Diabetes mellitus exacerbates citrin deficiency via glucose toxicity

Deciphering the Mutational Background in Citrin Deficiency Through a Nationwide Study in Japan and Literature Review

Citrin deficiency (CD) is an autosomal recessive disorder caused by the absence or dysfunction of the mitochondrial transporter citrin, resulting from mutations in SLC25A13. The disease presents with age-dependent clinical manifestations: neonatal intrahepatic cholestasis caused by CD (NICCD), failure to thrive and dyslipidemia by CD (FTTDCD), and an adult-onset form (formerly called Type II citrullinemia, CTLN2, recently renamed to "adolescent and adult citrin deficiency," AACD). We performed this study to compile known genotypes found in CD patients and investigate their impact on the clinical course. Through a nationwide survey in Japan as well as a literature review, we collected information regarding 68 genetic variants of a total of 345 patients with CD (285 NICCD, 19 post-NICCD, and 41 AACD). In this cohort, the pathogenic variants, arising from nonsense, insertion/deletion, and splice site mutations, are expected to have severe functional or biogenesis defects. Of 82 alleles in patients with AACD, the two most common variants, c.852_855del and c.1177+1G>A, accounted for 25 alleles (30.5%) and 15 alleles (18.3%), respectively. The c.852_855del variant, even when present as part of compound heterozygosity, often presented with hyperammonemia (≥ 180 μmol/L), cognitive impairment, short stature (< -2SD), liver cirrhosis, and pancreatitis, with some patients requiring liver transplantation. In conclusion, certain SLC25A13 genotypes are particularly frequent, especially those that result in severely truncated citrin proteins with often a significant impact on the clinical outcome of the patient. The most prevalent variant is c.852_855del, which was found in 42% (128/304) of NICCD/post-NICCD cases and 49% (20/41) of AACD patients.

The therapeutic landscape of citrin deficiency

Citrin deficiency (CD) is a recessive, liver disease caused by sequence variants in the SLC25A13 gene encoding a mitochondrial aspartate-glutamate transporter. CD manifests as different age-dependent phenotypes and affects crucial hepatic metabolic pathways including malate-aspartate-shuttle, glycolysis, gluconeogenesis, de novo lipogenesis and the tricarboxylic acid and urea cycles. Although the exact pathophysiology of CD remains unclear, impaired use of glucose and fatty acids as energy sources due to NADH shuttle defects and PPARα downregulation, respectively, indicates evident energy deficit in CD hepatocytes. The present review summarizes current trends on available and potential treatments for CD. Baseline recommendation for CD patients is dietary management, often already present as a self-selected food preference, that includes protein and fat-rich food, and avoidance of excess carbohydrates. At present, liver transplantation remains the sole curative option for severe CD cases. Our extensive literature review indicated medium-chain triglycerides (MCT) as the most widely used CD treatment in all age groups. MCT can effectively improve symptoms across disease phenotypes by rapidly supplying energy to the liver, restoring redox balance and inducing lipogenesis. In contrast, sodium pyruvate restored glycolysis and displayed initial preclinical promise, with however limited efficacy in adult CD patients. Ursodeoxycholic acid, nitrogen scavengers and L-arginine treatments effectively address specific pathophysiological aspects such as cholestasis and hyperammonemia and are commonly administered in combination with other drugs. Finally, future possibilities including restoring redox balance, amino acid supplementation, enhancing bioenergetics, improving ureagenesis and mRNA/DNA-based gene therapy are also discussed.

Pathogenesis and Management of Citrin Deficiency

Citrin deficiency (CD) is a hereditary disorder caused by SLC25A13 mutations that manifests as neonatal intrahepatic cholestasis caused by CD (NICCD), failure to thrive and dyslipidemia caused by CD (FTTDCD), and adult-onset type 2 citrullinemia (CTLN2). Citrin, an aspartate-glutamate carrier primarily expressed in the liver, is a component of the malate-aspartate shuttle, which is essential for glycolysis. Citrin-deficient hepatocytes have primary defects in glycolysis and de novo lipogenesis and exhibit secondarily downregulated PPARα, leading to impaired β-oxidation. They are unable to utilize glucose and free fatty acids as energy sources, resulting in energy deficiencies. Medium-chain triglyceride (MCT) supplements are effective for treating CD by providing energy to hepatocytes, increasing lipogenesis, and activating the malate-citrate shuttle. However, patients with CD often exhibit growth impairment and irreversible brain and/or liver damage. To improve the quality of life and prevent irreversible damage, MCT supplementation with a diet containing minimal carbohydrates is recommended promptly after the diagnosis.

Adult-onset Type II Citrullinemia Developed under Dietary Restrictions during Imprisonment

A 29-year-old man presented with liver damage, and a liver biopsy was performed, but the cause was unclear. Thereafter, he was referred to our hospital. We found that he had been unable to consume carbohydrates in his diet and preferred fried chicken since childhood. In addition, he had shown disturbance of consciousness and abnormal behavior while he had been in prison, where dietary intake had been restricted. A plasma amino acid analysis revealed hypercitrullinemia. Therefore, we suspected adult-onset type II citrullinemia (CTLN2). Genetic testing showed pathologic variations in the SLC25A13 gene, which allowed us to make a definite diagnosis of CTLN2.

Metabolic basis and treatment of citrin deficiency

Citrin deficiency is a hereditary disorder caused by SLC25A13 mutations and manifests as neonatal intrahepatic cholestasis (NICCD), failure to thrive and dyslipidemia (FTTDCD), and adult-onset type II citrullinemia (CTLN2). Citrin is a component of the malate-aspartate nicotinamide adenine dinucleotide hydrogen (NADH) shuttle, an essential shuttle for hepatic glycolysis. Hepatic glycolysis and the coupled lipogenesis are impaired in citrin deficiency. Hepatic lipogenesis plays a significant role in fat supply during growth spurt periods: the fetal period, infancy, and puberty. Growth impairment in these periods is characteristic of citrin deficiency. Hepatocytes with citrin deficiency cannot use glucose and fatty acids as energy sources due to defects in the NADH shuttle and downregulation of peroxisome proliferator-activated receptor α (PPARα), respectively. An energy deficit in hepatocytes is considered a fundamental pathogenesis of citrin deficiency. Medium-chain triglyceride (MCT) supplementation with a lactose-restricted formula and MCT supplementation under a low-carbohydrate diet are recommended for NICCD and CTLN2, respectively. MCT supplementation therapy can provide energy to hepatocytes, promote lipogenesis, correct the cytosolic NAD⁺ /NADH ratio via the malate-citrate shuttle and improve ammonia detoxification, and it is a reasonable therapy for citrin deficiency. It is very important to administer MCT at a dose equivalent to the liver's energy requirements in divided doses with meals. MCT supplementation therapy is certainly promising for promoting growth spurts during infancy and adolescence and for preventing CTLN2 onset. Intravenous administration of solutions containing fructose is contraindicated, and persistent hyperglycemia should be avoided due to glucose intoxication for patients receiving hyperalimentation or with complicating diabetes.