Genetics, pathobiology and therapeutic opportunities of polycystic liver disease

Sex, Genotype, and Liver Volume Progression as Risk of Hospitalization Determinants in Autosomal Dominant Polycystic Liver Disease

BACKGROUND & AIMS

Autosomal dominant polycystic liver disease is a rare condition with a female preponderance, based mainly on pathogenic variants in 2 genes, PRKCSH and SEC63. Clinically, autosomal dominant polycystic liver disease is characterized by vast heterogeneity, ranging from asymptomatic to highly symptomatic hepatomegaly. To date, little is known about the prediction of disease progression at early stages, hindering clinical management, genetic counseling, and the design of randomized controlled trials. To improve disease prognostication, we built a consortium of European and US centers to recruit the largest cohort of patients with PRKCSH and SEC63 liver disease.

METHODS

We analyzed an international multicenter cohort of 265 patients with autosomal dominant polycystic liver disease harboring pathogenic variants in PRKCSH or SEC63 for genotype-phenotype correlations, including normalized age-adjusted total liver volumes and polycystic liver disease-related hospitalization (liver event) as primary clinical end points.

RESULTS

Classifying individual total liver volumes into predefined progression groups yielded predictive risk discrimination for future liver events independent of sex and underlying genetic defects. In addition, disease severity, defined by age at first liver event, was considerably more pronounced in female patients and patients with PRKCSH variants than in those with SEC63 variants. A newly developed sex-gene score was effective in distinguishing mild, moderate, and severe disease, in addition to imaging-based prognostication.

CONCLUSIONS

Both imaging and clinical genetic scoring have the potential to inform patients about the risk of developing symptomatic disease throughout their lives. The combination of female sex, germline PRKCSH alteration, and rapid total liver volume progression is associated with the greatest odds of polycystic liver disease-related hospitalization.

Polycystic liver diseases: from molecular basis to development of effective treatments

Polycystic liver diseases (PLDs) comprise a heterogeneous group of congenital genetic disorders that mainly affect bile duct epithelial cells, known as cholangiocytes. Patients with PLD usually present bile duct dilatation and/or progressive develop intrahepatic, fluid-filled biliary cysts (more than 10), which is the main cause of morbidity.

Panic at the Bile Duct: How Intrahepatic Cholangiocytes Respond to Stress and Injury

In the liver, biliary epithelial cells (BECs) line intrahepatic bile ducts (IHBDs) and are primarily responsible for modifying and transporting hepatocyte-produced bile to the digestive tract. BECs comprise only 3% to 5% of the liver by cell number but are critical for maintaining choleresis through homeostasis and disease. To this end, BECs drive an extensive morphologic remodeling of the IHBD network termed ductular reaction (DR) in response to direct injury or injury to the hepatic parenchyma. BECs are also the target of a broad and heterogenous class of diseases termed cholangiopathies, which can present with phenotypes ranging from defective IHBD development in pediatric patients to progressive periductal fibrosis and cancer. DR is observed in many cholangiopathies, highlighting overlapping similarities between cell- and tissue-level responses by BECs across a spectrum of injury and disease. The following core set of cell biological BEC responses to stress and injury may moderate, initiate, or exacerbate liver pathophysiology in a context-dependent manner: cell death, proliferation, transdifferentiation, senescence, and acquisition of neuroendocrine phenotype. By reviewing how IHBDs respond to stress, this review seeks to highlight fundamental processes with potentially adaptive or maladaptive consequences. A deeper understanding of how these common responses contribute to DR and cholangiopathies may identify novel therapeutic targets in liver disease.

Simultaneous Echinococcal superinfection in a patient with polycystic liver disease

Key Clinical Message

Concurrent polycystic liver disease and echinococcus infection can hinder diagnosis. Surgery may be needed for accurate diagnosis and treatment. Multidisciplinary collaboration is crucial.

Abstract

Cystic echinococcosis, caused by Echinococcus granulosus eggs, is a parasitic zoonosis that typically affects humans through accidental ingestion. Polycystic liver disease is a condition characterized by the presence of multiple liver cysts and is often associated with polycystic kidney disease. Here, we present a case of a man in his 70s with a pre-existing diagnosis of polycystic liver disease. Radiological findings of a suspicious cyst in the S4 segment initially lacked serological evidence of echinococcosis; however, intraoperative confirmation revealed the presence of an echinococcal cyst. This article aims to explore both clinical conditions and highlight the therapeutic considerations for their management. Moreover, we discuss the significance of this unique case, emphasizing the possibility of the coexistence of these two pathologies.

A TGFβ-ECM-integrin signaling axis drives structural reconfiguration of the bile duct to promote polycystic liver disease

The formation of multiple cysts in the liver occurs in a number of isolated monogenic diseases or multisystemic syndromes, during which bile ducts develop into fluid-filled biliary cysts. For patients with polycystic liver disease (PCLD), nonsurgical treatments are limited, and managing life-long abdominal swelling, pain, and increasing risk of cyst rupture and infection is common. We demonstrate here that loss of the primary cilium on postnatal biliary epithelial cells (via the deletion of the cilia gene Wdr35) drives ongoing pathological remodeling of the biliary tree, resulting in progressive cyst formation and growth. The development of cystic tissue requires the activation of transforming growth factor-β (TGFβ) signaling, which promotes the expression of a procystic, fibronectin-rich extracellular matrix and which itself is perceived by a changing profile of integrin receptors on the cystic epithelium. This signaling axis is conserved in liver cysts from patients with either autosomal dominant polycystic kidney disease or autosomal dominant polycystic liver disease, indicating that there are common cellular mechanisms for liver cyst growth regardless of the underlying genetic cause. Cyst number and size can be reduced by inhibiting TGFβ signaling or integrin signaling in vivo. We suggest that our findings represent a therapeutic route for patients with polycystic liver disease, most of whom would not be amenable to surgery.

Heterozygosity of ALG9 in Association with Autosomal Dominant Polycystic Liver Disease

α-1,2-mannosyltransferase (ALG9) germline variants are linked to autosomal dominant polycystic kidney disease (ADPKD). Many individuals affected with ADPKD possess polycystic livers as a common extrarenal manifestation. We performed whole exome sequencing in a female with autosomal dominant polycystic liver disease (ADPLD) without kidney cysts and established the presence of a heterozygous missense variant (c.677G>C p.(Gly226Ala)) in ALG9. In silico pathogenicity prediction and 3D protein modeling determined this variant as pathogenic. Loss of heterozygosity is regularly seen in liver cyst walls. Immunohistochemistry indicated the absence of ALG9 in liver tissue from this patient. ALG9 expression was absent in cyst wall lining from ALG9- and PRKCSH-caused ADPLD patients but present in the liver cyst lining derived from an ADPKD patient with a PKD2 variant. Thus, heterozygous pathogenic variants in ALG9 are also associated with ADPLD. Somatic loss of heterozygosity of the ALG9 enzyme was seen in the ALG9 patient but also in ADPLD patients with a different genetic background. This expanded the phenotypic spectrum of ADPLD to ALG9.

Genetic Spectrum of Polycystic Kidney and Liver Diseases and the Resulting Phenotypes

Polycystic kidney diseases are a group of monogenically inherited disorders characterized by cyst development in the kidney with defects in primary cilia function central to pathogenesis. Autosomal dominant polycystic kidney disease (ADPKD) has progressive cystogenesis and accounts for 5-10% of kidney failure (KF) patients. There are two major ADPKD genes, PKD1 and PKD2, and seven minor loci. PKD1 accounts for ∼80% of patients and is associated with the most severe disease (KF is typically at 55-65 years); PKD2 accounts for ∼15% of families, with KF typically in the mid-70s. The minor genes are generally associated with milder kidney disease, but for DNAJB11 and ALG5, the age at KF is similar to PKD2. PKD1 and PKD2 have a high level of allelic heterogeneity, with no single pathogenic variant accounting for >2% of patients. Additional genetic complexity includes biallelic disease, sometimes causing very early-onset ADPKD, and mosaicism. Autosomal dominant polycystic liver disease is characterized by severe PLD but limited PKD. The two major genes are PRKCSH and SEC63, while GANAB, ALG8, and PKHD1 can present as ADPKD or autosomal dominant polycystic liver disease. Autosomal recessive polycystic kidney disease typically has an infantile onset, with PKHD1 being the major locus and DZIP1L and CYS1 being minor genes. In addition, there are a range of mainly recessive syndromic ciliopathies with PKD as part of the phenotype. Because of the phenotypic and genic overlap between the diseases, employing a next-generation sequencing panel containing all known PKD and ciliopathy genes is recommended for clinical testing.

NOTCH signalling - a core regulator of bile duct disease?

The Notch signalling pathway is an evolutionarily conserved mechanism of cell-cell communication that mediates cellular proliferation, fate determination and maintenance of stem/progenitor cell populations across tissues. Although it was originally identified as a critical regulator of embryonic liver development, NOTCH signalling activation has been associated with the pathogenesis of a number of paediatric and adult liver diseases. It remains unclear, however, what role NOTCH actually plays in these pathophysiological processes and whether NOTCH activity represents the reactivation of a conserved developmental programme that is essential for adult tissue repair. In this Review, we explore the concepts that NOTCH signalling reactivation in the biliary epithelium is a reiterative and essential response to bile duct damage and that, in disease contexts in which biliary epithelial cells need to be regenerated, NOTCH signalling supports ductular regrowth. Furthermore, we evaluate the recent literature on NOTCH signalling as a critical factor in progenitor-mediated hepatocyte regeneration, which indicates that the mitogenic role for NOTCH signalling in biliary epithelial cell proliferation has also been co-opted to support other forms of epithelial regeneration in the adult liver.

Novel α-1,3-Glucosyltransferase Variants and Their Broad Clinical Polycystic Liver Disease Spectrum

Protein-truncating variants in α-1,3-glucosyltransferase (ALG8) are a risk factor for a mild cystic kidney disease phenotype. The association between these variants and liver cysts is limited. We aim to identify pathogenic ALG8 variants in our cohort of autosomal dominant polycystic liver disease (ADPLD) individuals. In order to fine-map the phenotypical spectrum of pathogenic ALG8 variant carriers, we performed targeted ALG8 screening in 478 ADPLD singletons, and exome sequencing in 48 singletons and 4 patients from two large ADPLD families. Eight novel and one previously reported pathogenic variant in ALG8 were discovered in sixteen patients. The ALG8 clinical phenotype ranges from mild to severe polycystic liver disease, and from innumerable small to multiple large hepatic cysts. The presence of <5 renal cysts that do not affect renal function is common in this population. Three-dimensional homology modeling demonstrated that six variants cause a truncated ALG8 protein with abnormal functioning, and one variant is predicted to destabilize ALG8. For the seventh variant, immunostaining of the liver tissue showed a complete loss of ALG8 in the cystic cells. ALG8-associated ADPLD has a broad clinical spectrum, including the possibility of developing a small number of renal cysts. This broadens the ADPLD genotype-phenotype spectrum and narrows the gap between liver-specific ADPLD and kidney-specific ADPKD.

Liver Only Living Donor Transplantation for Polycystic Disease in a Patient on Chronic Hemodialysis: Case Report

BACKGROUND

Polycystic liver disease (PLD) is characterized by the progressive development of polycystic lesions in the kidney and the liver, possibly resulting in dual organ failure. We indicated living donor liver transplantation (LDLT) for a patient with end-stage liver and kidney disease (ELKD) due to PLD on uncomplicated chronic hemodialysis.

CASE PRESENTATION

A 63-year-old man with ELKD and uncontrolled massive ascites due to PLD and hepatitis B on uncomplicated chronic hemodialysis was referred to us with a single possible 47-year-old female living donor. Because of the necessity of right lobe liver procurement from this small middle-aged donor and uncomplicated hemodialysis on this recipient, we considered LDLT, rather than dual organ transplantation, could be the most well-balanced option to save the life of this recipient with acceptable risk limits for this donor. A right lobe graft with 0.91 for graft recipient weight ratio was implanted with an uneventful operative procedure under intra- and postoperative continuous hemodiafiltration. The recipient was rescheduled on routine hemodialysis on day 6 after transplantation and recovered with a gradual decrease in ascites output. He was discharged on day 56. He continues to have a very good liver function and quality of life without ascites and uncomplicated routine hemodialysis 1 year after transplantation. The living donor was discharged 3 weeks after surgery and is also doing well.

CONCLUSION

Although combined liver-kidney transplantation from a deceased donor could be the best option for ELKD due to PLD, LDLT can also be an acceptable option for ELKD with uncomplicated hemodialysis, considering the double equipoise theory for both lifesaving of the recipient and acceptable donor risk.

Inhibition of deubiquitinase USP28 attenuates cyst growth in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease, which is characterized by progressive growth of multiple renal cysts in bilateral kidneys. In the past decades, mechanistic studies have entailed many essential signalling pathways that were regulated through post-translational modifications (PTMs) during cystogenesis. Among the numerous PTMs involved, the effect of ubiquitination and deubiquitination remains largely unknown. Herein, we identified that USP28, a deubiquitinase aberrantly upregulated in patients with ADPKD, selectively removed K48-linked polyubiquitination and reversed protein degradation of signal transducer and activator of transcription 3 (STAT3). We also observed that USP28 could directly interact with and stabilize c-Myc, a transcriptional target of STAT3. Both processes synergistically enhanced renal cystogenesis. Furthermore, pharmacological inhibition of USP28 attenuated the cyst formation both in vivo and in vitro. Collectively, USP28 regulates STAT3 turnover and its transcriptional target c-Myc in ADPKD. USP28 inhibition could be a novel therapeutic strategy against ADPKD.