Citrin deficiency (CD) is an inherited autosomal recessive metabolic condition and a urea cycle disorder. Citrin deficiency is a complex disorder with several age-dependent phenotypes. A hallmark symptom of citrin deficiency is a strong dietary preference for foods rich in protein and fat, while being low in carbohydrates. Infants affected by citrin deficiency often present with prolonged jaundice and cholestasis. After the first year of life, patients may develop symptoms such as hypoglycemia, failure to thrive (growth impediments), fatigue, dyslipidemia, gastrointestinal discomfort, and fatty liver. If the condition is not well managed, patients may develop more serious complications such as hyperammonemia leading to hepatic encephalopathy that may be fatal. First line treatment is dietary management with a high protein, high fat, and low carbohydrate diet. Supplementing the diet with medium-chain triglyceride (MCT) may also be beneficial for patients. There is currently no cure for citrin deficiency other than liver transplantation if patients do not respond well to treatment.
Causes Citrin deficiency is caused by genetic mutations in the SLC25A13 gene encoding citrin. Citrin is an aspartate-glutamate carrier protein localized at the inner mitochondrial membrane and is mainly expressed in the liver. It is an important component of the malate-aspartate shuttle that facilitates the transfer of NADH from the cytosol to the mitochondria to maintain redox balance and adenosine triphosphate (ATP) production. Citrin also supplies aspartate to the cytosol (in exchange for glutamate) used in the urea cycle and de novo nucleotide synthesis.
In citrin deficiency, the malate-aspartate shuttle is impaired and leads to an excessive buildup of cytosolic NADH that disrupts multiple metabolic processes such as glycolysis, gluconeogenesis, de novo lipogenesis, beta-oxidation, the tricarboxylic acid (TCA) cycle and the urea cycle. This cumulatively results in a chronic energy deficit in the liver due to affected hepatocytes being unable to efficiently utilize energy sources such as carbohydrates and fatty acids to produce ATP.
One of the hallmarks of citrin deficiency is sweet aversion. It has been proposed that sweet aversion might be due to elevated hepatic expression of FGF21, and in a mouse model of citrin deficiency FGF21 was reported to be over-expressed because of the activation of the ChREBP transcription factor by glycerol-3-phosphate, a newly identified ChREBP-activating ligand. As ChREBP is also a transcriptional activator of de novo lipogenesis, this mechanism could provide an explanation for MASLD, which is very common in citrin deficient patients despite their leanness.
Aralar is an isoform of citrin and possesses similar transport functions. It is encoded by the SLC25A12 gene. Although functionally similar, aralar and citrin have different tissue distributions in humans, with aralar being found primarily in brain, skeletal muscle, kidney, and heart whereas citrin is expressed predominantly in the liver, kidney, and small intestine.
Clinical phenotypes and associated symptoms Citrin deficiency has three primary phenotypes that are age dependent. They are neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD) that affects infants, followed by the silent/adaptation period or failure to thrive and dyslipidemia caused by citrin deficiency (FTTDCD) during childhood and then, in a small percentage of patients, adolescent and adult citrin deficiency (AACD) (formerly termed CTLN2). AACD represents the most severe form of the condition and is characterized by acute episodes of hyperammonemia, citrullinemia, and psychiatric symptoms. The clinical presentations of citrin deficiency may be heterogenous, with patients exhibiting varying symptoms and severities despite similar demographics and SLC25A13 genetic mutations. Details of the signs and symptoms for each clinical phenotype are shown in Table 1.
In NICCD, frequently observed symptoms are cholestasis, prolonged jaundice, and fatty liver. With the appropriate treatment and management, most symptoms associated with NICCD tend to resolve after 1 year of age.
Post-NICCD, patients enter the silent (or adaptation) phase. While patients appear to be mostly healthy, some patients may have one or more of the following as clinical symptoms: fatty liver, hypoglycemia, fatigue, hyperlipidemia and occasional abdominal pain. A small percentage of patients may develop FTTDCD, with a tendency to show more pronounced symptoms, such as poor growth, hypoglycemia, fatigue, hyperlipidemia, fatty liver, and abdominal pain.
AACD patients typically present with low BMI, citrullinemia, hyperlipidemia, fatigue, fatty liver, and hyperammonemia that may result in neurological symptoms (e.g., nocturnal delirium, disorientation, coma). The onset of AACD can be gradual or sudden and is caused by triggers such as alcohol consumption, long-term overconsumption or binging on foods high in sugar/carbohydrate content, use of contraindicated medication, major surgery, or serious infections. Despite the severity of AACD, not all patients with citrin deficiency will develop AACD if their condition is managed properly.
A hallmark symptom that most citrin deficiency patients develop post-infancy is a strong dietary preference for foods rich in protein and fats and low in carbohydrates. Patients dislike carbohydrate-rich or sweet-tasting foods and will often avoid consuming them. Over-consumption of high carbohydrate and/or sugary foods may cause patients to feel unwell and could lead to metabolic decompensation and hyperammonemia in severe cases.
Table 1. The different phenotypes of citrin deficiency & associated signs & symptoms. Patient age Phenotype Frequent signs & symptoms Other possible signs & symptoms Newborn Neonatal Intrahepatic Cholestasis caused by citrin deficiency (NICCD) Prolonged jaundice, cholestasis, failure to thrive, fatty liver Prolonged bleeding time, galactosemia, vitamin K deficiency, hypoproteinemia >1 year old / Infant to child Silent or adaptation period Strong preference for protein/fat-rich foods, aversion to carbohydrate/sugar-rich foods, fatty liver Hypoglycemia, fatigue, occasional abdominal painFailure to thrive and dyslipidemia caused by citrin deficiency (FTTDCD) Strong preference for protein/fat-rich foods, aversion to carbohydrate/sugar-rich foods, failure to thrive, hypoglycemia, fatigue, abdominal pain, growth impairment, hyperlipidemia Fatty liver, pancreatitis, hepatoma Adolescent/adult Adolescent and adult citrin deficiency (AACD) Strong food preference for protein/fat-rich foods, aversion to carbohydrate/sugar-rich foods, citrullinemia, hyperammonemia, hyperlipidemia, fatigue, fatty liver Pancreatitis, hepatoma, low BMI
Epidemiology Citrin deficiency was originally described in the Japanese population and considered to be primarily an East Asian condition. In recent years, however, patients have been identified across North America and Europe, and the condition is now viewed as a pan-ethnic disease.
In Japan, citrin deficiency has an incidence rate of 1 in 17,000 based on the Japanese carrier rate of 1 in 65. The observed frequency of NICCD cases in Japan is similar to this incidence rate. However, the observed frequency of AACD cases is 1 in 100,000 to 1 in 230,000, which suggests that not all citrin deficiency patients will develop AACD.
Estimated carrier rates have been reported in several Asian countries, as shown below:
Japan (1:65) China (1:45) Taiwan (1:57) Korea (1:50) Singapore (1:41) Thailand (1:90) Vietnam (1:31)
Based on published carrier rates, there remains a large gap in the number of estimated patients versus the number of reported cases, which emphasizes the significant underdiagnosis of the condition.
History
Early clinical recognition and differentiation from classical citrullinemia The first documented cases resembling what is now recognized as citrin deficiency appeared in the 1970s, when Tsujii et al. described two adult siblings in Japan with chronic hepatocerebral symptoms, hyperammonemia, fatty liver, and elevated citrulline levels. The patients also exhibited a dietary aversion to carbohydrates and a preference for protein-rich foods; a unique trait later associated with citrin deficiency. At the time, the disorder was referred to as a "chronic recurrent hepatocerebral disease with hypercitrullinemia" and considered to be a late-onset variant of classical citrullinemia type I, caused by genetic defects in the enzyme argininosuccinate synthetase (ASS).
By the early 1980s, researchers began to propose that citrullinemia may consist of more than one disorder with distinct genetic causes. Saheki et al. observed that some cases in Japan involved a quantitative rather than qualitative deficiency of ASS enzyme, with normal activity observed in extrahepatic tissues such as fibroblasts and kidneys. This observation led to the suggestion of a possible defect in the regulation or expression of ASS enzyme that was limited to the liver, rather than a genetic defect in the ASS gene.
Discovery of the SLC25 transporter protein family Around the same period, Nobel laureate John E. Walker, best known for elucidating the mechanisms of ATP production through his seminal studies of ATP synthase, made foundational contributions to the discovery of the mitochondrial carrier family, forming the basis for subsequent work on citrin deficiency. In the 1970s and 1980s, studies led by Walker on mitochondrial carrier proteins led to the identification of the SLC25 family, which comprises transporters located in the inner mitochondrial membrane. Using comparative sequence analysis methods, they demonstrated that these proteins share a common structural signature, characterized by six transmembrane α-helices organized as three related sequences of about 100 amino acids, each containing two hydrophobic helices separated by a relatively extensive sequence of more polar amino acids. This discovery characterized them as a distinct family of mitochondrial transporters with a conserved architecture. Citrin, the protein encoded by SLC25A13, was eventually recognized as a member of this family.
Citrin deficiency gene discovery In 1999, Kobayashi and colleagues used positional cloning and homozygosity mapping in DNA samples from affected individuals to identify SLC25A13 as the causative gene in citrin deficiency. Analyses of the protein sequence revealed that citrin protein, encoded by SLC25A13, is a member of the SLC25 mitochondrial carrier protein family defined by Walker's earlier work. Although its precise biochemical function had not yet been determined at the time, this finding linked citrin deficiency to a specific genetic defect involving mitochondrial transporter function.
Functional characterization of citrin In 2001, an international collaboration involving the research groups of Takeyori Saheki, Jorgina Satrústegui, Ferdinando Palmieri, and John Walker elucidated the biochemical function of citrin and its paralog aralar. Using the EPRA (expression, purification, reconstitution, and assay) methodology developed by Walker's and Palmieri's groups, they demonstrated that both carrier proteins mediate the exchange of aspartate from the mitochondrial matrix for cytosolic glutamate and a proton. This function is essential for the malate-aspartate shuttle, helping to explain the liver-specific metabolic disturbances observed in citrin deficiency.
Expanding the clinical spectrum