Intrahepatic cholestasis of pregnancy and theory of inheritance of the disease. Literature review

Abstract Intrahepatic cholestasis during pregnancy is associated with a higher risk of prenatal and maternal complications. There are several new publications and guidelines on the detection and thresholds of intrahepatic cholestasis during pregnancy. However, the genetic background of this disease has rarely been investigated. This is a comprehensive review of the roles of genes in intrahepatic cholestasis during pregnancy.


Introduction
Intrahepatic cholestasis of pregnancy (ICP) is an exceptionally interesting phenomenon.Thus far, only a limited number of markers and possible medical susceptibility to ICP have been described.It also has a variable occurrence and is the most frequent cause of liver dysfunction in pregnant women [1].The prevalence of ICP varies significantly, ranging from less than 1% to 27.6%, for causes that are only partly clear [2].In addition to abnormal hepatic results, there is an increased incidence of dyslipidemia, impaired glucose tolerance, gestational diabetes, and pre-eclampsia [3][4][5].
The diagnosis is mainly based on pruritus and increased total bile acid (TBA) levels [6].Some variations in the laboratory criteria for the maximal limit of the normal reference range for TBA exist among the guidelines [7].However, severe cholestasis is consistently defined as a level over 40 lmol/l and accounts for approximately 20% of cases.In addition, Glantz et al. confirmed an increased risk of fetal complications in patients with fasting TBA levels above 40 mmol/l.Treatment with ursodeoxycholic acid (UDCA) is typically introduced in cases where the total bile acid or serum aminotransferase levels, or both, are elevated.
In most cases, pruritus is typically limited to the palms of the hands and soles of the feet.In most cases, the first symptoms develop in the late second or third trimester and resolve immediately after delivery.In some extremely sporadic cases, the first symptoms may appear as soon as eight weeks of gestation.In more severe cases, the serum bilirubin concentration may increase, and jaundice rarely appears as one of the symptoms.Owing to intense pruritus secondary to scratching, typical skin lesions may be observed (dermatitis artefacta) [8].
Intrahepatic cholestasis during pregnancy is usually a benign condition.However, high TBA levels are related to an increased probability of fetal complications, including preterm delivery, fetal distress, asphyxia, meconium staining, and even unexpected intrauterine fetal demise [9].These complications increase by 1-2% per additional micromol/l of TBA [10].
A meta-analysis analyzed individual patient data including 5557 fasting or postprandial samples from patients with ICP and revealed that only patients with TBA levels �100 mmol/l had a higher risk of intrauterine fetal demise, whereas patients with TBA �40 mmol/l had an increased risk of preterm birth [11].Serum bile acids > 100 lmol/l are associated with a 10-fold higher risk of intrauterine fetal demise [11,12].Some researchers suggest that placental elimination of bile acid during maternal cholestasis plays a key role in the defense of the fetus against toxic bile acids, whereas fetal bile acid metabolism is of minor importance [13].
Most biophysical methods used for fetal monitoring, including cardiotocography and Doppler indices, are insufficient for the risk assessment of intrauterine fetal demise in patients with ICP [14].Unfortunately, no known pharmacological management or maternal/ fetal monitoring approach has been proven effective in reducing intrauterine fetal demise in patients with ICP [15].Most monitoring policies include close maternal and fetal surveillance; however, in most cases, this leads to iatrogenic delivery.This approach was intended to avoid the subsequent risk of stillbirth, which might be a result of impaired fetal myocardial performance and impaired fetal cardiac conduction system [16].In most cases, recommended gestations for delivery are typically scheduled between 36 and 40 weeks of gestation, depending on TBA concentration [11,17].The Royal College of Obstetricians and Gynecologists (RCOG) guidelines on ICP recommends [18]: � For peak bile acids 19-39 lmol/l without other risk factors, the risk of stillbirth is similar to the background risk.Options included planned birth by 40 weeks of gestation or ongoing antenatal care according to national guidelines.� For peak bile acids 40-99 lmol/l without other risk factors, the risk of stillbirth is similar to the background risk until 38-39 weeks of gestation.Planned births should be considered at 38-39 weeks gestation.� For peak bile acid levels �100 lmol/l, the risk of stillbirth was higher than the background risk.Therefore, planned birth should be considered at 35-36 weeks of gestation.
Most cases of ICP improve soon after delivery; however, there is growing evidence that in many cases, ICP has permanent effects on the mother and child.This indicates a considerable increase in the risk of hepatobiliary disease later in life and a higher risk of hepatobiliary cancer and immune disease [19,20].Physiological consequences and adverse fetal outcomes remain the most significant clinical concerns; however, they are still poorly understood.It is likely that bile acid-induced fetal arrhythmia and placental dysfunction are the most important factors leading to unexpected intrauterine fetal demise [21].
Because the cause of ICP is unknown, this potentially dangerous disease is commonly known to have many different theories of its origin.We decided to summarize the existing evidence on the etiology of ICP.This is a review of theories and possible causes of ICP with particular emphasis on the potential genetic background.

Disease of complex etiology
The etiology of ICP is most likely multifactorial, and genetic, endocrine, and environmental factors play a major role and interact with each other.The background and markers of ICP are not entirely understood but are most likely complex and multifactorial [22].The main role of bile acids is dietary emulsification; however, they are progressively being acknowledged as significant metabolic signaling molecules.Bile acids control the homeostasis of cholesterol, lipids, carbohydrates, and the immune system through different receptor-controlled metabolic pathways [21].The most common risk factors associated with ICP are shown in Table 1.There are also several new biomarkers described for ICP such as lysyl oxidase-like protein 2 (LOXL-2).LOXL-2 is an enzyme that is involved in the development of hepatic fibrosis and bile duct epithelial injury in hepatic cholestasis.Interestingly LOXL-2 may be both an initiating factor in the pathophysiology of ICP and a marker in the prediction of the disease [23].

Geographic variations
One of the unsolved phenomena of ICP is the geographic variation and differences in predisposition between ethnic groups [19].The highest European ICP rate is reported in Scandinavia, but overall the incidence of this pregnancy-related disease across Europe varies from 0.5 to 1.5% [2].The Indian Asian and Pakistani Asian populations have an average reported incidence of 1.2 to 1.5% [24].The highest frequency worldwide was reported among Araucanos Indians in Chile (27.6%) [25].Most likely, this distinctive predisposition is due to the ethnic admixture with this South American Indian (ethnic) group.There have been theories suggesting local and geographical predisposition due to macro-and micro-element insufficiency or some other dietary deficiencies.However, it has to be recognized that one of possible explanation for such extensive and severely different prevalence (0.2% to 28%) might be a genetic predisposition.

Year interval variations
The incidence rate of intrahepatic cholestasis also varies annually, with most cases occurring more frequently in the winter months.There is also a higher risk of ICP in subsequent pregnancies in women with a history of this disease in previous pregnancies [2].To date, there is not enough medical and scientific evidence to explain the seasonal occurrence of ICP.However, the recurrence of the disease in subsequent pregnancies might also prove the gene theory of the disease.

Hormonal theory
Hepatobiliary transport proteins, as well as pregnancyrelated rising estrogen and progesterone levels, may play a significant role in the etiology of intrahepatic cholestasis of pregnancy.Usually, ICP shows its first symptoms when these hormones reach their peak levels, which fall in the late second and third trimesters.Peak levels of hormones may trigger ICP in patients with genetic susceptibility.The occurrence of gene mutations in Caucasian populations with ICP is as high as 16%.The genetic theory of ICP is further supported by familial clustering, increased risk in first-degree relatives, and a high recurrence rate [21,26].Particular progesterone metabolites, which are raised during normal pregnancy, can also interact with bile acid signaling pathways, which may also influence the cholestatic phenotype.Progesterone metabolites are also elevated in intrahepatic cholestasis during pregnancy [21,26].Several studies demonstrated that administration of natural progestin to women with threatened preterm labor resulted in a higher risk of ICP [27] Furthermore, in twin pregnancies, progesterone levels are on average higher than singletons and have a higher frequency of cholestasis (20.9% vs. 4.7% in singletons) [28].It has been suggested that in some genetically predisposed women, the formation of large amounts of progesterone metabolites during pregnancy results in the supersaturation of the hepatic transport system utilized for biliary elimination [29].
In addition, 17a-estradiol can induce pregnancyinduced cholestasis by trans-inhibiting the tubulomembrane targeting activity of the bile salt export pump (BSEP).BSEP is the major transporter for the secretion of bile acids from hepatocytes into bile in humans and is expressed primarily in the liver [30].The metabolites of 17a-estradiol are then exported from multidrug resistance-associated protein 2 (MRP2) to the tubule lumen.Multidrug resistance-associated protein mutations are also closely associated with ICP, and approximately 16% of ICP cases are caused by mutations in this gene [31].

Diet and ICP
Specific causal factors in the environment, particularly in diet, have not yet been identified.Different eating habits and nutritional deficiencies may alter the oxidative metabolism in the liver.Some researchers have reported that serum selenium concentrations were significantly lower in women with ICP than in women with normal pregnancies during the last trimester of pregnancy and postpartum.The same study reported a significant positive correlation between the activity of the selenoenzyme glutathione peroxidase and the selenium concentration.In addition, intrahepatic cholestasis of pregnancy is common in the Scandinavia and Chile regions, which have a low dietary intake of selenium [32].A low vitamin D concentration has also been reported in women with ICP, although its role is yet to be defined.Bile acid, estrogen, and progesterone metabolism or elimination in the liver and intestine is partly regulated by the vitamin D nuclear receptor (VDR).Therefore, vitamin D might play a major role in binding to its receptors.VDR controls calcium homeostasis, but also regulates important phases of bile acid detoxification [33].As pregnancy advances, several nutritional concentrations usually decrease, but this causes no harm if the body's storage and nutritional intake are normal; therefore, the risk of ICP may increase in cases of chronic nutritional insufficiency.

Liver enzymes activity
The liver is a critical organ in the human body that is responsible for metabolism, immunity, digestion, detoxification, vitamin storage and many other.The major route of liver metabolism and detoxification is through biotransformation.Changes in enzyme activity in liver cells in the course of hepatic diseases are reflected by the alterations of its activities.Some researchers found that the total alcohol dehydrogenase (ADH) activity was elevated in the serum of women with ICP [34].In particular alcohol dehydrogenase isoenzyme (ADH I) might be a promising marker of intrahepatic cholestasis of pregnancy [35].

Genetic susceptibility in ICP
A genetic predisposition for intrahepatic cholestasis during pregnancy has been proposed by many authors.Traditionally, ICP has been suggested to be inherited as a sex-limited dominant phenotype [36,37].Some reports specify a significant genetic factor in the pathophysiology [31,38].

Variants with low populational frequency
Biallelic mutations in particular hepatobiliary transporters have been recognized as essential in a number of severe pediatric hepatic diseases.These mutations are known as progressive familial intrahepatic cholestasis syndromes -PFIC [39].It is a genetically heterogeneous condition caused by deficiencies in bile acid transport.For these medical conditions, homozygous or compound heterozygous mutations in 12 genes have been reported to date, while ATP8B1 (a phosphatidyl serine flippase), ABCB11 (the bile salt export pump), and ABCB4 (a phosphatidyl choline floppase) have also been identified in ICP.There have been many reports on the canalicular transporters ABCB4 and ABCB11.A significant number of reports have extended the range of mutant alleles in ABCB4 associated with ICP [21].Mutations in this gene are responsible for approximately 15% of ICP cases [40] referred to as intrahepatic cholestasis of pregnancy-3 (ICP3; MIM 614972).ABCB4 changes have also been described in women who developed cholestasis induced by the use of hormonal contraception [41].
Approximately 5% or more ICP cases may be a result of a mutation in ABCB11 gene [21].Most reports have investigated ABCB11 and ABCB4 mutations across the European population (Germany, Sweden, Finland, France, and Great Britain), suggesting their role in ICP [42][43][44][45].Among the variants of interest, the common valine 444 alanine (V444A) polymorphism (rs2287622) of ABCB11 has been concluded to be a significant risk factor for ICP in the European population [42,46].
Other studies suggest a possible role for ATP8B1 mutations in limited cases (defined as intrahepatic cholestasis of pregnancy-1 (ICP1), MIM 147480) and a possible role for ABCC2, which encodes an integral membrane glycoprotein [47].It is expressed mainly in the apical membrane of liver cells and is part of the ATP-binding cassette transporter group.It plays a physiological role in the transport of endogenous and exogenous anionic conjugates from hepatocytes to the bile.The same group of genes was the target of interest in a recently published study by Liu et al. [48].The Authors used genotyping technologies (WES) in a cohort of 151 pregnant women with no other liver diseases who were diagnosed with ICP (based on skin pruritus and abnormal liver biochemistry results, e.g.TBA, ALT, and AST [aspartate transaminase]) and a TBA cutoff level of TBA of 10 lmol/l.Seven novel possible pathogenic mutations were identified within the known functional genes, including ABCB4 (variants Trp708Ter, Gly527Glu, and Lys386Glu), ABCB11 (variants Gln1194Ter, Gln605Pro, and Leu589Met), and ABCC2 (Ser1342Tyr variant).Interestingly, a genotypephenotype correlation was noted, which indicated higher average values of total bile acids (TBA), aspartate transaminase (AST), direct bilirubin (DBIL), total cholesterol (CHOL), triglycerides (TG), and high-density lipoprotein (HDL) in women with two mutations in ABC family genes than in those with one mutation, no ABC gene mutation, and local controls.
Moreover, several other loci have been inspected, usually in small reports, with low power to detect anything other than Mendelian-like effects [21].There are also associations between NR1H4 and TJP2 genes [49].The NR1H4 gene encodes a nuclear bile acid receptor named the farnesoid X receptor (FXR), which detects bile acid concentration.Increased bile acid levels activate FXR to stimulate a mechanism that limits bile acid biosynthesis in the hepatocytes.
TJP2 encodes tight junction protein-2, which is a member of a family of membrane-associated guanylate kinase (MAGUK) homologs.MAGKUK is thought to play an important role in epithelial and endothelial intercellular junctions.

Alleles with higher populational frequency
Some authors, by analogy with cholestasis observed in children with inherited alpha-1 antitrypsin deficiency, hypothesized that the SERPINA1 PI � Z deficiency variant might be linked to a higher risk of cholestasis in pregnancy [50].The significance of rs28929474 (Z allele, an amino acid substitution of Glu342Lys) in SERPINA1, which encodes alpha-1-antitrypsin, was also supported by data from a GWAS meta-analysis [51].In addition, this study revealed two other missense variants of the GCKR (glucokinase regulatory protein) and HNF4A (hepatic nuclear factor 4 alpha) genes, which encode proteins essential for hepatic metabolic homeostasis and are associated with ICP susceptibility.The rs1260326 (Leu446Pro) variant in GCKR is associated with altered serum fasting plasma glucose and triglyceride concentrations and nonalcoholic fatty liver disease (NAFLD).The rs1800961 (Thr139Ile) variant in HNF4A has been defined as deleterious to protein function based on MetaDome analysis [52].

Literature-based recommendation for genetic testing
The currently available data provide evidence that genetic testing should be considered in the assessment of pregnancies with ICP.Stronger support indicates that ABCB4, ABCB11, ABCC2, ATP8B1, NR1H4 have been analyzed [53][54][55].For other genes, further research should be conducted -TJP2 [53], ANO8 [56] and FXR [57].Genetic susceptibility of ICP is summarized in Table 2.Moreover, based on our experience, we suggest that the SERPINA1 Z allele should be considered, especially in families affected with unspecified liver disease, asthma, or emphysema.The clinical management of genetic testing in intrahepatic cholestasis of pregnancy in high-risk patients for the genetic origin of cholestasis is presented in Flowchart 1.In cases of intrahepatic cholestasis of pregnancy coexisting with symptoms presented in the Flowchart additional genetic testing might be recommended.

Summary
Intrahepatic cholestasis during pregnancy has a variable prevalence and is the most common pregnancy-related liver disorder.The clinical course of ICP is usually benign; however, fetal complications (including preterm labor, fetal distress, and stillbirth) may lead to significant perinatal morbidity and mortality.Postprandial TBA levels are required to identify high- Fanconi renotubular syndrome 4, with maturity-onset diabetes of the young/AD; MODY, type I/AD; (Diabetes mellitus, noninsulin-dependent)/AD AD: autosomal dominant; AR: autosomal recessive.Monitoring of subsequent pregnancy þ ICP risk for other female family members (in AD phenotypes) or risk of inherited condition not directly related to pregnancy -AATD diabetes (both AR and AD inheritance)-> rationale for genetic counseling to discuss family genetic testing.
risk ICP pregnancies (TBA �40 lmol/l).The postprandial rise in TBA in normal pregnancies indicates that a non-fasting threshold of �19 lmol/l improves diagnostic accuracy.Total serum bile acids above 100 lmol/l are associated with an approximately 10-fold higher risk of sudden intrauterine fetal death.On the other hand, prenatal exposure to high bile acids (programming effect) may also have an important influence on the metabolic health of adolescent offspring [58].
The underlying cause of intrahepatic cholestasis in pregnancy remains unclear, but more evidence is needed to support the genetic theory behind this disease.Therefore, keeping in mind the abovementioned potential risk for pregnancy outcome, we suggest including genetic testing in pregnancies with a previous history of ICP or clinical symptoms, which may suggest a genetic background of the disease.The prevalence of genetic mutations in Caucasian patients with ICP has been reported to be 16%.Genetic susceptibility is supported by evidence of familial clustering, increased risk in first-degree relatives, and a high recurrence rate [26].However, despite the origin of the disease (genetic background, environmental, or other), clinical management remains the same, which in most cases is iatrogenic preterm delivery as the only treatment to prevent unfavorable maternal and fetal outcomes.However, knowledge about genetic variants may be beneficial for future screening protocols and scientific purposes.

Table 1 .
Risk factors related to intrahepatic cholestasis of pregnancy.

Table 2 .
Genetic susceptibility in ICP.