Molecular testing in newborn screening: VUS burden among true positives and secondary reproductive limitations via expanded carrier screening panels.

PURPOSE: Expanded carrier screening (ECS) gene panels have several limitations, including variable content, current knowledge of disease-causing variants, and differing reporting policies. This study evaluated if the disease-associated variants identified in affected neonates who screened positive by California newborn screening (NBS) for an inherited metabolic disorder (IMD) by tandem mass spectrometry (MS/MS) would likely be reported by ECS gene panels. METHODS: Retrospective review of neonates referred by the California Department of Public Health for a positive NBS by multianalyte MS/MS from January 1, 2020 through June 30, 2021. RESULTS: One hundred thirty-six neonates screened positive for ≥1 NBS MS/MS indication. Nineteen neonates (14%) were ultimately diagnosed with an IMD, all of whom had abnormal biochemical testing. Eighteen of the 19 underwent molecular testing; 10 (56%) neonates had ≥1 variants of uncertain significance, 9 of whom were of non-White ancestry. ECS panels would have been negative for 56% (20/36) of parents with an affected neonate, 85% (17/20) of whom were of non-White ancestry. CONCLUSION: The number of variants of uncertain significance identified in this cohort highlights the need for more diversified variant databases. Due in part to the lack of diversity in currently sequenced populations, genomic sequencing cannot replace biochemical testing for the diagnosis of an IMD.

A homozygous Gly470Ala variant in PEX6 causes severe Zellweger spectrum disorder.

Zellweger spectrum disorder (ZSD) is a group of autosomal recessive disorders caused by biallelic pathogenic variants in any one of the 13 PEX genes essential for peroxisomal biogenesis. We report a cohort of nine infants who presented at birth with severe neonatal features suggestive of ZSD and found to be homozygous for a variant in PEX6 (NM_000287.4:c.1409G > C[p.Gly470Ala]). All were of Mixtec ancestry and identified by the California Newborn Screening (NBS) Program to have elevated C26:0-lysophosphatidylcholine but no reportable variants in ABCD1. The clinical and biochemical features of this cohort are described within. Gly470Ala may represent a founder variant in the Mixtec population of Central California. ZSD should be considered in patients who present at birth with severe hypotonia and enlarged fontanelles, especially in the setting of an abnormal NBS, Mixtec ancestry, or family history of infant death. There is a need to further characterize the natural history of ZSD, the Gly470Ala variant, and expand upon possible genotype-phenotype correlations.

Neonatal lupus is a novel cause of positive newborn screening for X-linked adrenoleukodystrophy.

We report three unrelated individuals, each exposed to maternal autoantibodies during gestation and found to have elevated very long-chain fatty acids (VLCFAs) in the newborn period after screening positive by California newborn screening (NBS) for X-linked adrenoleukodystrophy (ALD). Two probands presented with clinical and laboratory features of neonatal lupus erythematosus (NLE); the third had features suggestive of NLE and a known maternal history of Sjogren's syndrome and rheumatoid arthritis. In all three individuals, subsequent biochemical and molecular evaluation for primary and secondary peroxisomal disorders was nondiagnostic with normalization of VLCFAs by 15 months of age. These cases add to the expanding differential diagnosis to consider in newborns who screen positive for ALD via elevated C26:0-lysophosphatidylcholine. Though the pathophysiology of how transplacental maternal anti-Ro antibodies damage fetal tissue is not well-understood, we postulate that the VLCFA elevations reflect a systemic inflammatory response and secondary peroxisomal dysfunction that improves once maternal autoantibodies wane after birth. Additional evaluation of this phenomenon is warranted to better understand the intricate biochemical, clinical, and possible therapeutic overlap between autoimmunity, inflammation, peroxisomal dysfunction, and human disease.

MT-ATP6 mitochondrial disease identified by newborn screening reveals a distinct biochemical phenotype.

Although decreased citrulline is used as a newborn screening (NBS) marker to identify proximal urea cycle disorders (UCDs), it is also a feature of some mitochondrial diseases, including MT-ATP6 mitochondrial disease. Here we describe biochemical and clinical features of 11 children born to eight mothers from seven separate families who were identified with low citrulline by NBS (range 3-5 µM; screening cutoff >5) and ultimately diagnosed with MT-ATP6 mitochondrial disease. Follow-up testing revealed a pattern of hypocitrullinemia together with elevated propionyl-(C3) and 3-hydroxyisovaleryl-(C5-OH) acylcarnitines, and a homoplasmic pathogenic variant in MT-ATP6 in all cases. Single and multivariate analysis of NBS data from the 11 cases using Collaborative Laboratory Integrated Reports (CLIR; https://clir.mayo.edu) demonstrated citrulline <1st percentile, C3 > 50th percentile, and C5-OH >90th percentile when compared with reference data, as well as unequivocal separation from proximal UCD cases and false-positive low citrulline cases using dual scatter plots. Five of the eight mothers were symptomatic at the time of their child(ren)'s diagnosis, and all mothers and maternal grandmothers evaluated molecularly and biochemically had a homoplasmic pathogenic variant in MT-ATP6, low citrulline, elevated C3, and/or elevated C5-OH. All molecularly confirmed individuals (n = 17) with either no symptoms (n = 12), migraines (n = 1), or a neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP) phenotype (n = 3) were found to have an A or U mitochondrial haplogroup, while one child with infantile-lethal Leigh syndrome had a B haplogroup.

Creatine Transporter Deficiency Presenting as Failure to Thrive: A Case Report of a Novel SLC6A8 Variant Causing a Treatable but Likely Underdiagnosed Genetic Disorder.

Cerebral creatine deficiency syndromes (CCDS) are a rare group of inherited metabolic disorders (IMDs) that often present with nonspecific findings including global developmental delay (GDD), intellectual disability (ID), seizures, hypotonia, and behavioral differences. Creatine transporter (CRTR) deficiency is the most common CCDS, exhibiting X-linked inheritance and an estimated prevalence as high as 2.6% in individuals with neurodevelopmental disorders. Here, we present a 20-month-old boy with worsening failure to thrive (FTT) and GDD admitted for evaluation. He was found to have persistently low serum creatinine levels and a family history notable for a mother with learning disabilities and a maternal male cousin with GDD. Urine analyses revealed a marked elevation of creatine and elevated creatine:creatinine ratio suggestive of CRTR deficiency. Molecular genetic testing of SLC6A8 identified a maternally inherited hemizygous variant and brain magnetic resonance spectroscopy (MRS) showed diffusely diminished creatine peaks, further supporting the diagnosis of CRTR deficiency. The proband was started on creatine, arginine, and glycine supplementation and has demonstrated improved development. This case highlights that CRTR deficiency should be considered in all patients presenting with FTT and abnormal neurodevelopmental features, particularly if creatinine levels are low on serum chemistry studies. The nonspecific presentation of this condition in males and females likely has resulted in CRTR deficiency being underdiagnosed. There are existing therapies for individuals affected with CRTR deficiency and other CCDS, highlighting the importance of early diagnosis and intervention for affected individuals.

Optimization of the biochemical genetics laboratory rotation using a multidesign approach to curriculum.

PURPOSE: A biochemical genetics laboratory rotation is required for multiple genetics training programs. Traditionally, this rotation has been observational with experience being dependent upon cases released and availability of laboratory director(s), resulting in inconsistent learning opportunities. This curriculum was created to standardize the learning experience. METHODS: The revised rotation provides multiple teaching modalities including small group didactic sessions (flipped classroom model), case-based sessions, and hands-on laboratory experience. Trainees prepare a presentation (learning by teaching) and discuss the differential diagnosis, metabolic pathway, newborn screening, treatment, and molecular characteristics of the gene(s) implicated. Learner assessment is performed using pre- and post-tests, learner evaluations, and instructor feedback. RESULTS: Pre- and post-test scores were significantly different (P < .001) for learners from all programs. Participants found the course to be effective, increased their learning, and allowed them to interact with metabolic testing results in helpful ways. Faculty appreciated the use of prerecorded lectures and additional time for in-depth teaching on interesting cases. CONCLUSION: The revised rotation has been well received by trainees and faculty. Interaction of learners with the laboratory staff was optimized by ensuring all parties were prepared to teach and learn. Future directions include expanding the program to include remote learners from other centers.

Recent tPA administration can cause pseudo-hyperargininemia and may mimic arginase deficiency or arginine supplementation.

Individuals suspected of or diagnosed with a rare disorder, including inherited metabolic disorders (IMD), often need frequent and/or urgent vascular access for blood draws and treatment, making central indwelling catheters commonly used devices in this patient population. These indwelling catheters are prone to thrombosis, limiting vascular access. This complication is frequently resolved with the use of altepase, a recombinant tissue plasminogen activator (tPA). This report describes two individuals, one with a known IMD and one undergoing evaluation for an IMD, who were found to have hyperargininemia (>500 µM; reference 10-140 µM) by plasma amino acid (PAA) analysis of a specimen collected ~1.5-3 h after clearance of an indwelling catheter with tPA. In both cases, hyperargininemia resolved with repeat testing, suggesting pseudo-hyperargininemia secondary to tPA administration. Quantitative amino acid analysis of the administered tPA demonstrated an arginine level of ~200 mM, supporting tPA as the cause of pseudo-hyperargininemia. Certain formulations of tPA contain high concentrations of arginine, which if not cleared properly can result in marked elevations of arginine, mimicking arginase deficiency or suggesting arginine supplementation. Thus, the possibility of pseudohyperargininemia due to tPA administration should be considered when obtaining PAAs from an indwelling catheter in any individual being evaluated or managed for an IMD.

Genomic and biochemical analysis of repeatedly observed variants in DBT in individuals with maple syrup urine disease of Central American ancestry.

Maple syrup urine disease (MSUD) is an intoxication-type inherited metabolic disorder in which hyperleucinemia leads to brain swelling and death without treatment. MSUD is caused by branched-chain alpha-ketoacid dehydrogenase deficiency due to biallelic loss of the protein products from the genes BCKDHA, BCKDHB, or DBT, while a distinct but related condition is caused by loss of DLD. In this case series, eleven individuals with MSUD caused by two pathogenic variants in DBT are presented. All eleven individuals have a deletion of exon 2 (delEx2, NM_001918.3:c.48_171del); six individuals are homozygous and five individuals are compound heterozygous with a novel missense variant (NM_001918.5:c.916 T > C [p.Ser306Pro]) confirmed to be in trans. Western Blot indicates decreased amount of protein product in delEx2;c.916 T > C liver cells and absence of protein product in delEx2 homozygous hepatocytes. Ultrahigh performance liquid chromatography-tandem mass spectrometry demonstrates an accumulation of branched-chain amino acids and alpha-ketoacids in explanted hepatocytes. Individuals with these variants have a neonatal-onset, non-thiamine-responsive, classical form of MSUD. Strikingly, the entire cohort is derived from families who immigrated to the Washington, DC, metro area from Honduras or El Salvador suggesting the possibility of a founder effect.

Measurement of lysosomal enzyme activities: A technical standard of the American College of Medical Genetics and Genomics (ACMG).

Assays that measure lysosomal enzyme activity are important tools for the screening and diagnosis of lysosomal storage disorders (LSDs). They are often ordered in combination with urine oligosaccharide and glycosaminoglycan analysis, additional biomarker assays, and/or DNA sequencing when an LSD is suspected. Enzyme testing in whole blood/leukocytes, serum/plasma, cultured fibroblasts, or dried blood spots demonstrating deficient enzyme activity remains a key component of LSD diagnosis and is often prompted by characteristic clinical findings, abnormal newborn screening, abnormal biochemical findings (eg, elevated glycosaminoglycans), or molecular results indicating pathogenic variants or variants of uncertain significance in a gene associated with an LSD. This document, which focuses on clinical enzyme testing for LSDs, provides a resource for laboratories to develop and implement clinical testing, to describe variables that can influence test performance and interpretation of results, and to delineate situations for which follow-up molecular testing is warranted.

Aicardi-Goutières syndrome may present with positive newborn screen for X-linked adrenoleukodystrophy.

We report three unrelated probands, two male and one female, diagnosed with Aicardi-Goutières syndrome (AGS) after screening positive on California newborn screening (CA NBS) for X-linked adrenoleukodystrophy (X-ALD) due to elevated C26:0 lysophosphatidylcholine (C26:0-LPC). Follow-up evaluation was notable for elevated C26:0, C26:1, and C26:0/C22:0 ratio, and normal red blood cell plasmalogens levels in all three probands. Diagnoses were confirmed by molecular sequencing prior to 12 months of age after clinical evaluation was inconsistent with X-ALD or suggestive of AGS. For at least one proband, the early diagnosis of AGS enabled candidacy for enrollment into a therapeutic clinical trial. This report demonstrates the importance of including AGS on the differential diagnosis for individuals who screen positive for X-ALD, particularly infants with abnormal neurological features, as this age of onset would be highly unusual for X-ALD. While AGS is not included on the Recommended Universal Screening Panel, affected individuals can be identified early through state NBS programs so long as providers are aware of a broader differential that includes AGS. This report is timely, as state NBS algorithms for X-ALD are actively being established, implemented, and refined.

Laboratory analysis of acylcarnitines, 2020 update: a technical standard of the American College of Medical Genetics and Genomics (ACMG).

Acylcarnitine analysis is a useful test for identifying patients with inborn errors of mitochondrial fatty acid ß-oxidation and certain organic acidemias. Plasma is routinely used in the diagnostic workup of symptomatic patients. Urine analysis of targeted acylcarnitine species may be helpful in the diagnosis of glutaric acidemia type I and other disorders in which polar acylcarnitine species accumulate. For newborn screening applications, dried blood spot acylcarnitine analysis can be performed as a multiplex assay with other analytes, including amino acids, succinylacetone, guanidinoacetate, creatine, and lysophosphatidylcholines. Tandem mass spectrometric methodology, established more than 30 years ago, remains a valid approach for acylcarnitine analysis. The method involves flow-injection analysis of esterified or underivatized acylcarnitines species and detection using a precursor-ion scan. Alternative methods utilize liquid chromatographic separation of isomeric and isobaric species and/or detection by selected reaction monitoring. These technical standards were developed as a resource for diagnostic laboratory practices in acylcarnitine analysis, interpretation, and reporting.

Growth, development, and phenotypic spectrum of individuals with deletions of 2q33.1 involving SATB2.

SATB2-Associated syndrome (SAS) is an autosomal dominant, multisystemic, neurodevelopmental disorder due to alterations in SATB2 at 2q33.1. A limited number of individuals with 2q33.1 contiguous deletions encompassing SATB2 (ΔSAS) have been described in the literature. We describe 17 additional individuals with ΔSAS, review the phenotype of 33 previously published individuals with 2q33.1 deletions (n = 50, mean age = 8.5 ± 7.8 years), and provide a comprehensive comparison to individuals with other molecular mechanisms that result in SAS (non-ΔSAS). Individuals in the ΔSAS group were often underweight for age (20/41 = 49%) with a progressive decline in weight (95% CI = -2.3 to -1.1, p < 0.0001) and height (95% CI = -2.3 to -1.0, p < 0.0001) Z-score means from birth to last available measurement. ΔSAS individuals were often noted to have a broad spectrum of facial dysmorphism. A composite image of ΔSAS individuals generated by automated image analysis was distinct as compared to matched controls and non-ΔSAS individuals. We also present additional genotype-phenotype correlations for individuals in the ΔSAS group such as an increased risk for aortic root/ascending aorta dilation and primary pulmonary hypertension for those individuals with contiguous gene deletions that include COL3A1/COL5A2 and BMPR2, respectively. Based on these findings, we provide additional care recommendations for individuals with ΔSAS variants.

Biallelic variants in KYNU cause a multisystemic syndrome with hand hyperphalangism.

Catel-Manzke syndrome is characterized by the combination of Pierre Robin sequence and radial deviation, shortening as well as clinodactyly of the index fingers, due to an accessory ossification center. Mutations in TGDS have been identified as one cause of Catel-Manzke syndrome, but cannot be found as causative in every patient with the clinical diagnosis. We performed a chromosome microarray and/or exome sequencing in three patients with hand hyperphalangism, heart defect, short stature, and mild to severe developmental delay, all of whom were initially given a clinical diagnosis of Catel-Manzke syndrome. In one patient, we detected a large deletion of exons 1-8 and the missense variant c.1282C > T (p.Arg428Trp) in KYNU (NM_003937.2), whereas homozygous missense variants in KYNU were found in the other two patients (c.989G > A (p.Arg330Gln) and c.326G > C (p.Trp109Ser)). Plasma and urine metabolomic analysis of two patients indicated a block along the tryptophan catabolic pathway and urine organic acid analysis showed excretion of xanthurenic acid. Biallelic loss-of-function mutations in KYNU were recently described as a cause of NAD deficiency with vertebral, cardiac, renal and limb defects; however, no hand hyperphalangism was described in those patients, and Catel-Manzke syndrome was not discussed as a differential diagnosis. In conclusion, we present unrelated patients identified with biallelic variants in KYNU leading to kynureninase deficiency and xanthurenic aciduria as a very likely cause of their hyperphalangism, heart defect, short stature, and developmental delay. We suggest performance of urine organic acid analysis in patients with suspected Catel-Manzke syndrome, particularly in those with cardiac or vertebral defects or without mutations in TGDS.

Laboratory diagnosis of disorders of peroxisomal biogenesis and function: a technical standard of the American College of Medical Genetics and Genomics (ACMG).

Peroxisomal disorders are a clinically and genetically heterogeneous group of diseases caused by defects in peroxisomal biogenesis or function, usually impairing several metabolic pathways. Peroxisomal disorders are rare; however, the incidence may be underestimated due to the broad spectrum of clinical presentations. The inclusion of X-linked adrenoleukodystrophy to the Recommended Uniform Screening Panel for newborn screening programs in the United States may increase detection of this and other peroxisomal disorders. The current diagnostic approach relies heavily on biochemical genetic tests measuring peroxisomal metabolites, including very long-chain and branched-chain fatty acids in plasma and plasmalogens in red blood cells. Molecular testing can confirm biochemical findings and identify the specific genetic defect, usually utilizing a multiple-gene panel or exome/genome approach. When next-generation sequencing is used as a first-tier test, evaluation of peroxisome metabolism is often necessary to assess the significance of unknown variants and establish the extent of peroxisome dysfunction. This document provides a resource for laboratories developing and implementing clinical biochemical genetic testing for peroxisomal disorders, emphasizing technical considerations for sample collection, test performance, and result interpretation. Additionally, considerations on confirmatory molecular testing are discussed.

Clinical and molecular spectrum of CHOPS syndrome.

CHOPS syndrome is a multisystem disorder caused by missense mutations in AFF4. Previously, we reported three individuals whose primary phenotype included cognitive impairment and coarse facies, heart defects, obesity, pulmonary involvement, and short stature. This syndrome overlaps phenotypically with Cornelia de Lange syndrome, but presents distinct differences including facial features, pulmonary involvement, and obesity. Here, we provide clinical descriptions of an additional eight individuals with CHOPS syndrome, as well as neurocognitive analysis of three individuals. All 11 individuals presented with features reminiscent of Cornelia de Lange syndrome such as synophrys, upturned nasal tip, arched eyebrows, and long eyelashes. All 11 individuals had short stature and obesity. Congenital heart disease and pulmonary involvement were common, and those were seen in about 70% of individuals with CHOPS syndrome. Skeletal abnormalities are also common, and those include abnormal shape of vertebral bodies, hypoplastic long bones, and low bone mineral density. Our observation indicates that obesity, pulmonary involvement, skeletal findings are the most notable features distinguishing CHOPS syndrome from Cornelia de Lange syndrome. In fact, two out of eight of our newly identified patients were found to have AFF4 mutations by targeted AFF4 mutational analysis rather than exome sequencing. These phenotypic findings establish CHOPS syndrome as a distinct, clinically recognizable disorder. Additionally, we report three novel missense mutations causative for CHOPS syndrome that lie within the highly conserved, 14 amino acid sequence of the ALF homology domain of the AFF4 gene, emphasizing the critical functional role of this region in human development.

FGF21 underlies a hormetic response to metabolic stress in methylmalonic acidemia.

Methylmalonic acidemia (MMA), an organic acidemia characterized by metabolic instability and multiorgan complications, is most frequently caused by mutations in methylmalonyl-CoA mutase (MUT). To define the metabolic adaptations in MMA in acute and chronic settings, we studied a mouse model generated by transgenic expression of Mut in the muscle. Mut-/-;TgINS-MCK-Mut mice accurately replicate the hepatorenal mitochondriopathy and growth failure seen in severely affected patients and were used to characterize the response to fasting. The hepatic transcriptome in MMA mice was characterized by the chronic activation of stress-related pathways and an aberrant fasting response when compared with controls. A key metabolic regulator, Fgf21, emerged as a significantly dysregulated transcript in mice and was subsequently studied in a large patient cohort. The concentration of plasma FGF21 in MMA patients correlated with disease subtype, growth indices, and markers of mitochondrial dysfunction but was not affected by renal disease. Restoration of liver Mut activity, by transgenesis and liver-directed gene therapy in mice or liver transplantation in patients, drastically reduced plasma FGF21 and was associated with improved outcomes. Our studies identify mitocellular hormesis as a hepatic adaptation to metabolic stress in MMA and define FGF21 as a highly predictive disease biomarker.

Tissue acylcarnitine status in a mouse model of mitochondrial ß-oxidation deficiency during metabolic decompensation due to influenza virus infection.

Despite judicious monitoring and care, patients with fatty acid oxidation disorders may experience metabolic decompensation due to infection which may result in rhabdomyolysis, cardiomyopathy, hypoglycemia and liver dysfunction and failure. Since clinical studies on metabolic decompensation are dangerous, we employed a preclinical model of metabolic decompensation due to infection. By infecting mice with mouse adapted influenza and using a pair-feeding strategy in a mouse model of long-chain fatty acid oxidation (Acadvl-/-), our goals were to isolate the effects of infection on tissue acylcarnitines and determine how they relate to their plasma counterparts. Applying statistical data reduction techniques (Partial Least Squares-Discriminant Analysis), we were able to identify critical acylcarnitines that were driving differentiation of our experimental groups for all the tissues studied. While plasma displayed increases in metabolites directly related to mouse VLCAD deficiency (e.g. C16 and C18), organs like the heart, muscle and liver also showed involvement of alternative pathways (e.g. medium-chain FAO and ketogenesis), suggesting adaptive measures. Matched correlation analyses showed strong correlations (r > 0.7) between plasma and tissue levels for a small number of metabolites. Overall, our results demonstrate that infection as a stress produces perturbations in metabolism in Acadvl-/- that differ greatly from WT infected and Acadvl-/- pair-fed controls. This model system will be useful for studying the effects of infection on tissue metabolism as well as evaluating interventions aimed at modulating the effects of metabolic decompensation.

Extending the phenotypic spectrum of Sengers syndrome: Congenital lactic acidosis with synthetic liver dysfunction.

Sengers syndrome is a rare autosomal recessive mitochondrial disease characterized by lactic acidosis, hypertrophic cardiomyopathy and bilateral cataracts. We present here a case of neonatal demise, within the first day of life, who initially presented with severe lactic acidosis, with evidence of both chorioamnionitis and cardiogenic shock. Initial metabolic labs demonstrated a severe lactic acidosis prompting genetic testing which revealed a homozygous pathogenic variant for Sengers syndrome in AGK, c.979A >  T; p.K327*. In addition to the canonical features of Sengers syndrome, our patient is the first reported case with liver dysfunction extending the phenotypic spectrum both in terms of severity and complications. This case also highlights the importance of maintaining a broad differential for congenital lactic acidosis.

Correction of hyperleucinemia in MSUD patients on leucine-free dietary therapy.

PURPOSE: Maple Syrup Urine Disease (MSUD) is a rare disorder of branched-chain amino acid catabolism associated with encephalopathy from accumulation of leucine. Leucine is closely monitored during normal growth and particularly during acute illness. As most hospitals do not have access to rapid plasma amino acid quantification, the initial management is often empirical. A model describing the reduction of plasma leucine in hyperleucinemic patients on leucine-free formula would help to guide management and optimize testing frequency. METHODS: We retrospectively reviewed charts from 15 MSUD patients comprising 29 episodes of hyperleucinemia that were managed with leucine-free formula. Episodes were categorized by clinical presentation. RESULTS: Upon leucine restriction, plasma leucine concentrations fell exponentially at a rate proportional to approximately 50% of the starting value over each 24-hour period. Recovery appears to be sensitive to clinical status and triggering event of the hyperleucinemic episode. Patients with upper respiratory infections generally recovered slowly, while cases of dietary non-adherence resolved more quickly. CONCLUSION: This general model may help anticipate leucine levels during clinical management of MSUD patients when using nutritional support and leucine-free formula. The response of individual patients may vary depending on clinical status and triggering factors.

Confirmation that MAT1A p.Ala259Val mutation causes autosomal dominant hypermethioninemia.

Methionine adenosyltransferase (MAT) I/III deficiency is an inborn error of metabolism caused by mutations in MAT1A, encoding the catalytic subunit of MAT responsible for the synthesis of S-adenosylmethionine, and is characterized by persistent hypermethioninemia. While historically considered a recessive disorder, a milder autosomal dominant form of MAT I/III deficiency occurs, though only the most common mutation p.Arg264His has ample evidence to prove dominant inheritance. We report a case of hypermethioninemia caused by the p.Ala259Val substitution and provide evidence of autosomal dominant inheritance by showing both maternal inheritance of the mutation and concomitant hypermethioninemia. The p.Ala259Val mutation falls in the dimer interface, and thus likely leads to dominant inheritance by a similar mechanism to that described in the previously reported dominant negative mutation, that is, by means of interference with subunits encoded by the wild-type allele.