EXPERIMENTAL AND CLINICAL TRANSPLANTATION

Objectives: Inborn errors of bile acid synthesis are rare genetic disorders that usually present as neonatal cholestasis and liver disease in older children and adults. The symptomatology of inborn errors of bile acid synthesis can markedly vary among individuals, ranging from mild to severe conditions. Diagnosis is based on genetic tests and/or urine liquid secondary ionization mass spectrometry. Here, we have described characteristics of patients who were diagnosed with inborn errors of bile acid synthesis in our department.
Materials and Methods: We retrospectively evaluated data from patients diagnosed with inborn errors of bile acid synthesis by urine bile acid analysis and/or genetic tests between 2013 and 2023.
Results: Ten patients (8 boys, 2 girls) born to consanguineous parents were diagnosed with inborn errors of bile acid synthesis during the study period. Six patients were diagnosed with 3β-hydroxy-Δ5-C27-steroid dehydrogenase deficiency, 2 patients with peroxisomal acyl-CoA oxidase 2 deficiency, and 2 patients with peroxisome biogenesis disorder. In patients with 3β-hydroxy-Δ5-C27-steroid dehydrogenase deficiency, 3 patients were monitored with cholic acid treatment, 2 underwent liver transplant due to liver failure, and 1 patient died from liver failure. Ursodeoxycholic acid treatment was given to patients with acyl-CoA oxidase 2 deficiency. Cholic acid was given to patients with peroxisome biogenesis disorder.
Conclusions: Inborn errors of bile acid synthesis can cause a variety of liver diseases, from asymptomatic liver enzyme elevation to cirrhosis. Clinical findings may include neurological symptoms and fat and fat-soluble vitamin malabsorption complications. Deficiency of 3β-hydroxy-Δ5-C27-steroid dehydrogenase is the most common bile acid synthetic defect presenting in cholestasis in infancy and childhood. Cholic acid is effective for most patients with inborn errors of bile acid synthesis. If patients do not receive an early diagnosis, progressive liver disease or other serious complications may develop.

Key words : Acyl-CoA oxidase 2 deficiency, Cholestasis, Cholic acid, 3β-Hydroxy-Δ5-C27-steroid dehydrogenase deficiency

Inborn errors of bile acid synthesis (IEBAS) are rare genetic metabolic disorders characterized by defects in the synthesis of bile acids. The disorders affecting the metabolic pathway are hereditary in an autosomal recessive manner.¹ The liver synthesizes bile acids from cholesterol in complex reactions involving many enzymatic steps. Blocking any of these reactions prevents normal bile acid production, leading to the accumulation of abnormal bile acids and intermediary metabolites. These metabolites may be toxic to hepatocytes.²

The symptomatology of the IEBAS can markedly vary among individuals, ranging from mild to severe conditions. Patients with IEBAS can present from the neonatal period to adulthood. Some clinical features include neonatal cholestasis, hepatomegaly, malabsorption of fat-soluble vitamins, and neurological findings.³ Here, we aimed to share the characteristics of patients diagnosed with IEBAS in our department.

Materials and Methods

Between 2013 and 2023, we retrospectively assessed 10 patients with IEBAS in the Division of Pediatric Gastroenterology at Gazi University. The diagnosis of IEBAS was made according to urine bile acid analysis and/or genetic tests by whole exome sequencing (WES). The other causes of liver diseases, such as anatomic, genetic, metabolic, autoimmune, and infectious disorders, had been carefully ruled out by appropriate investigations. We obtained clinical, laboratory, radiologic, and pathologic findings of patients from medical records. We recorded age at diagnosis, sex, type of IEBAS, presenting symptoms, liver function tests, liver biopsy results, treatment, and long-term clinical outcomes. One patient diagnosed with peroxisomal acyl-CoA oxidase 2 (ACOX2) deficiency was reported previously.⁴ The study protocols were approved by the ethics committees at Gazi University Medical School.

Ten patients (8 boys, 2 girls) born to Turkish consanguineous parents were diagnosed with IEBAS during this period. The median age at diagnosis for the patients was 3.1 years (range, 2 months to 14 years). Among the 10 patients, 6 were diagnosed with 3β-hydroxy-Δ5-C27-steroid dehydrogenase (3β-HSD) deficiency, 2 patients with ACOX2 deficiency, and 2 with peroxisome biogenesis disorder (PBD). The 2 patients with 3β-HSD deficiency were diagnosed through urinary bile acid analysis, but the remaining 8 patients were diagnosed with genetic analysis. We reported 3 novel homozygous pathogenic variants in the HSD3B7 gene (c.976_981delGTGGCC [p.Val1326_Ala327del]; homozygous c.598C>T [p.Gln200Ter]; c.914T>C [p.Leu305Pro]) and 1 novel homozygous pathogenic variant in the ACOX2 gene (c.484C>T [p.Leu162Phe]). The clinical and laboratory characteristics of patients are shown in Table 1.

Patients with 3β-HSD deficiency had median age of 3.1 years (range, 3 months to 14 years). The main initial signs and symptoms were fat and fat-soluble vitamin malabsorption and associated complications (n = 6), hepatosplenomegaly (n = 4), and neonatal cholestasis (n = 2). One patient had bilateral cysts. No patient had pruritus (Table 1). Median serum aspartate amino-transferase and alanine aminotransferase levels were 176 IU/L (range, 41-1276 IU/L) and 124 IU/L (range, 25-319 IU/L), respectively. The γ-glutamyl transpeptidase (GGT) values were found to be within normal ranges for all patients. Median serum total and direct bilirubin levels were 7 mg/dL (range, 1.8-19 mg/dL) and 3.5 mg/dL (range, 0.6-16 mg/dL), respectively. In 5 of 6 patients, prothrombin time was prolonged. Five had hypoalbuminemia. Four patients had vitamin deficiencies (2 had vitamin A and E deficiencies, 1 had only vitamin A deficiency, and 1 had only vitamin D deficiency).

Four patients had percutaneous liver biopsy. Histopathologic findings showed cirrhosis with or without giant cell formation (n = 3) and pre-cirrhosis (n = 1). A biopsy was not performed on the other 2 patients because they were diagnosed early with genetic analysis. Whereas 2 of 3 patients with cirrhosis underwent liver transplant, 1 patient (patient 1) died of progressive liver failure at the age of 6 months before being diagnosed with 3β-HSD deficiency. The postoperative follow-up findings at 4 and 10 years of the 2 patients (patient 2 and patient 3) who underwent liver transplant were uneventful under immunosuppressive treatment.

The remaining 3 patients with 3β-HSD deficiency were started on cholic acid at a dose of 10 mg/kg/day. The clinical and laboratory findings of patient 4, who had been taking cholic acid for 3 years, completely improved. The clinical and laboratory findings of 2 patients (patients 5 and 6), who received cholic acid for less than 6 months, were stable, and there was no need for liver transplant.

Patients with ACOX2 deficiency (patients 7 and 8) presented with elevated transaminase levels. Patient 8, a 5-year-old male patient, had intermittent elevated transaminase levels, incomplete septal cirrhosis, mild ataxia, dysmetria, and cognitive impairment. Patient 7, along with intermittently elevated transaminase levels, had concomitant attention deficit and hyperactivity disorder. His liver biopsy revealed normal microanatomy and nonspecific findings. Throughout the 10-year follow-up, their transaminase levels and hepatic synthetic functions remained within normal limits under ursodeoxycholic acid (UDCA) at a dose of 20 mg/kg/day, and there has been no progression in their neurological findings.

Patients with PBD (patients 9 and 10) at the age of 2 months exhibited evidence of compromised synthetic liver function along with notable neurological manifestations, including nystagmus, dysphagia, and cognitive impairment. The patients were given cholic acid orally at a dose of 10 mg/kg/day. The patient with PBD type 1A has been followed for 2 years with cholic acid therapy, despite the absence of improvement in hepatic and neurological symptoms. The patient with PBD type 3A died as a result of progressive hepatological and neurological complications 2 years after diagnosis.

This retrospective case series showed that patients diagnosed with IEBAS may have different clinical presentations. Toxic bile acid intermediates resulting from enzymatic defects can lead to liver involvement ranging from asymptomatic elevation of liver enzymes to neonatal cholestasis and end-stage liver disease and its complications. Inborn errors of bile acid synthesis may also cause acute hepatitis, acute liver failure, and noncirrhotic portal hypertension. Hepatologic findings may be accompanied by steatorrhea, deficiencies in fat-soluble vitamins, malnutrition, and neurological manifestations. Steatorrhea, deficiencies in fat-soluble vitamins, and subsequent malnutrition occur because of inadequate bile salt synthesis. As shown in our patients with ACOX2 and PBD, neurological findings and involvement of other systems are commonly observed in disorders of bile acid side-chain oxidation and peroxisome biogenesis disorders (secondary bile acid synthesis disorders). Xanthomas, cataracts, dysmorphic features, and renal cysts are other clinical findings that can be observed in IEBAS. For this reason, in the diagnosis of IEBAS, which presents with a spectrum of clinical signs and symptoms, clinical suspicion is crucial. All of our patients were children born from first-degree cousin marriages. All patients also shared the common feature of both the absence of pruritus and normal GGT levels.

Infants with IEBAS typically do not experience pruritus, a common symptom of chronic cholestasis, and often have normal or slightly elevated levels of GGT.^1,5-7 Pruritus plays a crucial role in the differential diagnosis of GGT-normal cholestasis, particularly in conditions like progressive familial intrahepatic cholestasis types 1 and 2, where severe pruritus may manifest.⁷ In children with cholestasis, normal levels of GGT, the absence of pruritus, and consanguinity are suggestive indicators of IEBAS, similar to findings in our patients. Inborn errors of bile acid synthesis can result in asymptomatic elevation of liver enzymes. In addition, documented cases exist where asymptomatic siblings have been diagnosed with IEBAS at an early stage through family screening.⁶ Consequently, the screening of asymptomatic siblings of patients with IEBAS is crucial for the prompt diagnosis and management of this autosomal recessive disorder.

The diagnosis of IEBAS is established through urine bile acid analysis and/or molecular genetic analysis. Furthermore, tests for normal levels of fasting serum bile acids can also contribute to supporting the diagnosis of IEBAS. Because bile acid analyses of serum and urine are only available in a few specialized referral laboratories and the use of UDCA can also interfere with bile acid analyses, the use of these tests for the diagnosis of IEBAS becomes challenging in clinical practice. However, advances in technology, such as WES and whole genome sequencing, have made it easier and more cost-effective to diagnose genetic disorders, and the widespread availability of molecular genetic analysis in many centers has facilitated the diagnosis of these diseases. Whole exome sequencing has been increasingly used recently in pediatric hepatology for the diagnosis of liver diseases of unknown cause. If the clinician suspects IEBAS, genetic analyses may be used before any further tests. In recent years, the use of molecular genetic investigations has been progressively increasing in our clinical practice for diagnosis of patients with metabolic and genetic liver conditions.^8,9

In recent years, with improved accessibility to genetic testing within our institution, WES has become our preferred diagnostic modality in cases of suspected IEBAS. With the use of WES, we successfully diagnosed IEBAS in 8 of 10 patients, with 5 patients harboring novel pathogenic variants in the HSD3B7 and ACOX2 genes. In addition, we identified previously reported pathological variants in the HSD3B7, PEX1, and PEX12 genes in other cases.

Consistent with the literature, 3β-HSD deficiency is the most commonly encountered form of IEBAS in our clinical practice. Bile acids are produced in the liver from cholesterol by a series of multiple enzymatic reactions. The HSD3B7 gene encodes an enzyme that plays a role in the initial stages of bile acid synthesis.² Presence of 3β-HSD deficiency, a well-known form of IEBAS caused by mutations in the HSD3B7 gene, is characterized by neonatal cholestasis, hepatomegaly, malabsorption of fat-soluble vitamins, and neurological findings.³ In our series, most patients with 3B-HSD deficiency manifested symptoms and signs of fat and fat-soluble vitamin deficiency, along with hepatosplenomegaly, neonatal cholestasis, and renal cysts.

Bile acid replacement therapy, preferably with use of cholic acid, constitutes the definitive treatment for 3β-HSD deficiency. The combination of cholic acid and chenodeoxycholic acid has been used for the treatment of 3B-HSD deficiency. However, when comparing side effects, cholic acid is favored because of its lower hepatotoxicity and teratogenicity compared with chenodeoxycholic acid.¹⁰ Patients who are diagnosed early have favorable responses to cholic acid therapy. Studies have shown that cholic acid replacement therapy may lead to regression of fibrosis in individuals with chronic liver disease. Remarkably, many patients with IEBAS are highly treatable and, as a result, have an excellent prognosis if diagnosed and treated early in their lives.^6,7,11,12

A recently identified bile acid synthesis disorder is ACOX2 deficiency; ACOX2, a peroxisomal enzyme involved in bile acid biosynthesis, participates in the shortening process of the side chain of cholesterol to produce C24 bile acids. Since 2016, 7 cases have been documented. The first reported case (patient 8) was earlier described by Vilarinho and colleagues.⁴ Subsequently, a 16-year-old male patient presenting with persistent hypertransaminasemia was identified to harbor a homozygous ACOX2 mutation.^13,14 Ferdinandusse and colleagues reported an infant with severe multisystem disease who died before the age of 1 year.¹⁵ Recently, Alonso-Peña and colleagues demonstrated that UDCA led to the normalization of serum aminotransferase levels in 4 patients with ACOX2 deficiency and hyper-transaminasemia.¹⁶ We identified a patient harboring a novel homozygous variant (c.484C>T) in the ACOX2 gene (NM_003500.4:c.484C>T [p.Leu162Phe]) associated with ACOX2 deficiency, thereby expanding the clinical spectrum of this rare syndrome. This case involved a 10-year-old boy who presented with elevated liver enzymes and attention deficit and hyperactivity disorder. Notably, another patient previously reported by us exhibited neurological symptoms alongside unexplained transaminase elevations.⁴ In both cases, liver function tests remained within normal limits with over 10 years of UDCA treatment. Furthermore, no progression of neurological findings was observed in either patient during long-term follow-up.

Peroxisome biogenesis disorder is a genetic disorder caused by mutations in multiple PEX genes, which affect peroxisome organization and function. Patients with PBD can present with a wide range of clinical symptoms, such as neurologic involvement, hypotonia, liver dysfunction, and development delay. Treatment should be personalized to the patient’s needs, with a focus on symptomatic and supportive treatment. It is often observed that patients tend to have a reduced life expectancy.^17-19 In most patients with PBD, cholic acid therapy has been reported to ameliorate abnormal liver chemistry. However, the efficacy of treatment is influenced by the presence of early liver disease. Patients with liver damage, particularly those without cirrhosis, are more likely to benefit from treatment.^20,21 Nevertheless, certain publications in the literature suggest that cholic acid treatment does not lead to regression of neurological findings.²² Our 2 patients had elevated liver enzymes detected during the newborn period, with neurological symptoms emerging within a few months. Despite both patients receiving cholic acid treatment, 1 died as a result of worsening neurological and hepatological involvement. The other patient, however, showed no substantial improvement in central nervous system and liver involvement despite cholic acid treatment.

Inborn errors of bile acid synthesis can cause a variety of liver diseases, from asymptomatic liver enzyme elevation to cirrhosis. Clinical findings may include neurological symptoms and complications of fat and fat-soluble vitamin malabsorption. The most common of the bile acid synthetic defects is 3β-HSD deficiency, which presents as cholestasis in infancy and childhood. Patients with appropriate clinical scenarios should undergo genetic testing and/or bile acid analysis promptly to obtain a rapid diagnosis. Bile acid replacement therapies such as cholic and chenodeoxycholic acid are highly effective and life-saving treatment options for most patients with IEBAS.

Volume: 22 Issue: 10 October 2024 - Supplement - 5

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