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NTCP deficiency: A new inherited disease of bile acid transport
Clinics and Research in Hepatology and Gastroenterology (2015) 39, 7—8
Available online at
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COMMENTARY
NTCP deficiency: A new inherited disease of bile acid transport Serge Erlinger ∗ University of Paris 7, 5, rue Thomas Mann, 75013 Paris, France Available online 2 September 2014
Summary The authors report the case of a child with extreme elevation of serum bile acid concentration, without pruritus, symptomatic cholestasis, liver disease, or abnormalities of liver function tests. Sequencing of the SLC10A1 gene, encoding NTCP (the sinusoidal uptake transporter of conjugated bile acids) revealed a single homozygous point mutation in the coding sequence of the gene resulting in an arginine to histidine substitution at position 252. This mutation resulted in a markedly reduced uptake activity of taurocholic acid. This is the first report of a new inborn error of bile acid transport, due to a mutation of NTCP. © 2014 Elsevier Masson SAS. All rights reserved.
In the era of genomic medicine, study of a single case may bring more information than large genome-wide association studies. This is the case with the report of Vaz et al. who studied a child with extremely high concentrations of conjugated serum bile acids (conjugated hypercholanemia) [1]. Bile acids are steroid molecules, which provide the main driving force for bile secretion, are essential for cholesterol solubilization into bile and for lipid absorption in the intestine. They are synthesized by the liver from cholesterol, secreted into canalicular bile (where they promote bile secretion) by an ATP-dependent transporter, named BSEP (bile salt export pump, or ABCB11, ATP-binding cassette, family B, number 11) and then into the intestine. They are reabsorbed in the terminal ileum and return to the liver, where they are taken up at the sinusoidal membrane, probably by a sodium dependent transporter named NTCP
∗
Tel.: +33 6 27 65 02 65. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.clinre.2014.07.011 2210-7401/© 2014 Elsevier Masson SAS. All rights reserved.
(Na-dependent co-transporting polypeptide, or SLC10A1). Inherited diseases of bile salt transport have been described, due to mutations of the ABCB11 gene [2,3]: progressive familial intrahepatic cholestasis, type 2, benign recurrent intrahepatic cholestasis, type 2 and cholestasis of pregnancy, but, so far, no genetic defect in NTCP was known. In this paper, Vaz et al. [1] report the case of the fourth female child of consanguineous Afghan parents who had, in her first year of life, growth and development retardation, and hypotonia. At 9 months, an extensive workup revealed normal liver function tests, a low 25-OH vitamin D level, and mildly reduced fat-soluble vitamins A and K. Abdominal ultrasonography showed a normal liver. A highly surprising finding was a markedly elevated serum total bile acid level of 445 M (normal < 16 M). At this time, secondary bile acids were absent, indicating absent entero-hepatic cycling. After introduction of a high-calorie diet, the patient demonstrated growth catch-up, but followup bile acid measurements in serum showed a further increase, with a maximum of 1531 M at 27 months, without
8 any sign of cholestasis. Later on, bile acid concentration tended to decrease, while secondary bile acids appeared in serum: at 5 years, taurodeoxycholic acid concentration reached 27% of taurocholic acid concentration, while glycodeoxycholic acid represented 13% of glycocholic acid. This indicated that some entero-hepatic cycling of bile acids was present. Hepatic bile acid synthesis [estimated by levels of C4 (7␣-OH-4-cholesten-3-one)] was normal. Autotaxin activity, associated with pruritus in cholestasis, was normal. At 3 years of age, the authors sequenced the SLC10A1 gene and found a single homozygous non-synonymous c.755G > a nucleotide change resulting in an arginine to histidine substitution at position 252 of the NTCP protein (R252H). Parents were heterozygous for the mutation and three siblings were either heterozygous or did not have the mutation. As the wild type allele was not observed in the patient, the authors assumed that she is homozygous for the mutant allele. The authors then expressed wild type and mutant NTCPR252H in 293 T cells (that do not express the transporter spontaneously). In cells expressing the mutant NTCP, Vmax for Na-dependent taurocholate uptake was considerably reduced (9 times) when compared to cells with the wild type NTCP, while Km was unaffected. Using immunofluorescence and biotinylation, they demonstrated that the mutant NTCP was poorly expressed on the plasma membrane and retained in the endoplasmic reticulum. These elegant studies on a single patient provide unambiguous evidence for the long presumed role of NTCP in hepatic uptake of conjugated bile acids. In addition, they show that alternative mechanisms play some role in the uptake process, since serum bile acid concentration decreased with time, while secondary bile acids appeared in the circulation. Candidates for this rescue role in hepatic uptake are OATP1B1/1B3, which transport both unconjugated and conjugated bile acids [4], or OST␣-OST, which functions as a bidirectional transporter [5]. However, the transport capacity of these alternative pathways of bile acid uptake seems largely insufficient to compensate for NTCP deficiency. The findings also show that deficient bile acid uptake does not affect bile acid synthesis, contrary to interruption of bile acid cycling by bile acid sequestrants. Therefore, mechanisms other than bile acid return to the liver are probably important in the regulation of bile acid synthesis. Signalling via the FGF (fibroblast growth factor) 19-FGFR4 pathway has been proposed [6]. Finally, the study provides strong evidence for the view that increased serum bile acid concentration without cholestasis is not responsible in itself for pruritus. Lysophosphatidic acid, a product of the circulating enzyme autotaxin,
S. Erlinger is a good candidate as pruritogen in cholestatic patients [7,8]. As stated in the accompanying editorial [9], the authors should be applauded for elucidating the mechanism of this severe hypercholanemia and identifying a new inherited disease of bile acid transport. They confirm the primary role of NTCP in bile acid uptake by hepatocytes. They show that alternate pathways for uptake exist, but only partially compensate for NTCP deficiency. Incidentally, they provide further evidence that serum bile acids are not directly responsible for pruritus in cholestatic patients. These findings are an important step in understanding the processes at work in hepatic transport of bile acids in man, and the role of bile acids in health and disease.
Disclosure of interest The author declares that he has no conflicts of interest concerning this article.
References [1] Vaz FM, Paulusma CC, Huidekoper H, de Ru M, Lim C, Koster J, et al. Sodium taurocholate co-transporting polypeptide (SLC10A1) deficiency: conjugated hypercholanemia without a clear clinical phenotype. Hepatology 2014, http://dx.doi.org/10.1002/hep.27240 [In press]. [2] Erlinger S. Pathophysiology of cholestasis. Hepatogastroenterology 2012;19:14—20. [3] Kubitz R, Dröge C, Stindt J, Weissenberger K, Haüssinger D. The bile salt export pump(BSEP) in health and disease. Clin Res Hepatol Gastroenterol 2012;36:536—53. [4] Hagenbuch B, Gui C. Xenobiotic transporters of the human organic anion transporting polypeptides (OATP) family. Xenobiotica 2008;38:778—801. [5] Dawson PA, Hubbert ML, Rao A. Getting the mOST from OST: role of organic solute transporter. OSTalpha-OSTbeta in bile acid and steroid metabolism. Biochim Biophys Acta 2010;1801:994—1004. [6] Galman C, Angelin B, Rudling M. Pronounced variation in bile acid synthesis in humans is related to gender, hypertriglyceridaemia and circulating levels of fibroblast growth factor 19. J Intern Med 2011;270:580—8. [7] Kremer AE1, Oude Elferink RP, Beuers U. Pathophysiology and current management of pruritus in liver disease. Clin Res Hepatol Gastroenterol 2011;35(2):89—97. [8] Kremer AE, van Dijk R, Leckie P, Schaap FG, Kuiper EM, Mettang T, et al. Serum autotaxin is increased in pruritus of cholestasis, but not of other origin, and responds to therapeutic interventions. Hepatology 2012;56:1391—400. [9] Karpen SJ, Dawson PA. Not all (bile acids) who wander are lost: the 1st report of a patient with an isolated NTCP defect. Hepatology 2014, http://dx.doi.org/10.1002/hep.27294 [In press].
Available online at
ScienceDirect www.sciencedirect.com
COMMENTARY
NTCP deficiency: A new inherited disease of bile acid transport Serge Erlinger ∗ University of Paris 7, 5, rue Thomas Mann, 75013 Paris, France Available online 2 September 2014
Summary The authors report the case of a child with extreme elevation of serum bile acid concentration, without pruritus, symptomatic cholestasis, liver disease, or abnormalities of liver function tests. Sequencing of the SLC10A1 gene, encoding NTCP (the sinusoidal uptake transporter of conjugated bile acids) revealed a single homozygous point mutation in the coding sequence of the gene resulting in an arginine to histidine substitution at position 252. This mutation resulted in a markedly reduced uptake activity of taurocholic acid. This is the first report of a new inborn error of bile acid transport, due to a mutation of NTCP. © 2014 Elsevier Masson SAS. All rights reserved.
In the era of genomic medicine, study of a single case may bring more information than large genome-wide association studies. This is the case with the report of Vaz et al. who studied a child with extremely high concentrations of conjugated serum bile acids (conjugated hypercholanemia) [1]. Bile acids are steroid molecules, which provide the main driving force for bile secretion, are essential for cholesterol solubilization into bile and for lipid absorption in the intestine. They are synthesized by the liver from cholesterol, secreted into canalicular bile (where they promote bile secretion) by an ATP-dependent transporter, named BSEP (bile salt export pump, or ABCB11, ATP-binding cassette, family B, number 11) and then into the intestine. They are reabsorbed in the terminal ileum and return to the liver, where they are taken up at the sinusoidal membrane, probably by a sodium dependent transporter named NTCP
∗
Tel.: +33 6 27 65 02 65. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.clinre.2014.07.011 2210-7401/© 2014 Elsevier Masson SAS. All rights reserved.
(Na-dependent co-transporting polypeptide, or SLC10A1). Inherited diseases of bile salt transport have been described, due to mutations of the ABCB11 gene [2,3]: progressive familial intrahepatic cholestasis, type 2, benign recurrent intrahepatic cholestasis, type 2 and cholestasis of pregnancy, but, so far, no genetic defect in NTCP was known. In this paper, Vaz et al. [1] report the case of the fourth female child of consanguineous Afghan parents who had, in her first year of life, growth and development retardation, and hypotonia. At 9 months, an extensive workup revealed normal liver function tests, a low 25-OH vitamin D level, and mildly reduced fat-soluble vitamins A and K. Abdominal ultrasonography showed a normal liver. A highly surprising finding was a markedly elevated serum total bile acid level of 445 M (normal < 16 M). At this time, secondary bile acids were absent, indicating absent entero-hepatic cycling. After introduction of a high-calorie diet, the patient demonstrated growth catch-up, but followup bile acid measurements in serum showed a further increase, with a maximum of 1531 M at 27 months, without
8 any sign of cholestasis. Later on, bile acid concentration tended to decrease, while secondary bile acids appeared in serum: at 5 years, taurodeoxycholic acid concentration reached 27% of taurocholic acid concentration, while glycodeoxycholic acid represented 13% of glycocholic acid. This indicated that some entero-hepatic cycling of bile acids was present. Hepatic bile acid synthesis [estimated by levels of C4 (7␣-OH-4-cholesten-3-one)] was normal. Autotaxin activity, associated with pruritus in cholestasis, was normal. At 3 years of age, the authors sequenced the SLC10A1 gene and found a single homozygous non-synonymous c.755G > a nucleotide change resulting in an arginine to histidine substitution at position 252 of the NTCP protein (R252H). Parents were heterozygous for the mutation and three siblings were either heterozygous or did not have the mutation. As the wild type allele was not observed in the patient, the authors assumed that she is homozygous for the mutant allele. The authors then expressed wild type and mutant NTCPR252H in 293 T cells (that do not express the transporter spontaneously). In cells expressing the mutant NTCP, Vmax for Na-dependent taurocholate uptake was considerably reduced (9 times) when compared to cells with the wild type NTCP, while Km was unaffected. Using immunofluorescence and biotinylation, they demonstrated that the mutant NTCP was poorly expressed on the plasma membrane and retained in the endoplasmic reticulum. These elegant studies on a single patient provide unambiguous evidence for the long presumed role of NTCP in hepatic uptake of conjugated bile acids. In addition, they show that alternative mechanisms play some role in the uptake process, since serum bile acid concentration decreased with time, while secondary bile acids appeared in the circulation. Candidates for this rescue role in hepatic uptake are OATP1B1/1B3, which transport both unconjugated and conjugated bile acids [4], or OST␣-OST, which functions as a bidirectional transporter [5]. However, the transport capacity of these alternative pathways of bile acid uptake seems largely insufficient to compensate for NTCP deficiency. The findings also show that deficient bile acid uptake does not affect bile acid synthesis, contrary to interruption of bile acid cycling by bile acid sequestrants. Therefore, mechanisms other than bile acid return to the liver are probably important in the regulation of bile acid synthesis. Signalling via the FGF (fibroblast growth factor) 19-FGFR4 pathway has been proposed [6]. Finally, the study provides strong evidence for the view that increased serum bile acid concentration without cholestasis is not responsible in itself for pruritus. Lysophosphatidic acid, a product of the circulating enzyme autotaxin,
S. Erlinger is a good candidate as pruritogen in cholestatic patients [7,8]. As stated in the accompanying editorial [9], the authors should be applauded for elucidating the mechanism of this severe hypercholanemia and identifying a new inherited disease of bile acid transport. They confirm the primary role of NTCP in bile acid uptake by hepatocytes. They show that alternate pathways for uptake exist, but only partially compensate for NTCP deficiency. Incidentally, they provide further evidence that serum bile acids are not directly responsible for pruritus in cholestatic patients. These findings are an important step in understanding the processes at work in hepatic transport of bile acids in man, and the role of bile acids in health and disease.
Disclosure of interest The author declares that he has no conflicts of interest concerning this article.
References [1] Vaz FM, Paulusma CC, Huidekoper H, de Ru M, Lim C, Koster J, et al. Sodium taurocholate co-transporting polypeptide (SLC10A1) deficiency: conjugated hypercholanemia without a clear clinical phenotype. Hepatology 2014, http://dx.doi.org/10.1002/hep.27240 [In press]. [2] Erlinger S. Pathophysiology of cholestasis. Hepatogastroenterology 2012;19:14—20. [3] Kubitz R, Dröge C, Stindt J, Weissenberger K, Haüssinger D. The bile salt export pump(BSEP) in health and disease. Clin Res Hepatol Gastroenterol 2012;36:536—53. [4] Hagenbuch B, Gui C. Xenobiotic transporters of the human organic anion transporting polypeptides (OATP) family. Xenobiotica 2008;38:778—801. [5] Dawson PA, Hubbert ML, Rao A. Getting the mOST from OST: role of organic solute transporter. OSTalpha-OSTbeta in bile acid and steroid metabolism. Biochim Biophys Acta 2010;1801:994—1004. [6] Galman C, Angelin B, Rudling M. Pronounced variation in bile acid synthesis in humans is related to gender, hypertriglyceridaemia and circulating levels of fibroblast growth factor 19. J Intern Med 2011;270:580—8. [7] Kremer AE1, Oude Elferink RP, Beuers U. Pathophysiology and current management of pruritus in liver disease. Clin Res Hepatol Gastroenterol 2011;35(2):89—97. [8] Kremer AE, van Dijk R, Leckie P, Schaap FG, Kuiper EM, Mettang T, et al. Serum autotaxin is increased in pruritus of cholestasis, but not of other origin, and responds to therapeutic interventions. Hepatology 2012;56:1391—400. [9] Karpen SJ, Dawson PA. Not all (bile acids) who wander are lost: the 1st report of a patient with an isolated NTCP defect. Hepatology 2014, http://dx.doi.org/10.1002/hep.27294 [In press].