The bile duct development may not be fully completed at birth, and this is quite a common event. Moreover, bile formation is immature, and there is a propensity for the neonate to develop cholestasis in the presence of a wide variety of insults that can damage the liver. A biliary atresia is a correctable form of infantile cholangiopathies. The Kasai hepatic portoenterostomy (HPE) is often performed and is successful if it is done at an early stage. However, HPE can fail, and the liver fate is inevitably a cirrhotic change. Biliary atresia is heterogeneous and may result from a combination of genetic factors, vascular, infective or toxic insults with activation of different genetic and immunological pathways. In this chapter, we will review some genes that may be highly relevant to biliary atresia, including not only PKHD1, JAG1, and CFTR, but also GPC1, ADD3 and others. Four genetic loci are considered as predisposition loci in biliary atresia, despite the absence of an etiologic mutation. The rare occurrence of biliary atresia in well-known genetic syndromes seems to suggest coincidental finding, but epigenetic aspects might play a significant role in contributing to the increase of biliary atresia rate.
- biliary atresia
- kasai failure
Biliary atresia (BA) is a necroinflammatory process of the intrahepatic (inside of the liver) and extrahepatic (outside of the liver) biliary system with a various etiologic background. Since the introduction of the yellow card registries in Taiwan in 2002 as a pilot study and then in other countries, the awareness and the proper management of this disease has increased. In the Western countries, BA occurs in about 1:18,000 live births, but the reported incidences vary from 5 to 32 per 105 live births . The incidence of BA is highest in Asia and in some countries of the Pacific region comparing to the rest of the world with male infants more often affected than females. The clinical triad includes neonatal jaundice (conjugated hyperbilirubinemia that is considered lasting beyond 2 weeks of postnatal life), acholic (pale) stools and dark urine, and hepatomegaly. The ultrasonography and the endoscopic retrograde cholangiopancreatography (ERCP) are essential to address the diagnosis, which relies on the microscopic examination. Histologically, BA relies on the inflammatory injury of the intra- and extrahepatic biliary tract with sclerosis, luminal narrowing, and final obliteration of the biliary tree. BA lead to liver cirrhosis, if no treatment is initiated. Also, death can occur within the 2 years of life or first infancy. Although BA is, currently, not known to be hereditary, some families with recurrence of this disease have been reported in the literature. BA requires an immediate shunt to promote the discharge of bilirubin from the progressive accumulation in the liver and Professor Morlo Kasai (1922–2008) invented a technique (Kasai hepatic portoenterostomy or HPE), which is currently in use and the first line of surgical therapy for these patients [2, 3, 4]. Subsequently, a liver transplantation may be needed, particularly in infants who underwent to surgery not early in life as it should be. The delay of the surgical hepatic portoenterostomy is still a problem in some regions not only in underdeveloped countries, but even in USA and Canada. Thus, the awareness of this condition is crucial to save lives and money. Liver transplantation may be crucial needed to restore the flow of bile or if liver cirrhotic complications occur (Figure 1). Currently, nearly 90% of BA infants survive. The majority of these patients have normal quality of life. Prof. Kasai is considered one of the greatest innovators in the field of pediatric surgery and numerous thousands of babies and their families will be always grateful to him.
2. Bile duct development
The development of the biliary tree is crucial to understand and interpret the categories of neonatal and infantile cholangiopathies. This aspect is particularly important if the patient is a preterm baby or a baby considered small for gestational age . At the 3rd week of the post-ovulatory period, endodermal cells sprout from the primitive foregut at the cranial portion. These cells grow towards a loosely arranged mesoderm. The direction is the
3. Liver biopsy
Currently, the liver biopsy remains the gold standard for determining the diagnosis of surgical conditions with neonatal cholestasis, such as BA, which is the most critical surgically correctable form of persistent conjugated hyperbilirubinemia. There is an incredible rate of accuracy that comes with the experience of the pathologist and liver biopsy is precise in probably more than 9 cases out of 10, in most tertiary centers of pediatric healthcare, provided that the liver tissue encompasses at least six portal tracts. The experience of the pathologist may be crucial to address the patient to a pediatric surgeon or genetic counseling for neonatal cholestatic liver diseases. The challenge may be quite high that some argued that there might be a continuum with a single underlying cause. According to our previously published data , patterns of ductular reactions may muddle the pathologist, and ductal plate remnants, often seen in the liver histology, may indeed recapitulate the embryonic anlage. As a pediatrician and pathologist, the urgency of critical reports is well understood . Medical and surgical causes of neonatal cholestasis need to be ruled out promptly. The central differential diagnoses of BA that need to be kept in mind are: Alagille syndrome, alpha-1-antitrypsin deficiency, cystic fibrosis, sclerosing cholangitis with neonatal onset, and more rarely progressive familial intrahepatic cholestasis (PFIC) types I-III. The differential diagnosis may be complicated and ancillary studies may need mass spectroscopy or other sophisticated techniques.
As indicated early, BA is usually not hereditary, but some cases and families with recurrence rate have been observed . Four genetic loci are considered as predisposition loci in BA, although no etiologic mutation was identified. The rare occurrence of BA in well-known genetic syndromes seems to suggest coincidental finding, but epigenetic aspects might play a major role in contributing to the increase of BA rate. Chromosomal alterations reported in patients with BA include duplication, deletion, and single copy number variation. Duplications include trisomy 18, trisomy or tetrasomy 22, trisomy 21, trisomy 10q and trisomy 11q23. In trisomy 18, BA has been associated in individuals with congenital heart disease and facial dysmorphism. In trisomy or tetrasomy 22, cat eye syndrome occurs. Facial dysmorphism, congenital heart disease, and cleft palate occurred in trisomy 11q23, while coarctation of the aorta, anal anteposition, and mental retardation have been recorded in patients harboring trisomy 10q. Patients with trisomy 21 syndrome (Down syndrome) showed accompanying atresia of the duodenum and esophagus as well as a heterotaxic setting, which is a condition in which the internal organs are abnormally arranged in the abdomen and chest. Deletion of chromosomal portions has been identified on 18p, 2q73.3, 18q21, 17q12, 1p36, and 20p11.21. Facial dysmorphism, mental retardation, hypothyroidism, and polysplenia were founds in deletion of the 2q37.2, while hyperechogenic kidney and pancreatic hypoplasia were found in the deletion of 17q12. Facial dysmorphism and classic Pitt-Hopkins syndrome were found in the deletion of 1p36 and 18q21, respectively. Laterality defects and panhypopituitarism were findings in the setting of deletion of 20p11.21. Pitt-Hopkins syndrome is a condition characterized by developmental delay and intellectual disability with individuals with breathing problems, recurrent seizures, and distinctive facial features [24, 25, 26, 27]. There is a delay in the development of mental and motor skills with numerous affected individuals having features of autistic spectrum disorders. Copy number variations (CNVs) are defined as a type of structural variation: specifically, i.e., a type of duplication or deletion event that affects a considerable number of base pairs. CNVs are a phenomenon in which sections of the genome are repeated, and the number of repeats in the genome varies between individuals in the human population with a specific neurological disorder as seen in Huntington’s disease. In this disease, CAG (i.e., cytosine-adenine-guanine) repeats of less than 26 have no effect, but between 27 and 35 there is a risk for the offspring, 36–29 CAG repeats may (or may not) be affected by the disease but harbors 50% of risk to offspring, while 40 or more has a consequence with full penetrance of the disease and a risk of 50% to offspring. CMVs have been identified in a cohort of patients with BA with 29 specific CNVs involving the
The original dichotomic description of BA in embryonic and fetal type or syndromic and non-syndromic forms is still valid . The reader may need to keep in mind that most probably the term “atresia” is a misnomer and a necroinflammatory process of obliterative nature is different from an atretic process in origin from the embryologic point of view. In consideration of the period in which “atresia” occurs, it may be classified as embryonic or fetal and perinatal. In approximately 20% of patients with BA, the embryonic form is responsible, while the rest is due to fetal form, which is also called perinatal form. In the early form, the extrahepatic biliary tree might have undergone abnormal morphogenetic processes. BA patients suffering from this form of BA have associated non-hepatic structural anomalies, of which the most common multiple congenital anomalies is the polysplenia syndrome, which is found in 8–12% of patients with BA. This syndrome is characterized by polysplenia/asplenia associated with cardiac defects (dextrocardia, tetralogy of Fallot, anomalies of the pulmonary vasculature, atrio-/ventricular septal defects) a midline liver, preduodenal portal vein, interruption of the inferior vena cava, and
If about 10% of patients with BA have multiple congenital anomalies, it has been argued that the laterality defects have a phenotype like that detected in the ciliopathies, which is a heterogeneous group of disorders caused by structural and functional abnormalities in genes that encode cilia proteins [23, 50, 51, 52, 53, 54, 55]. Embryologically, cilia are evolutionarily conserved. The cilia support in founding the left-right axis in vertebrates and are present on numerous cell types. The cilia are also present in the apical surface of biliary cells or cholangiocytes. In these cells, cilium function is integral to bile flow, bile ductular maturation, and neoductular formation . The
There is quite an overwhelming evidence against a Mendelian paradigm of inheritance of BA. The argument against a Mendelian inheritance is supported by the lack of familial penetrance, despite rare cases of familial BA [52, 53], and discordant presentation of BA among twins, including monozygotic twins [54, 55, 56, 57]. On the other side, a growing principle of developmental diseases exhibits unusual patterns of inheritance, including Bardet-Biedl syndrome. The Bardet-Biedl syndrome is a ciliopathy, which is clinically heterogeneous and specifically marked by oligogenic inheritance. The mutations in multiple genes cooperate to generate the phenotype of Bardet-Biedl syndrome. It has been suggested that a similar singularity related to gene-gene interactions, or epistasis, may account for the diversity of presentations in BA . Non-Mendelian inheritance is also detected in Alagille syndrome, which is the result of a haploid insufficiency of
Recently, genome-wide association studies (GWASs) have been milestones in identifying some genes and pathways of several diseases. A GWAS is an observational study of a genome-wide set of genetic variants in diverse individuals to understand if any modification is associated with a trait. Generally, GWASs emphasize the associations between single-nucleotide polymorphisms (SNPs) and traits like major human diseases but can equally be useful for any other genetic variants and any other organisms. SNPs are DNA sequence variations occurring probably 1 in every 100, 200–300 bases along the 3-billion-base human genomic sequence and at least 1% of the population. SNPs make up about 90% of DNA sequence variation as indicated by the Human Genome Project (http://www.ornl.gov, http://linkage.rockefeller.edu/soft/). On the other side, if one of the possible sequences is present in less than 1% of the population (99.9% of people have a C, and 0.1% have a G), then the variation is called a mutation [57, 58].
The Han Chinese population is particularly susceptible to GWAS because of the large size of individuals and homogeneity of this population. A recent study using a GWAS found a potential susceptibility locus for BA between the genes ADD3 and XPNPEP1 located with the chromosomal localization of 10q25.1 with replication in independent Chinese and Thai specimens and identification of BA in a Zebrafish model [59, 60, 61]. Interestingly, sequencing of a Han Chinese sample identified that a 5-SNP risk haplotype to be associated with BA and the genotype correlated with reduced levels of
The GWAS was also able to identify an additional putative gene, which is called glypican 1 (
5. Epigenetics and genetic modifiers of biliary atresia
Epigenetic modification (e.g., methylation) may be at the basis of some BA patients . Epigenetics is the investigation of inherited variations in the expression of genes that do not involve deviations to the underlying DNA sequence. It means that there is a change in phenotype without evidence of a deviation in genotype. Epigenetics specifically affects how cells translate the genes. There is a natural occurrence of epigenetics, but several factors including age, lifestyle, environment, and disease state influence the epigenetic flow in an organism. Epigenetic changes can affect how cells terminally differentiate or how cancer can occur. There are at least three systems including DNA methylation, modification of histones, and non-coding RNA (ncRNA)-associated gene silencing that are considered to initiate and endure epigenetic change. Epigenetic studies in BA may be particularly challenging because hepatocyte or cholangiocyte DNA of patients may be needed. It has been hypothesized that some genes that are relevant for the biliary function may be involved. These genes may be
6. Non-genetic contributive factors of biliary atresia
It would be incomplete this chapter, if BA would not be discussed exploring the non-genetic causes of this condition. Although poorly understood, some non-genetic factors may contribute to the development of this surgically correctable neonatal biliary disease. There are several categories, including viruses, toxins, immunological dysregulations, and feto-maternal factors [1, 78, 79, 80, 81, 82]. Cytomegalovirus (CMV) is a genus of viruses in the order
7. Final remarks and future perspectives
BA remains a very challenging disease not only from the clinical point of views but also for the pathological one and transcriptome analysis cannot provide a univocal answer . The importance to differentiate surgically correctable cholangiopathies in the neonatal age is crucial for the prognosis of these patients. The introduction of the Kasai HPE has revolutionized the outcome of patients harboring this condition. The timing when the Kasai HPE is performed has been emphasized as critical. Alternatively, liver transplantation is the only hope for liver cirrhosis developed on the ground of a BA. The underlying cause(s) and outcome contributor(s) to BA remain poorly understood, but new technologies are on the front to target this field. Two genes that we currently consider of significant interest for BA are
This work is dedicated to the career of Prof. Dr. J. Hager (Figure 4), formerly Director of the Department of Pediatric and Youth Surgery, of the University of Innsbruck, Austria. Prof. Hager was born in 1946 and his career has been fulfilled of enormous and vivid achievements as well as honors, such as the European pediatric surgery diploma, Theodor-Billroth Prize of the Austrian Society for Surgery, and the Spitzy Prize. He has more than 150 peer-reviewed publications with countless lectures and presentations. His mentoring and teaching activity resulted in numerous fellows, who came out from his school in Innsbruck.