, in a normal nutritional state. These results may have important implications for understanding the metabolic consequences of mitochondrial myopathies.

Effects of polyclonal IgG derived from patients with different clinical types of the antiphospholipid syndrome on monocyte signaling pathways.

A major mechanism of hypercoagulability in the antiphospholipid syndrome (APS) is antiphospholipid Ab-mediated upregulation of tissue factor (TF) on monocytes via activation of TLRs, p38 MAPK, and NF-kappaB pathways. We examined whether monocyte signaling pathways are differentially activated by IgG from patients with vascular thrombosis (VT) alone compared with IgG from patients with pregnancy morbidity (PM) alone. We purified IgG from 49 subjects. A human monocyte cell line and ex vivo healthy monocytes were treated with 100 microg/ml IgG for 6 h, and cell extracts were examined by immunoblot using Abs to p38 MAPK and NF-kappaB. To further investigate intracellular signaling pathways induced by these IgGs, specific inhibitors of p38 MAPK, NF-kappaB, TLR4, and TLR2 were used to determine their effect on TF activity. Only IgG from patients with VT but no PM (VT+/PM-) caused phosphorylation of NF-kappaBand p38 MAPK and upregulation of TF activity in monocytes. These effects were not seen with IgG from patients with PM alone (VT-/PM+), anti-phospholipid Ab-positive patients without APS, or healthy controls. TF upregulation caused by the VT+/PM- samples was reduced by inhibitors of p38 MAPK, NF-kappaB, and TLR4. The effects of VT+/PM- IgG on signaling and TF upregulation were concentrated in the fraction that bound beta-2-glycoprotein I. Our findings demonstrate that IgGs from patients with diverse clinical manifestations of APS have differential effects upon phosphorylation of NF-kappaB and p38 MAPK and TF activity that may be mediated by differential activation of TLR4.

An overview of international genetic evaluations of show jumping in sport horses.

The breeding of sport horses to compete in the Olympic disciplines of show jumping, eventing, and dressage is fast becoming a global industry with the increased use of reproductive technologies, including artificial insemination and embryo transfer. Reproductive technologies have facilitated the dissemination of genetics from elite horses across multiple countries and breeds as breeders are no longer limited by location. Due to this increased level of crossbreeding, there is an increased need for estimated breeding values (EBVs) for sport horse performance that can be compared across breeds and countries. However, the implementation of across-breed or across-country genetic evaluations has been limited by the differences in each studbook's individual breeding programs and genetic evaluations. Consequently, the aim of this review was to compare the genetic evaluations for show jumping of sport horse studbooks worldwide. The top sport horse studbooks in the world according to the World Breeding Federation for Sport Horses Studbook Rankings 2019 were contacted by email to request information on their current breeding programs and genetic evaluations. Twenty-six of the 51 studbooks contacted replied to this request but only 18 of these studbooks conducted their own genetic evaluations or were part of a larger genetic evaluation in their country of origin. The other eight studbooks were not involved in genetic evaluations at present but expressed an interest in the implementation of such in the future. Overall, many differences were identified among the genetic evaluations of each studbook or each country. The definition of show jumping performance differed within each evaluation and the methods and models utilized also differed. Despite some stallions and mares being registered in multiple studbooks or having progeny in multiple studbooks, these differences make comparison of EBVs across studbooks difficult. Further transparency and collaboration of sport horse studbooks with organizations such as Interstallion, will be essential to facilitate any future implementation of international genetic evaluations for show jumping performance.