# Citrin deficiency

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 for 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.
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 distribution profiles in humans, with aralar primarily being found in brain, skeletal muscle, kidney, and heart while citrin is predominantly expressed in the liver, kidney, and small intestines.

## Clinical phenotypes and associated symptoms

Citrin deficiency has four primary phenotypes that are age dependent. These include neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD) that affects infants, the adaptation or silent period, failure to thrive and dyslipidemia caused by citrin deficiency (FTTDCD) and adolescent and adult citrin deficiency (AACD) (formerly termed CTLN2), which represents the most severe form of the condition with acute episodes of hyperammonemia. 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 clinical symptoms for each phenotype are shown in Table 1.
In NICCD, frequently cholestasis, prolonged jaundice, and fatty liver. Most symptoms associated with NICCD tend to resolve after 1 year of age with the appropriate treatment and management.
Post-NICCD, patients enter the silent (or adaptation) phase. While patients appear 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, who tend to show more pronounced symptoms such as poor growth, as well as hypoglycemia, fatigue, hyperlipidemia, fatty liver, and abdominal pain.
AACD patients typically present with low BMI, citrullinemia, hyperlipidemia, 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 properly managed.
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 and associated symptoms. :
| Patient age | Phenotype | Frequent symptoms | Other possible 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 pain |
| >1 year old / Infant to child | Failure 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, fatty liver | Pancreatitis, hepatoma, low BMI |

## Epidemiology

Citrin deficiency was originally described in the Japanese population and considered a primarily East Asian condition. In recent years, however, patients across North America and Europe have been identified and the condition is now widely 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 underscores the significant underdiagnosis of the condition.

## Diagnosis

The diagnosis of citrin deficiency is based on clinical presentations and biochemical analysis (Table 1). However, genetic analysis for SLC25A13 gene variants remains the gold standard of diagnosis. The following sections detail the specific diagnosis associated with each citrin deficiency phenotype.

### NICCD

Suspected cases may be picked up if newborn screening shows increases in amino acids such as arginine, citrulline, isoleucine + leucine, methionine, and tyrosine. Other clinical presentations such as cholestasis, prolonged jaundice, elevated plasma alpha-fetoprotein, elevated citrulline without significant hyperammonemia, and increased galactose in blood or urine are strongly indicative of NICCD. Elevated serum gamma-glutamyl transferase (GGT, or GTP) may sometimes be observed in NICCD patients as well.
Although listed as a target condition in many NBS programs across Asia, not all NICCD cases are picked up at birth and highlights the need to improve the sensitivity and specificity of current NBS methods. Citrin deficiency is not included as a target condition in many NBS programs in the West, which further compounds the global underdiagnosis of this condition.

### Adaptation/compensation period

Without a previous NICCD diagnosis, the diagnosis of patients in the adaptation period may be challenging as most patients appear asymptomatic. However, patients who exhibit a specific dietary preference for foods high in protein and fat while avoiding carbohydrates and sugars may indicate citrin deficiency. Abdominal pain, dyslipidemia, fatty liver, and hypoglycemia may also be indicative of the condition. Recent reports have also shown that silent period patients may exhibit elevated levels of plasma BCAAs and ketogenic amino acids, and low glucogenic amino acids such as glycine.
FTTDCD may be suspected if, in addition to the above symptoms, low BMI, citrullinemia, and high lactate/pyruvate ratios are also observed.

### AACD

Elevated AST, ALT, GGT, arginine, ammonia, and citrulline levels are common biochemical presentations of AACD. Other clinical sympt.oms may include low BMI, hyperlipidemia, fatty liver, and consciousness disturbances caused by hyperammonemia. Combined with a distinct food preference characteristic to citrin deficiency patients, this strongly indicates AACD
If a patient is admitted with consciousness disturbances and hyperammonemia, doctors should first rule out the possibility of other urea cycle disorders. AACD may be differentiated from ASS deficiency if plasma levels of glutamine are not significantly elevated with an absence of urinary orotic acid and normal or slightly elevated arginine levels.

## Monitoring

Patients are advised to attend regular checkups to assess the progression of the condition. Monitoring of the following parameters are recommended:
* Plasma ammonia concentration
* Plasma amino acids
* Liver function tests
* Serum lipid levels
* Anthropometric measurements (for infant and adolescent patients)
An increase in plasma citrulline and serum pancreatic secretory trypsin inhibitor may indicate an onset of AACD, which should prompt medical treatment.

## Management and treatment

Aside from liver transplantation, there is currently no cure for citrin deficiency. Dietary management with a low carbohydrate, high protein, high fat diet is considered as first-line management for patients. Treatment with MCT has also been reported to improve or resolve some symptoms associated with citrin deficiency by rapidly providing energy to the liver.
All patients should refrain from consuming high-carbohydrate meals or alcohol, as they may trigger metabolic decompensation and may lead to AACD
The specific management and treatment for each phenotype are detailed below:

### NICCD

Dietary therapy is the mainstay treatment for NICCD. The use of MCT supplemented formula has been reported to improve symptoms associated with NICCD. MCT-enriched formula is frequently prescribed to NICCD patients, and for patients complicated with galactosemia, a lactose-free formula supplemented with MCT is recommended. Restricting lactose has also been reported to improve symptoms associated with NICCD. Fat-soluble vitamins (vitamins A, D, E, K) may sometimes be prescribed to patients

### Post-NICCD

All citrin deficiency patients post-NICCD should follow a low carbohydrate, high protein and fat diet. Based on clinical reports demonstrating the benefits of MCT supplementation for adolescent and adult patients, it may be advisable for patients to continue taking MCT supplements even after the resolution of clinical symptoms

### FTTDCD

A combination of dietary management and supplementation with MCT oil may be beneficial for patients with FTTDCD

### AACD

Dietary management and supplementation with MCT oil is recommended for AACD patients and has been shown to improve or resolve symptoms associated with citrin deficiency
Ammonia scavengers and arginine may be prescribed to better manage ammonia levels. Patients with brain edema caused by hepatic encephalopathy should avoid glycerol infusions because glycerol is metabolized in the liver to generate NADH, which worsens the condition and may be fatal. Infusions with mannitol to treat brain edema have been reported to be safe. Infusions with high-concentration glucose should also be avoided as it may exacerbate hyperammonemia. In serious cases where patients do not respond well to treatment, liver transplantation may be required as a final solution and has been shown to correct the metabolic disturbances caused by citrin deficiency.