BACKGROUND:Female infertility occurs in ∼37% of infertile couples, while premature ovarian insufficiency (POI) impacts 1-3.7% of women under the age of 40. POI is clinically heterogeneous, with various genetic pathways associated with its pathogenesis. Mitochondrial diseases (MDs) are a broad group of clinically heterogeneous genetic conditions characterized by aberrantly functioning mitochondria. MDs have a disproportionate burden on organs and tissues with increased aerobic/energy demands, such as the heart, skeletal muscles, brain, and ovaries. The role of mitochondria in female fertility and ovarian reserve is increasingly being recognized. OBJECTIVE AND RATIONALE:A comprehensive understanding of the role of mitochondria in the maintenance of female fertility is pertinent to better understanding female reproductive potential. In a world with increasing demand for assisted reproductive technologies (ART), due to a considerable rate of global infertility, there is a need to better understand the genes and pathways involved in female reproduction. This review summarizes, evaluates, and explores the current knowledge of mitochondria-associated genes and variants that are implicated in POI, including their function and dysfunction in female reproduction. SEARCH METHODS:We searched articles in the PubMed database, containing the following key words: premature ovarian insufficiency, mitochondria, mitochondrial, premature ovarian failure, genetics, mitochondrial DNA, mtDNA, mitochondrial protein, infertility, premature menopause, mitochondrial donation, assisted reproductive technologies, electron transport chain, oxidative phosphorylation (OXPHOS), mitochondrial disease, oocyte, oogenesis, meiosis, in vitro fertilization, mitoribosome, Perrault syndrome, and ovarioleukodystrophy, in the English-language literature until March 2026. OUTCOMES:Genetic variants that affect mitochondrial genes/proteins can negatively impact ovarian function. Various mitochondrial pathways are associated with female infertility, reflecting the broad sensitivity of ovarian reserve to mitochondrial dysfunction. Mitochondrial dysfunction and infertility can present in isolation or as part of a syndrome. Infertility in women may be the first clinical sign of a MD. Conversely, POI may be an underappreciated symptom of MDs. WIDER IMPLICATIONS:This review draws attention to the fact that females with MDs should be monitored for POI so it can be detected early for prompt and appropriate therapeutic interventions, such as hormone replacement therapy. This is known to mitigate the risk of comorbidities such as cardiovascular and bone disease and will optimize long-term health outcomes. Of equal importance, our review highlights the potential for girls and women presenting with apparently 'isolated' POI to harbour pathogenic variants in MD-associated genes, therefore putting these individuals at risk of developing further clinical manifestations of MDs. We emphasize the need for surveillance in these cases for hearing loss, vision disturbance, cardiomyopathy, muscle weakness and neurodegeneration, depending on the genetic cause. Given that mitochondrial function is essential to female fertility, future therapies for mitochondria-associated infertility could involve mitochondrial supplementation to improve the mitochondrial fraction or mitochondrial donation to optimize the likelihood of reproductive success. Finally, we also discuss the current landscape of biomarkers as potential early diagnostic tools for POI. Whilst currently rudimentary in their clinical utility, the further development of early screening methods will be invaluable for the detection, diagnosis, and early intervention of POI. REGISTRATION NUMBER:N/A.

XRN1 encodes a highly conserved cytoplasmic 5’-3’ exoribonuclease involved in mRNA decay and quality control. It localizes to transient ribonucleoprotein aggregates, P-bodies and stress granules, where it interacts with other mRNA decay proteins and is involved in various cellular responses, including an emerging role in viral infection responses. Complete knockout of XRN1 in multicellular organisms is lethal, most likely due to its central role in mRNA homeostasis, with no prior human disease association reported. Here, we characterize seven individuals from six unrelated families with a lethal infantile form of mitochondrial cardiomyopathy caused by heterozygous de novo frameshift truncation variants clustering in the C-terminal region of XRN1 , each predicted to evade nonsense-mediated mRNA decay. Each variant results in a near-identical XRN1 C-terminal sequence predicted to alter a characterized binding domain that interacts with the mRNA decapping enhancer EDC4. Biochemical investigations of striated muscle revealed combined oxidative phosphorylation deficiencies, demonstrated by decreased respiratory chain enzyme activities, decreased proteomics abundances, and abnormal histochemical reactivities. Despite having no established mitochondrial function in mammals, clinical and molecular findings across the cohort were consistent with mitochondrial disease. The precise mechanism by which the altered XRN1 proteins cause disease remains to be elucidated.

MRPS22-related mitochondrial disease (MIM#611719) is a rare autosomal recessive disorder caused by defects in the mitochondrial ribosomal protein S22, a component of the small mitoribosomal subunit essential for mitochondrial translation. Of the few reported cases, most present antenatally with a severe phenotype, conveying a poor prognosis. We describe a fetus with severe antenatal-onset MRPS22-related mitochondrial disease and the use of multi-omics in the molecular diagnosis. A primigravida underwent termination of pregnancy following identification of multiple congenital anomalies (hydrops fetalis, microcephaly, corpus callosal agenesis, periventricular cysts and cardiac hypertrophy) on ultrasound at 20 + 2 weeks' gestation, confirmed on fetal magnetic resonance imaging. Trio genome sequencing revealed compound heterozygous variants in MRPS22 (NM_020191.4: c.509G>A; p.(Arg170His) and c.565C>G; p.(Arg189Gly)). Rapid proteomic analysis demonstrated destabilisation of the small mitoribosomal subunit and combined reduction of OXPHOS complexes, supporting the pathogenicity of the variants. This case consolidates the antenatal phenotype of severe MRPS22-related disease and highlights the importance of considering mitochondrial disease in the differential diagnosis of congenital anomalies, especially hydrops fetalis and corpus callosum anomalies. This study provides evidence for the utility of multi-omic approaches (trio genome sequencing, proteomics) in confirming variant pathogenicity following pregnancy loss, enabling accurate diagnosis, and informing reproductive counselling for affected families.

Genomic sequencing has transformed the diagnostic approach for mitochondrial disease, yet integration into standard clinical practice is limited by access and funding. We conducted a post-implementation evaluation of genome sequencing (GS) for mitochondrial disease in Australia, which became publicly funded through the Medicare Benefits Scheme (MBS) in November 2023, to allow for broader access to testing. Test request data, including demographics, phenotypic information, and the diagnostic outcomes, were collected from November 2023 to May 2025 from the Victorian Clinical Genetics Services, the current laboratory provider of the MBS-funded service. Test uptake was 26% of predicted, with lower test rates in regional and remote areas. Over the first 19 months, 300 individuals suspected of mitochondrial disease underwent GS with a median turnaround time of 84 days (8 days-218 days). The diagnostic yield was 20%, with 56% of diagnoses in known mitochondrial disease genes. Of these, 70% (24 of 34) were in mitochondrial DNA. Seventeen diagnoses were in individuals who had prior non-diagnostic testing (exome sequencing or gene panel). We demonstrate that publicly-funded GS can deliver meaningful diagnostic outcomes for mitochondrial disease on a national scale. To maximise its impact, attention must now shift towards ensuring equitable access, particularly for regional and remote areas, and embedding sustainable mainstreaming models that support both genetic and non-genetic clinicians.

Mitochondrial disease is a diverse group of clinically and genetically complex disorders caused by pathogenic variants in nuclear or mitochondrial DNA-encoded genes that disrupt mitochondrial energy production or other important mitochondrial pathways. Mitochondrial disease can present with a wide spectrum of clinical features and can often be difficult to recognize. These conditions can be devastating; however, for the majority, there is no targeted treatment. In the last 60 years, mitochondrial medicine has experienced significant evolution, moving from the pre-molecular era to the Age of Genomics in which considerable gene discovery and advancement in our understanding of the pathophysiology of mitochondrial disease have been made. In the last decade, in response to the urgent need for effective treatments, a wide range of emerging therapies have been developed, driven by innovative approaches addressing both the genetic and cellular mechanisms underpinning the diseases. Emerging therapies include dietary intervention, small molecule therapies aimed to restore mitochondrial function, stem cell or liver transplantation, and gene or RNA-based therapies. However, despite these advances, translation to clinical practice is complicated by the sheer genetic and clinical complexity of mitochondrial disease, difficulty in efficient and precise delivery of therapies to affected tissues, rarity of individual genetic conditions, lack of reliable biomarkers and clinically relevant outcome measures, and the dearth of natural history data. This review examines the latest developments in the pursuit to identify effective treatments for mitochondrial disease and discusses the barriers impeding their success in translation to clinical practice. While treatment for mitochondrial disease may be on the horizon, many challenges must be addressed before it can become a reality.

Variants in the mitochondrial genome (mtDNA) cause a diverse collection of mitochondrial diseases and have extensive phenotypic overlap with Mendelian diseases encoded on the nuclear genome. The mtDNA is not always specifically evaluated in patients with suspected Mendelian disease, resulting in overlooked diagnostic variants. Here, we analyzed a cohort of 6,660 rare disease families (5,625 genetically undiagnosed [84%]) from the Genomics Research to Elucidate the Genetics of Rare diseases (GREGoR) Consortium, as well as other rare disease cohorts. Using dedicated pipelines to address the technical challenges posed by the mtDNA-circular genome, variant heteroplasmy, and nuclear misalignment-we called single nucleotide variants, small insertions/deletions, and large mtDNA deletions from exome and/or genome sequencing data, in addition to RNA sequencing data when available. Diagnostic mtDNA variants were identified in 10 previously genetically undiagnosed families (1 large deletion, 8 reported pathogenic variants, and 1 previously unreported likely pathogenic variant), as well as candidate diagnostic variants in a further 11 undiagnosed families. In one additional undiagnosed proband, detection of >900 heteroplasmic variants provided functional evidence of pathogenicity to a de novo variant in the nuclear gene POLG (DNA polymerase gamma), responsible for mtDNA replication and repair. Overall, mtDNA variant calling from data generated by exome and genome sequencing-primarily for nuclear variant analysis-resulted in a genetic diagnosis for 0.2% of undiagnosed families affected by a broad range of rare diseases, as well as the identification of additional promising candidates in 0.2%.

Only half of individuals with suspected rare diseases receive a genetic diagnosis following genomic testing. A genetic diagnosis allows access to appropriate care, restores reproductive confidence and reduces the number of potentially unnecessary interventions. A major barrier is the lack of disease agnostic functional tests suitable for implementation in routine diagnostics that can provide evidence supporting pathogenicity of novel variants, especially those refractory to RNA sequencing. Focusing on mitochondrial disease, we describe an untargeted mass-spectrometry based proteomics pipeline that can quantify proteins encoded by > 50 https://rdms.app/ ) allow mitochondrial dysfunction to be identified in proteomics data with high confidence. Application of proteomics to undiagnosed individuals led to 6 additional diagnoses, including a mitochondrial phenocopy disorder, highlighting the disease agnostic nature of proteomics. Use of PBMCs as a sample type allowed rapid return of proteomics data supporting pathogenicity of novel variants identified through ultra-rapid genome sequencing in as little as 54 h. This study provides a framework to support the integration of a single untargeted proteomics test into routine diagnostic practice for the diagnosis of mitochondrial and potentially other rare genetic disorders in clinically actionable timelines, offering a paradigm shift for the functional validation of genetic variants.

Purpose: To characterize the diagnostic and clinical outcomes of a cohort of critically ill infants and children with suspected mitochondrial disorders (MD) undergoing ultrarapid genomic testing as part of a national program. Methods: Ultrarapid genomic sequencing was performed in 454 families (genome sequencing: n = 290, exome sequencing + / - mitochondrial DNA sequencing: n = 164). In 91 individuals, MD was considered, prompting analysis using an MD virtual gene panel. These individuals were reviewed retrospectively and scored according to modified Nijmegen Mitochondrial Disease Criteria. Results: A diagnosis was achieved in 47% (43/91) of individuals, 40% (17/43) of whom had an MD. Seven additional individuals in whom an MD was not suspected were diagnosed with an MD after broader analysis. Gene-agnostic analysis led to the discovery of 2 novel disease genes, with pathogenicity validated through targeted functional studies ( CRLS1 and MRPL39). Functional studies enabled diagnosis in another 4 individuals. Of the 24 individuals ultimately diagnosed with an MD, 79% had a change in management, which included 53% whose care was redirected to palliation. Conclusion: Ultrarapid genetic diagnosis of MD in acutely unwell infants and children is critical for guiding decisions about the need for additional investigations and clinical management. (c) 2024 American College of Medical Genetics and Genomics. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

TIMM50 is a core subunit of the TIM23 complex, the mitochondrial inner membrane translocase responsible for the import of pre-sequence-containing precursors into the mitochondrial matrix and inner membrane. Here we describe a mitochondrial disease patient who is homozygous for a novel variant in TIMM50 and establish the first proteomic map of mitochondrial disease associated with TIMM50 dysfunction. We demonstrate that TIMM50 pathogenic variants reduce the levels and activity of endogenous TIM23 complex, which significantly impacts the mitochondrial proteome, resulting in a combined oxidative phosphorylation (OXPHOS) defect and changes to mitochondrial ultrastructure. Using proteomic data sets from TIMM50 patient fibroblasts and a TIMM50 HEK293 cell model of disease, we reveal that laterally released substrates imported via the TIM23SORT complex pathway are most sensitive to loss of TIMM50. Proteins involved in OXPHOS and mitochondrial ultrastructure are enriched in the TIM23SORT substrate pool, providing a biochemical mechanism for the specific defects in TIMM50-associated mitochondrial disease patients. These results highlight the power of using proteomics to elucidate molecular mechanisms of disease and uncovering novel features of fundamental biology, with the implication that human TIMM50 may have a more pronounced role in lateral insertion than previously understood.

Short-chain enoyl-coA hydratase (SCEH) deficiency due to biallelic pathogenic ECHS1 variants was first reported in 2014 in association with Leigh syndrome (LS) and increased S-(2-carboxypropyl)cysteine excretion. It is potentially treatable with a valine-restricted, high-energy diet and emergency regimen. Recently, Simon et al. described four Samoan children harbouring a hypomorphic allele (c.489G > A, p.Pro163 =) associated with reduced levels of normally-spliced mRNA. This synonymous variant, missed on standard genomic testing, is prevalent in the Samoan population (allele frequency 0.17). Patients with LS and one ECHS1 variant were identified in NZ and Australian genomic and clinical databases . ECHS1 sequence data were interrogated for the c.489G > A variant and clinical data were reviewed. Thirteen patients from 10 families were identified; all had Pacific ancestry including Samoan, Ma ori, Cook Island Ma ori, and Tokelauan. All developed bilateral globus pallidi lesions, excluding one pre-symptomatic infant. Symptom onset was in early childhood, and was triggered by illness or starvation in 9/13. Four of 13 had exercise-induced dyskinesia, 9/13 optic atrophy and 6/13 nystagmus. Urine S-(2-carboxypropyl)cysteine-carnitine and other SCEH-related metabolites were normal or mildly increased. Functional studies demonstrated skipping of exon four and markedly reduced ECHS1 protein. These data provide further support for the pathogenicity of this ECHS1 variant which is also prevalent in Ma ori, Cook Island Ma ori, and Tongan populations (allele frequency 0.14 -0.24). It highlights the need to search for a second variant in apparent heterozygotes with an appropriate phenotype, and has implications for genetic counselling in family members who are heterozygous for the more severe ECHS1 alleles. Synopsis: Short -chain enoyl-CoA hydratase deficiency is a frequent cause of Leigh -like disease in Ma ori and wider -Pacific populations, due to the high carrier frequency of a hypomorphic ECHS1 variant c.489G > A, p. [Pro163 =, Phe139Valfs*65] that may be overlooked by standard genomic testing.

Around 60% of individuals with neurodevelopmental disorders (NDD) remain undiagnosed after comprehensive genetic testing, primarily of protein-coding genes1. Increasingly, large genome-sequenced cohorts are improving our ability to discover new diagnoses in the non-coding genome. Here, we identify the non-coding RNA RNU4-2 as a novel syndromic NDD gene. RNU4-2 encodes the U4 small nuclear RNA (snRNA), which is a critical component of the U4/U6.U5 tri-snRNP complex of the major spliceosome2. We identify an 18 bp region of RNU4-2 mapping to two structural elements in the U4/U6 snRNA duplex (the T-loop and Stem III) that is severely depleted of variation in the general population, but in which we identify heterozygous variants in 119 individuals with NDD. The vast majority of individuals (77.3%) have the same highly recurrent single base-pair insertion (n.64_65insT). We estimate that variants in this region explain 0.41% of individuals with NDD. We demonstrate that RNU4-2 is highly expressed in the developing human brain, in contrast to its contiguous counterpart RNU4-1 and other U4 homologs, supporting RNU4-2's role as the primary U4 transcript in the brain. Overall, this work underscores the importance of non-coding genes in rare disorders. It will provide a diagnosis to thousands of individuals with NDD worldwide and pave the way for the development of effective treatments for these individuals.

Mitochondrial DNA (mtDNA) has an important, yet often overlooked, role in health and disease. Constraint models quantify the removal of deleterious variation from the population by selection, representing a powerful tool for identifying genetic variation underlying human phenotypes 1–4 . However, a constraint model for the mtDNA has not been developed, due to its unique features. Here we describe the development of a mitochondrial constraint model and its application to the Genome Aggregation Database (gnomAD), a large-scale population dataset reporting mtDNA variation across 56,434 humans 5 . Our results demonstrate strong depletion of expected variation, suggesting most deleterious mtDNA variants remain undiscovered. To aid their identification, we compute constraint metrics for every mitochondrial protein, tRNA, and rRNA gene, revealing a spectrum of intolerance to variation. We characterize the most constrained regions within genes via regional constraint, and positions across the entire mtDNA via local constraint, showing their enrichment in pathogenic variation and functionally critical sites, including topological clustering in 3D protein and RNA structures. Notably, we identify constraint at often overlooked sites, such as rRNAs and non-coding regions. Lastly, we demonstrate how these metrics can improve the discovery of mtDNA variation underlying rare and common human phenotypes.

Purpose Families living with mitochondrial diseases (MD) often endure prolonged diagnostic journeys and invasive testing, yet many remain without a molecular diagnosis. Through a national team of clinicians, diagnostic, and research scientists, the Australian Genomics Mitochondrial disease flagship conducted a prospective study to identify the diagnostic utility of singleton genomic sequencing using blood samples as a first step to diagnose MD. Methods 140 children and adults living with suspected MD were recruited using modified Nijmegen criteria (MNC) and randomised to either exome + mtDNA sequencing (ES+mtDNAseq) or genome sequencing (GS). Results Diagnostic yield was 55% (n=77) with variants in nuclear (n=37) and mtDNA (n=18) MD genes, as well as phenocopy genes (n=22). A nuclear gene aetiology was identified in 77% of diagnoses, irrespective of disease onset. Diagnostic rate was higher in paediatric-onset (71%) than adult-onset (31%) cases. For children, higher MNC scores correlated with increased diagnostic yield and fewer diagnoses in phenocopy genes. Additionally, three adult patients had a mtDNA deletion discovered in skeletal muscle that was not initially detected in blood. Conclusion Genomic sequencing from blood can simplify the diagnostic pathway for individuals living with suspected MD, especially those with childhood onset diseases and high MNC scores.

MRPL39 encodes one of 52 proteins comprising the large subunit of the mitochondrial ribosome (mitoribosome). In conjunction with 30 proteins in the small subunit, the mitoribosome synthesizes the 13 subunits of the mitochondrial oxidative phosphorylation (OXPHOS) system encoded by mitochondrial Deoxyribonucleic acid (DNA). We used multi-omics and gene matching to identify three unrelated individuals with biallelic variants in MRPL39 presenting with multisystem diseases with severity ranging from lethal, infantile-onset (Leigh syndrome spectrum) to milder with survival into adulthood. Clinical exome sequencing of known disease genes failed to diagnose these patients; however quantitative proteomics identified a specific decrease in the abundance of large but not small mitoribosomal subunits in fibroblasts from the two patients with severe phenotype. Re-analysis of exome sequencing led to the identification of candidate single heterozygous variants in mitoribosomal genes MRPL39 (both patients) and MRPL15. Genome sequencing identified a shared deep intronic MRPL39 variant predicted to generate a cryptic exon, with transcriptomics and targeted studies providing further functional evidence for causation. The patient with the milder disease was homozygous for a missense variant identified through trio exome sequencing. Our study highlights the utility of quantitative proteomics in detecting protein signatures and in characterizing gene-disease associations in exome-unsolved patients. We describe Relative Complex Abundance analysis of proteomics data, a sensitive method that can identify defects in OXPHOS disorders to a similar or greater sensitivity to the traditional enzymology. Relative Complex Abundance has potential utility for functional validation or prioritization in many hundreds of inherited rare diseases where protein complex assembly is disrupted.

Critically ill infants and children with rare diseases need equitable access to rapid and accurate diagnosis to direct clinical management. Over 2 years, the Acute Care Genomics program provided whole-genome sequencing to 290 families whose critically ill infants and children were admitted to hospitals throughout Australia with suspected genetic conditions. The average time to result was 2.9 d and diagnostic yield was 47%. We performed additional bioinformatic analyses and transcriptome sequencing in all patients who remained undiagnosed. Long-read sequencing and functional assays, ranging from clinically accredited enzyme analysis to bespoke quantitative proteomics, were deployed in selected cases. This resulted in an additional 19 diagnoses and an overall diagnostic yield of 54%. Diagnostic variants ranged from structural chromosomal abnormalities through to an intronic retrotransposon, disrupting splicing. Critical care management changed in 120 diagnosed patients (77%). This included major impacts, such as informing precision treatments, surgical and transplant decisions and palliation, in 94 patients (60%). Our results provide preliminary evidence of the clinical utility of integrating multi-omic approaches into mainstream diagnostic practice to fully realize the potential of rare disease genomic testing in a timely manner.

Mitochondrial respiratory chain disorders (MRC) are amongst the most common group of inborn errors of metabolism. MRC, of which complex I deficiency accounts for approximately a quarter, are very diverse, causing a wide range of clinical problems and can be difficult to diagnose. We report an illustrative MRC case whose diagnosis was elusive. Clinical signs included failure to thrive caused by recurrent vomiting, hypotonia and progressive loss of motor milestones. Initial brain imaging suggested Leigh syndrome but without expected diffusion restriction. Muscle respiratory chain enzymology was unremarkable. Whole-genome sequencing identified a maternally inherited NDUFV1 missense variant [NM_007103.4 (NDUFV1):c.1157G > A; p.(Arg386His)] and a paternally inherited synonymous variant [NM_007103.4 (NDUFV1):c.1080G > A; (p.Ser360=)]. RNA sequencing demonstrated aberrant splicing. This case emphasizes the diagnostic odyssey of a patient in whom a confirmed diagnosis was elusive because of atypical features and normal muscle respiratory chain enzyme (RCE) activities, along with a synonymous variant, which are often filtered out from genomic analyses. It also illustrates the following points: (1) complete resolution of magnetic resonance imaging changes may be part of the picture in mitochondrial disease; (2) analysis for synonymous variants is important for undiagnosed patients; and (3) RNA-seq is a powerful tool to demonstrate pathogenicity of putative splicing variants.

Premature ovarian insufficiency (POI) is a common cause of infertility in women, characterised by amenorrhea and elevated FSH under the age of 40 years. In some cases, POI is syndromic in association with other features such as sensorineural hearing loss in Perrault syndrome. POI is a heterogeneous disease with over 80 causative genes known so far; however, these explain only a minority of cases. Using whole-exome sequencing (WES), we identified a MRPL50 homozygous missense variant (c.335T > A; p.Val112Asp) shared by twin sisters presenting with POI, bilateral high-frequency sensorineural hearing loss, kidney and heart dysfunction. MRPL50 encodes a component of the large subunit of the mitochondrial ribosome. Using quantitative proteomics and western blot analysis on patient fibroblasts, we demonstrated a loss of MRPL50 protein and an associated destabilisation of the large subunit of the mitochondrial ribosome whilst the small subunit was preserved. The mitochondrial ribosome is responsible for the translation of subunits of the mitochondrial oxidative phosphorylation machinery, and we found patient fibroblasts have a mild but significant decrease in the abundance of mitochondrial complex I. These data support a biochemical phenotype associated with MRPL50 variants. We validated the association of MRPL50 with the clinical phenotype by knockdown/knockout of mRpL50 in Drosophila, which resulted abnormal ovarian development. In conclusion, we have shown that a MRPL50 missense variant destabilises the mitochondrial ribosome, leading to oxidative phosphorylation deficiency and syndromic POI, highlighting the importance of mitochondrial support in ovarian development and function.

Variants of uncertain significance (VUS) are commonly found following genomic sequencing, particularly in ethnically diverse populations that are underrepresented in large population databases. Functional characterization of VUS may assist in variant reclassification, however these studies are not readily available and often rely on research funding and good will. We present four individuals from three families at different stages of their diagnostic trajectory with recurrent acute liver failure (RALF) and biallelic NBAS variants, confirmed by either trio analysis or cDNA studies. Functional characterization was undertaken, measuring NBAS and p31 levels by Western blotting, demonstrating reduced NBAS levels in two of three families, and reduced p31 levels in all three families. These results provided functional characterization of the molecular impact of a missense VUS, allowing reclassification of the variant and molecular confirmation of NBAS-associated RALF. Importantly, p31 was decreased in all individuals, including an individual with two missense variants where NBAS protein levels were preserved. These results highlight the importance of access to timely functional studies after identification of putative variants, and the importance of considering a range of assays to validate variants whose pathogenicity is uncertain. We suggest that funding models for genomic sequencing should consider incorporating capabilities for adjunct RNA, protein, biochemical, and other specialized tests to increase the diagnostic yield which will lead to improved medical care, increased equity, and access to molecular diagnoses for all patients.