What Is Known/What Is New
What Is Known
- Hypercholanemia caused by impaired canalicular bile acid transport, as in ABCB11 deficiency, for example, is associated with severe liver injury.
- Sodium taurocholate cotransporting polypeptide is the primary basolateral conjugated bile acid transporter. Its deficiency is associated with long-lasting hypercholanemia.
What Is New
- Sodium taurocholate cotransporting polypeptide deficiency is the exclusive or major cause, and c.800C>T, p. S267F in SLC10A1 is the major contributing variation, of long-lasting isolated hypercholanemia in the Han Chinese pediatric population.
- Sodium taurocholate cotransporting polypeptide deficiency may present as transit neonatal cholestasis, with no apparent long-term consequences.
Bile acids play a key role in bile formation and fat-soluble vitamin absorption. Interrupting canalicular bile acid transport causes hypercholanemia and severe liver injury, as seen in patients with progressive familial intrahepatic cholestasis type II (PFIC2, ABCB11 deficiency). The sodium-taurocholate co-transporting polypeptide (NTCP), encoded by SLC10A1, transports conjugated bile acids across the basolateral membrane of hepatocytes. The consequence of a deficiency in NTCP is, however, not well understood. In 2015, the first case of NTCP deficiency was reported by Vaz et al (1) and to date, only 37 cases of NTCP deficiency have, thus far been reported (1–6). The main manifestation of this genetic defect was described as a persistent hypercholanemia but there is a paucity of information on other phenotypic features of NTCP deficiency.
To better understand the clinical manifestations and long-term consequences of NTCP deficiency, we sequenced SLC10A1 in Chinese children that presented with a persistent isolated hypercholanemia. Here we report on the follow-up results of patients with NTCP deficiency.
MATERIALS AND METHODS
Patients presenting to the Children's Hospital of Fudan University and Jinshan Hospital Fudan University between January 2012 and December 2018 with isolated hypercholanemia were recruited for this study. In China, a serum total bile acid concentration is routinely measured in many hospitals and medical centers as a component of the panel of liver function tests. Furthermore, liver function tests are usually performed on all hospital admissions, on all children entering school and college, and on adults undergoing annual physical check-ups. Isolated hypercholanemia was defined by elevated serum total bile acids (sTBA, measured by an enzymatic method) of more than 5× their age-matched upper normal range (UNR) with other liver function tests (LFTs), including serum bilirubin, transaminases, and gamma glutamyl transpeptidase (GGT) levels less than 2× UNR on at least 3 consecutive measurements with a normal albumin level and coagulation function. Patients in whom isolated hypercholanemia was found after liver transplantation were excluded. The study protocol was approved by the Children's Hospital of Fudan University and Jinshan Hospital Fudan University Ethics Committees at both Institutions and written informed consent was obtained from parents, guardians, and/or patients.
Thirty-four patients fulfilled the definition of isolated hypercholanemia but 1 was excluded as a liver transplant recipient. Finally, 33 patients were enrolled in this study. Details of the LFTs measured at every visit are presented in Supplementary Table S1 (Supplemental Digital Content, https://links.lww.com/MPG/B894).
Gene Sequencing and Variant Pathogenicity Prediction
Genomic DNA from peripheral blood was obtained from each child and whenever possible from the parents. All exons and their flanking regions of SLC10A1 (NM_003049) were amplified by PCR with primer pairs listed in Supplementary Table S2 (Supplemental Digital Content, https://links.lww.com/MPG/B894). Multigenetic panel sequencing of cholestasis-associated genes was performed as described previously (7) on the 10 patients with documented conjugated hyperbilirubinemia (9 cases presented as transient neonatal cholestasis [TNC] and the other presented with long-lasting dark urine, see Supplement Table S1, Supplemental Digital Content, https://links.lww.com/MPG/B894, and results). The pathogenicity of variants was analyzed by Mutation Taster (http://www.mutationtaster.org) and Polyphen-2 (http://genetics.bwh.harvard.edu/pph-2).
All subjects received a thorough medical history review and physical examination at referral. Their past laboratory results were retrospectively collected. No medication was given after the diagnosis of NTCP deficiency was established, except for fat-soluble vitamin supplementation given in the stage of neonatal cholestasis. Laboratory tests were ordered based on the clinician's medical judgment, with physical examination and medical history review at each follow-up. Percentiles of height and weight were compared with the national standard published in 2013 (8). Vitamin D deficiency was defined as <12 ng/mL, and insufficiency <20 ng/mL (9).
Age- and Sex-Matched Controls for Direct Bilirubin Comparison
On the basis of the observation that 1 patient had a persistent mild conjugated hyperbilirubinemia, we questioned whether NTCP deficiency may affect the metabolism of bilirubin in the long-term. If this is true, the patients with NTCP deficiency may have higher conjugated bilirubin levels, and even direct bilirubin (DB) levels within normal range after recovery from TNC. To test this hypothesis, we followed the DB levels in patients with NTCP deficiency. To examine the relationship between conjugated bilirubin and NTCP deficiency, we compared the most recent serum DB levels measured at the last follow-up visit with DB levels in 3 randomly selected age- and sex-matched controls that were hospitalized for nonliver-related disorders (bronchitis or acute upper respiratory infection) at the same hospital and on the same day. To avoid possible bias, the patient presenting with long lasting dark urine (P5) and those whose last LFTs measurements were performed at other centers (P9, P20, P21, P31–33) were excluded from the final analysis.
Statistical analysis was performed with SPSS 19 (IBM, Armonk, NY). Graphs were generated using GraphPad Prism (version 6.0; GraphPad Software Inc.). Data were expressed as mean ± standard deviation for a normal distribution and median (inter-quartile range) for skewed distribution. One sample t test was performed to compare the mean between patient and population values. The differences between groups were analyzed using the Mann-Whitney U-test or Kruskal-Wallis test. Statistical significance was considered at P < 0.05 bilateral.
Demographic and Presenting Patterns
A total of 33 patients (23 boys, 10 girls) were enrolled in this study. The mean birth weight was 3105 ± 125 g, which included 4 children born preterm at gestational weeks 30 + 5, 35, 35 + 5, and 36 + 4, respectively. The age at which hypercholanemia was first documented ranged from 1 day to 19 years 5 months (median 8 weeks). Nine patients were identified with hypercholanemia during their work-up for TNC, one for long-lasting dark urine at ages 5 years 4 months, 7 for prolonged neonatal unconjugated hyperbilirubinemia, 2 during a routine health checkup at age 9 and 12 years, respectively, and 12 from routine LFTs whereas hospitalized for nonliver-related conditions. Two other subjects were identified following genetic screening of siblings; P32 is the 19-year-old sister of P31, and P33 is the elder sister of P27. None of subjects had a family history of liver diseases except for 2 fathers who were asymptomatic hepatitis B carriers.
Isolated Hypercholanemia Attributed to Sodium Taurocholate Cotransporting Polypeptide Deficiency
A rare variant, c.800C>T, p. S267F in SLC10A1 was detected in all 33 patients, of which 30 were homozygous, and 3 were compound heterozygous with c.776G>A/p.G259E (P1), c.682–683del/p.L228Dfs × 49 (P25) or c.263T>C/p. I88T (P26). The variant c.800C>T is a known functional single nucleotide polymorphism (SNP, rs2296651) affecting bile acid transport, leading to 98% reduction in the total uptake of taurocholate in vitro (10). Variant c.263T>C has previously been reported in NTCP patients (6), variants c.776G>A and c.682-683del were novel findings. All 3 missense variants were predicted as probably damaging (Table 1). All parents were simple heterozygous when tested variants in SLC10A1.
Results from panel sequencing, to exclude the possibility that the conjugated hyperbilirubinemia was caused by other cholestasis-associated genes showed none harbored a putatively pathogenic mutation in accordance to the inherent mode, save P8, a compound heterozygote with one known disease-causing mutation c.334C>T, p. R112X (11) paternal, and a rare missense variant c.776G>T, p. G259 V with uncertain pathogenicity maternal in CYP7B1. A disorder in the cholesterol-bile acid biosynthetic pathway was, however, ruled out by fast atom bombardment ionization-mass spectrometry (FAB-MS) analysis of urine (data not shown) at the age of 3 years 4 months.
Altered Conjugated Bilirubin Metabolism in Sodium Taurocholate Cotransporting Polypeptide Deficiency
One patient (P5) had a persistently mild conjugated hyperbilirubinemia. Despite having been treated with ursodeoxycholic acid (UDCA, 10–15 mg−1· kg−1· day−1) for almost 3 years before referral, his serum bilirubin remained elevated, with a total bilirubin (TB) concentration of 23.63 ± 5.2 μmol/L (ref: 5.1∼17.1 μmol/L) and DB of 10.7 ± 3.0 μmol/L (ref: 0∼6 μmol/L). No changes in LFTs were observed after UDCA therapy was terminated.
Nine patients (P6–11, P21–23) had TNC with onset of jaundice at 1 to 4 days of life. Their TB level normalized at 3.1 months (range 1.7–4.0 months) of age and alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels normalized by age 1.0 year (0.2–1.3 year) (Supplementary Table S1, Supplemental Digital Content, https://links.lww.com/MPG/B894). Generally, the neonatal cholestasis presented with a high serum GGT. The highest serum GGT level in every patient was above 150U/L and the median (interquartile range) was 166.1 U/L (69,226 U/L) during the cholestatic period.
The effect on conjugated bilirubin metabolism was long-lasting. The serum DB concentration at the last follow-up was still significantly higher than age- and sex-matched controls (Fig. 1, 2.85 ± 1.50 vs 1.49 ± 0.7 μmol/L, P = 0.00008). Other LTFs were unremarkable. Serum cholesterol and triglycerides were within the normal range in all patients. Serum 25-OH vitamin D measured in 13 patients (Table 2) and vitamin D deficiency observed in 2 patients during their TNC period, all others within the normal range.
Growth Milestones and Follow-up Outcomes
The overall follow-up time ranged from 6 weeks to 4 years 10 months (median 1 year). Compared with the population standard, the percentiles for weight for patients with NTCP deficiency were significantly lower during the first year of life (35.0 ± 30.2, P = 0.004), but not after 1 year of age (49.1 ± 29.6, P = 0.886). The lower weight of NTCP patients in the first year may be attributed to TNC. Before 1 year of age, patients with TNC had significantly lower weight percentiles when compared with patients without TNC (24.7 ± 21.5 vs 46.5 ± 34.6, P = 0.029). The percentiles for height in patients with NTCP deficiency were not significantly different from the national standard, either before (41.2 ± 32.9, P = 0.130), or after 1 year of age (43.6 ± 32.4, P = 0.349).
In follow-up, hypercholanemia persisted in all patients, but all thrived well except for 2 patients with comorbidities. P16 was first found to have hypercholanemia during a check-up for delayed neuropsychological development and persistent muscle hypertonia. She had a history of neonatal meconium aspiration syndrome and degree I enhanced echo in brain parenchyma by cranial ultrasound at birth. P20, born at 35+2 week, had delayed psychomotor development at age 9 months. Except for temporary hypothyroidism in one premature baby (P11), no other subjects displayed comorbidities or complications, such as cardiomyopathy or severe heart disease, kidney disease or endocrine disorders.
NTCP is encoded by SLC10A1 and plays a key role in the enterohepatic circulation of bile acids. Patients with NTCP deficiency may thus serve as a natural model to evaluate the long-term consequences and safety of blocking NTCP function. This study followed a relatively large cohort of patients with genetically confirmed NTCP deficiency because of variants in SLC10A1, and concluded that NTCP deficiency may present as neonatal transient cholestasis or mild persistent hyperbilirubinemia, but with no severe long-term consequences.
In this report, of 33 patients with genetically confirmed NTCP deficiency because of mutations in SLC10A1, the largest cohort yet to be systematically followed, the predominant recurrent genetic variant was c.800C>T, p.S267F. Transient neonatal cholestasis had been previously described in 2 patients with a homozygous c.800C>T variant in SLC10A1(3,5). In our patients, 10 cases were associated with a conjugated hyperbilirubinemia, including 9 with transient neonatal cholestasis, and 1 with mild persistent conjugated hyperbilirubinemia. Apart from 1 case (P11), a premature infant with a history of total parenteral nutrition for more than 10 days, which may partly explain the transient cholestasis, no other cause or risk factors for neonatal cholestasis or persistent conjugated hyperbilirubinemia were identified. Even after the resolution of TNC, the serum DB level, although in normal range, was still significantly higher than age- and sex-matched controls. These findings strongly suggest that NTCP deficiency mildly influences bilirubin metabolism, to account for TNC as a specific clinical presentation.
Growth retardation was a feature in the world's first reported case of NTCP deficiency (1). In our cohort, only 2 patients experienced growth retardation, which was attributed to brain injury in utero, or a premature birth. Overall, we observed a temporary lower body weight in NTCP patients in the first year of age, which we attributed to the effects of neonatal cholestasis. After 1 year of age, percentiles for weight and height were no different from population standards suggesting that the early influence on growth in patients with NTCP deficiency appears related only to the associated TNC, which is temporary. These findings are consistent with findings from the Slc10a1-/- mouse model of NTCP deficiency (12).
There were no other clinically significant abnormalities observed in our subjects with NTCP deficiency. Low 25-OH vitamin D levels have been noted previously (2), but this was not apparent in our patients, all of whom had normal serum 25-OH vitamin D levels, except for 2 during the TNC stage.
In conclusion, this report of the largest cohort of patients with documented NTCP deficiency, indicates that NTCP deficiency is a significant cause of isolated persistent hypercholanemia, and that the c.800C>T, p.S267F is the major contributing variant in the Chinese population. NTCP deficiency affects bilirubin metabolism and presents as transient neonatal cholestasis with no apparent or severe long-term clinical consequences.
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