Adult-onset citrullinemia type II (CTLN2) is a rare autosomal recessive urea cycle disorder caused by mutations in the solute carrier family 25 member 13 (SLC25A13) gene, which encodes citrin-a mitochondrial transporter involved in the malate-aspartate shuttle. In adults, CTLN2 may present atypically as isolated hypertriglyceridemia, often misattributed to secondary dyslipidaemia. A 30-year-old Vietnamese male with longstanding severe hypertriglyceridemia, first identified at age 13, was referred after an episode of acute pancreatitis at age 28. At presentation, plasma triglyceride levels reached 34.61 mmol/l. Secondary causes were excluded. Genetic testing via next-generation sequencing revealed compound heterozygous SLC25A13 mutations: c.2T>C and c.1638_1660dup, resulting in p.Met1Thr and p.Ala554GlyfsTer17, confirming the diagnosis of CTLN2. Initiation of a low-carbohydrate, high-protein, low-fat diet combined with fenofibrate (145 mg/day) led to a rapid reduction in triglyceride levels, normalizing within two weeks and remaining stable over three months. The treatment was well tolerated, with no reported adverse effects. CTLN2 should be considered in young adults with persistent, unexplained, and severe hypertriglyceridemia. Dietary modification constitutes the cornerstone of management, with fibrates playing a supportive role. Persistent, unexplained, and severe hypertriglyceridemia in young adults should prompt consideration of inherited metabolic disorders, including adult-onset citrullinemia type II (CTLN2).Genetic testing to detect compound heterozygous or homozygous SLC25A13 variants is essential for definitive diagnosis of CTLN2.In CTLN2, dietary management with a low-carbohydrate, high-protein, low-fat represents the cornerstone, with fibrates serving as adjunctive lipid-lowering therapy.

Citrin deficiency is a rare metabolic disorder prevalent in East and Southeast Asia that affects liver or neurological function throughout various life stages. While early diagnosis and dietary management can improve prognosis for infant onset disease, data on long-term neurocognitive outcomes is scarce. This study aimed to clarify whether transient metabolic disturbances during early childhood have a lasting effect on the neurocognitive function of individuals with citrin deficiency. Thirty patients diagnosed with citrin deficiency prior to 1 year of age underwent neuropsychological assessments including attention deficit/hyperactivity disorders (ADHD) and intelligence quotient (IQ). We compared the peak laboratory values during infancy between children who were versus were not later diagnosed with ADHD. Neurocognitive assessments of 30 individuals with citrin deficiency aged 3-25 years revealed that full-scale IQ scores were normally distributed. Of this cohort, 47% (14 of 30) were diagnosed with ADHD: 6, 6, and 2 with the combined, inattentive, and hyperactive-impulsive types, respectively. This prevalence was higher than that in the general population (1.7%-16%). Moreover, a one-unit increase in ammonia levels before 1 year of age was associated with a 1.023-fold increase in the likelihood of future hyperactivity-impulsivity symptoms (P=0.038; 95%confidence interval, 1.001-1.046). Despite these findings, this long-term follow-up of individuals with citrin deficiency indicated that it had minimal impact on neurocognitive function, allowing for a generally normal life. Patients with a history of cholestasis caused by citrin deficiency during infancy have a greater incidence of ADHD than the general population, suggesting that metabolic disturbances during early childhood in individuals with citrin deficiency may have a long-term negative impact on their neurocognitive function.

Multiple acyl-CoA dehydrogenase deficiency (MADD) is a rare autosomal recessive disorder. Riboflavin-responsive MADD (RR-MADD) represents a treatable subtype, though its molecular mechanisms are incompletely characterized. Two patients presented to department of Rheumatology, the 3rd Affiliated Hospital of Sun Yet-sen University with progressive proximal muscle weakness. A 2-fluorine-18-fluoro-2-deoxy-D-glucose positron emission tomography/computed tomography (18F-FDG PET/CT) scan revealed marked metabolic hyperactivity in cervical and paraspinal muscles. Histopathology confirmed lipid storage myopathy, and exome sequencing (ES) identified electron transfer flavoprotein dehydrogenase gene (ETFDH) mutations. Both patients harbored the common variant NM_004453.4:c.250G > A (p.Ala84Thr). Case 1 exhibited homozygosity for this variant and potential XYY syndrome, while Case 2 carried a compound heterozygous mutation including a novel frameshift variant, NM_004453.4: c.265_266del (p. Gln89Valfs*6), and concurrent SLC25A13 mutation linked to adult-onset citrullinemia type II. Riboflavin supplementation achieved complete biochemical remission in both cases. Retrospective analysis for previous reports of homozygous single-locus missense ETFDH gene mutations revealed that patients with mutations in the flavin adenine dinucleotide binding domain (FAD) or the iron-sulfur cluster domain (Fe-S) exhibited a better response to riboflavin and better prognosis. 18F-FDG PET/CT is a promising tool for evaluating myopathic involvement in MADD. ES enables rapid and comprehensive diagnosis of MADD and detection of coexisting genetic disorders. The prognosis is related to mutation forms, mutation pathogenicity, and the located structural domain. The online version contains supplementary material available at 10.1186/s12920-025-02210-8. Citrin deficiency can manifest in newborns or infants as neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD), in older children as failure to thrive and dyslipidemia caused by citrin deficiency (FTTDCD), and in adults as recurrent hyperammonemia with neuropsychiatric symptoms in citrullinemia type II (CTLN2). Often citrin deficiency is characterized by strong preference for protein-rich and/or lipid-rich foods and aversion to carbohydrate-rich foods. NICCD: Children younger than age one year have a history of low birth weight with growth restriction and transient intrahepatic cholestasis, hepatomegaly, diffuse fatty liver, and parenchymal cellular infiltration associated with hepatic fibrosis, variable liver dysfunction, hypoproteinemia, decreased coagulation factors, anemia, and/or hypoglycemia. NICCD is generally not severe, and clinical manifestations are often resolved by age one year with appropriate treatment, although liver failure may still occur; liver transplantation has been required in rare instances. FTTDCD: Beyond age one year, many children with citrin deficiency develop a protein-rich and/or lipid-rich food preference and aversion to carbohydrate-rich foods. Clinical abnormalities may include poor weight gain, growth deficiency, severe fatigue, anorexia, and impaired quality of life. Laboratory changes are dyslipidemia, recurrent hypoglycemia, increased lactate-to-pyruvate ratio, higher levels of urinary oxidative stress markers, and considerable deviation in tricarboxylic acid cycle metabolites. One or more decades later, some adults with NICCD or FTTDCD may progress and develop features of CTLN2. CTLN2: Presentation is sudden and usually between ages 20 and 50 years. Clinical manifestations include recurrent hyperammonemia with neuropsychiatric (aggression, irritability, restlessness, hyperactivity, delusions, nocturnal delirium) and neurologic manifestations (flapping tremors, memory loss, disorientation, drowsiness, convulsive seizures, coma). Clinical manifestations are often caused by alcohol and sugar intake, medication, and/or surgery. Complications include severe liver steatosis and pancreatitis. Affected individuals may or may not have a prior history of NICCD or FTTDCD. The diagnosis of citrin deficiency is established in an individual with characteristic biochemical analytes (increased blood or plasma concentration of ammonia, plasma or serum concentration of citrulline and arginine, plasma or serum threonine-to-serine ratio) and biallelic pathogenic variants in SLC25A13 identified by molecular genetic testing. Treatment of manifestations: NICCD: lactose-free and medium-chain triglyceride (MCT)-enriched formula supplemented with fat-soluble vitamins. FTTDCD: protein- and lipid-rich, low-carbohydrate diet; in addition to dietary treatment, administration of sodium pyruvate and MCT oil may improve growth. CTLN2: reduced calorie/carbohydrate intake and increased protein intake lessens hypertriglyceridemia. Sodium pyruvate can increase weight and decrease frequency of hyperammonemia; arginine administration decreases blood ammonia concentration; MCT oil can decrease frequency of hyperammonemia; use of arginine, sodium pyruvate, and MCT oil may delay the need for liver transplantation; liver transplantation prevents hyperammonemic crises, corrects metabolic disturbances, and eliminates preferences for protein-rich foods. Surveillance: Periodic measurement of plasma concentration of ammonia and citrulline for all phenotypes associated with citrin deficiency. Assess growth and development throughout childhood; assessment of liver and pancreatic function as clinically indicated; neuropsychologic testing and quality of life assessment as clinically indicated; complete blood count and ferritin as clinically indicated. Agents/circumstances to avoid: Low-protein and high-carbohydrate diets; glycerol, fructose, and glucose infusions due to risk of brain edema; alcohol. Evaluation of relatives at risk: It is appropriate to identify affected sibs of a proband so that appropriate dietary management can be instituted before symptoms occur. Citrin deficiency is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for an SLC25A13 pathogenic variant, each sib of an affected individual has at conception a 25% chance of inheriting biallelic pathogenic variants, a 50% chance of inheriting one pathogenic variant and being a carrier, and a 25% chance of inheriting neither of the familial SLC25A13 pathogenic variants. If one parent is known to be heterozygous for an SLC25A13 pathogenic variant and the other parent is known to have biallelic SLC25A13 pathogenic variants, each sib of an affected individual has at conception a 50% chance of inheriting biallelic SLC25A13 pathogenic variants and a 50% chance of inheriting one SLC25A13 pathogenic variant. In general, sibs who inherit biallelic SLC25A13 pathogenic variants will be affected and have clinical manifestations of citrin deficiency similar to those of the proband in the family. Once the SLC25A13 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible.

Newborn screening (NBS) is an important public health program that aims to identify pre-symptomatic healthy babies that will develop significant disease if left undiagnosed and untreated. The number of conditions being screened globally is expanding rapidly in parallel with advances in technology, diagnosis, and treatment availability for these conditions. In Hong Kong, NBS for inborn errors of metabolism (NBSIEM) began as a pilot program in October 2015 and was implemented to all birthing hospitals within the public healthcare system in phases, with completion in October 2020. The number of conditions screened for increased from 21 to 24 in April 2016 and then to 26 in October 2019. The overall recruitment rate of the NBS program was 99.5%. In the period between October 2015 and December 2022, 125,688 newborns were screened and 295 were referred back for abnormal results. The recall rate was reduced from 0.26% to 0.12% after the implementation of second-tier testing. An inherited metabolic disorder (IMD) was eventually confirmed in 47 infants, making the prevalence of IMD in Hong Kong 1 in 2674. At the time of the NBS result, 78.7% of the newborns with IMD were asymptomatic. There were two deaths reported: one newborn with methylmalonic acidemia cobalamin B type (MMACblB) died after the initial crisis and another case of carnitine palmitoyltransferase II deficiency (CPTII) died at 18 months of age after metabolic decompensation. The most common IMD noted were disorders of fatty acid oxidation metabolism (40%, 19 cases), closely followed by disorders of amino acid metabolism (38%, 18 cases), with carnitine uptake defect (19.1%, 9 cases) and citrullinemia type II (17%, 8 cases) being the two most common IMD picked up by the NBSIEM in Hong Kong. Out of the all the IMDs identified, 19.1% belonged to diverse ethnic groups. False negative cases were reported for citrullinemia type II and congenital adrenal hyperplasia during this period.

In this study, we evaluated the implementation of a second-tier genetic screening test using an amplicon-based next-generation sequencing (NGS) panel in our laboratory during the period of 1 September 2021 to 31 August 2022 for the newborn screening (NBS) of six conditions for inborn errors of metabolism: citrullinemia type II (MIM #605814), systemic primary carnitine deficiency (MIM #212140), glutaric acidemia type I (MIM #231670), beta-ketothiolase deficiency (#203750), holocarboxylase synthetase deficiency (MIM #253270) and 3-hydroxy-3-methylglutaryl-CoA lyase deficiency (MIM # 246450). The custom-designed NGS panel can detect sequence variants in the relevant genes and also specifically screen for the presence of the hotspot variant IVS16ins3kb of SLC25A13 by the copy number variant calling algorithm. Genetic second-tier tests were performed for 1.8% of a total of 22,883 NBS samples. The false positive rate for these six conditions after the NGS second-tier test was only 0.017%, and two cases of citrullinemia type II would have been missed as false negatives if only biochemical first-tier testing was performed. The confirmed true positive cases were citrullinemia type II (n = 2) and systemic primary carnitine deficiency (n = 1). The false positives were later confirmed to be carrier of citrullinemia type II (n = 2), carrier of glutaric acidemia type I (n = 1) and carrier of systemic primary carnitine deficiency (n = 1). There were no false negatives reported. The incorporation of a second-tier genetic screening test by NGS greatly enhanced our program's performance with 5-working days turn-around time maintained as before. In addition, early genetic information is available at the time of recall to facilitate better clinical management and genetic counseling.