ADPKD is caused by mutations in either Pkd1 or Pkd2 gene, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively. PC1 and PC2 are involved in the development and maintenance of kidney cells, and their mutations can lead to the growth of fluid-filled cysts [31].

Mutations of Pkd1 gene on chromosome 16p13.3 and Pkd2 gene on chromosome 4q22.1 account for almost 80% and 15% of ADPKD cases. The remaining 5–10% of ADPKD cases are not genetically determined or occur due to rare mutations at other loci [31]. Some cases of PKD can be explained by mutations in at least one of the endoplasmic reticulum protein-encoding genes. The loss of any of these genes, such as GANAB, DNAJB11, and ALG9, results in the production of non-functional PC1 [31,32,33].

GANAB, also known as Pkd3, encodes the alpha subunit of glucosidase II. The main function of glucosidase II is to promote protein folding by catalyzing the hydrolysis of glucose residues of immature glycoproteins. GANAB mutation can disrupt protein maturation and cell surface localization of PC1 and PC2 [34]. Studies have shown that GANAB variants cause mild polycystic kidney and liver cysts in most patients [35]. DNAJB11 is a co-factor of binding immunoglobulin protein (BiP), which is a major chaperone in the endoplasmic reticulum and regulates the folding, trafficking, and degradation of secreted and membrane proteins [36]. DNAJB11 deletion was shown to impair PC1 maturation and trafficking [36]. Likewise, heterozygous loss of function mutation of the ALG9 gene, which encodes an enzyme needed for adding specific mannose molecules to produce N-glycan precursors in the endoplasmic reticulum, can impair PC1 maturation and lead to the development of kidney and liver cysts [33].

Pkd1 or Pkd2 deletion promotes renal tubular cell proliferation, which was shown to be associated with higher intracellular concentrations of Ca2+37. PC2 mainly localizes on the endoplasmic reticulum, primary cilia, and plasma membrane, acts as a cation channel, and forms the PC1-PC2 complex in a 1:3 ratio [38, 39]. PC2 acts as an ion channel on the plasma membrane and allows a small but detectable Ca2+ influx in renal primary cilia; therefore, mutated PC2 is deemed to decrease intracellular Ca2+ concentration [40]. PC2 acts as a potassium channel in the endoplasmic reticulum to facilitate potassium–calcium counterion exchange for inositol trisphosphate–mediated endoplasmic reticulum Ca2+ release [41]. PC2 also directly functions as a calcium-activated, high-conductance ER channel mediating Ca2+ release from the endoplasmic reticulum [42], and Pkd2 knockout impairs Ca2+ release from the endoplasmic reticulum in kidney cells [41]. In addition, PC1 was shown to decrease Ca2+ leak from the endoplasmic reticulum and increase endoplasmic reticulum Ca2+ uptake [43, 44]. It has been hypothesized that PC1 may physically block cation transfer by PC2 [39, 45]. Membrane depolarization and increased intraciliary Ca2+ concentration both can activate monovalent cation transfer by PC2 39. In addition, PC2 is needed for PC1 localization in the cilia, and PC2 deletion not only promotes cystogenesis but also inhibits ciliary localization of PC1 [46]. Furthermore, Yao et al. reported that Pkd1 knockout can enhance PC2 expression by upregulating GRP94, an endoplasmic reticulum chaperone [47]. Enhancing Pkd2 expression in Pkd1-mutant cells may improve PC1 trafficking or promote the formation of heteromeric PC1-PC2 protein complexes (Table 1 and Fig. 1) [48].

The role of PLD-causing genes in cholangiocytes. As shown in the figure, PLD-causing genes are primarily involved in ciliogenesis and quality control of protein folding, transport, and maturation in the endoplasmic reticulum

The morphological assessment of hepatic cyst epithelium in patients with ADPKD illuminated that small (< 1 cm) hepatic cysts had normal epithelium, medium-sized (1–3 cm) hepatic cysts had rare or shortened cilia, and large (> 3 cm) hepatic cysts lacked both primary cilia and microvilli [19]. Normally, primary cilia are assumed to promote cellular quiescence and delay cell cycle progression to the S or M phase [49]. In addition, ciliary disassembly was shown to induce cell-cycle reentry [49]. Consistently, it was shown that decreased ciliogenesis in cancer cells enhances their proliferative capacity and promotes their invasive behavior [50].

The classical hypothesis for cyst formation claims that in addition to a germline inactivating mutation in one allele of the Pkd gene, there is somatic inactivation (referred to as the second hit) in another allele, causing the complete loss of polycystin expression. However, recent studies claimed that the function of the Pkd gene has a threshold for cystogenesis [51, 52]. Based on this hypothesis, complete loss of Pkd1 function is not required, and partial malfunctioning of Pkd1 is enough to induce cystogenesis [53]. Consistently, many individuals with ADPKD still have residual PC1 expression because they carry missense rather than inactivating mutations [54]. Thus, promoting the expression of the normal Pkd1 allele may improve ADPKD even in the presence of an abnormal allele. The type of mutation not only determines the development and penetrance of ADPKD but also explains the severity of cystogenesis [16]. A study with 129 participants with ADPKD revealed that mutation position and mutation type (truncating mutation: nonsense, frameshift, and splicing mutation; or non-truncating mutation: substitution) can affect the severity of hepatic cystogenesis, and patients with PKD1 nonsense mutations exhibit more severe hepatic cystogenesis [16]. Furthermore, in this study, ADPKD patients with Pkd1 nonsense mutation located closer to the 5ʹ end of Pkd1 gene were more likely to have a maximum diameter index value of hepatic cyst ≥ 6 cm [16].

ARPKD is caused by mutations in the polycystic kidney and hepatic disease 1 (Pkhd1) gene, which encodes fibrocystin/polyductin. Different variants of the Pkhd1 gene (missense and truncating mutations) cause most cases of ARPKD. The mRNA of Pkhd1 is alternatively spliced to generate multiple transcripts [55, 56]. Pkhd1 knockout was shown to promote cholangiocyte proliferation in vitro [57]. Furthermore, it was found that Pkhd1 knockout induces connective tissue growth factor (CTGF) production by cholangiocytes, which can induce hepatic fibrosis [57]. Similar to PC1, fibrocystin forms a complex with PC2 on the plasma membrane and participates in Ca2+ transfer [58]. Previously, it was found that the COOH terminal of fibrocystin interacts with the NH2 terminal of PC2. The lack of fibrocystin decreased PC2 expression, but Pkd2 deletion did not alter fibrocystin expression 59. These findings suggest that fibrocystin binds to PC2 and maintains its normal levels, thereby preventing cystogenesis (Table 1 and Fig. 1) [59].

In another study, it was shown that children with clinically moderate ARPKD had a mutation in the Dzip1l gene [60]. Similar to the Pkhd1 gene, the Dzip1l gene is involved in ciliogenesis [61]. Dzip1l deletion downregulated ciliogenesis or led to the formation of dysmorphic cilia in mice [61]. Dzip1l gene encodes a ciliary transition zone protein that is responsible for ciliary membrane translocation of PC1 and PC2 (Table 1 and Fig. 1) [60].

Mutations in PRKCSH or Sec63 genes have been implicated in the development of ADPLD [62]. PRKCSH or Sec63 mutations are found in approximately 40% of patients with isolated ADPLD [9]. PRKCSH and Sec63 genes encode glucosidase IIβ and SEC63p, respectively, and are involved in endoplasmic reticulum quality control [62]. They are responsible for carbohydrate processing and folding and translocation of newly synthesized glycoproteins [62]. As a chaperone-like molecule, glucosidase II binds to the C-terminal domain of PC2 and inhibits Herp-mediated ubiquitination and subsequent degradation of PC2 [62]. Likewise, PRKCSH or Sec63 deletion was shown to impair normal PC1 folding and accelerate its ubiquitination and proteasomal degradation [63]. Sec63 conducts the post-translational transport of proteins in the endoplasmic reticulum (Table 1 and Fig. 1) [64]. Consistently, proteasome inhibition by MG132 and carfilzomib, two proteasome inhibitors, markedly upregulated PC1 and promoted cyst-lining cell apoptosis [63].

Using whole-exome sequencing data from 102 unrelated patients, Choi et al. demonstrated that heterozygous loss of function mutations in 3 additional genes, ALG8, GANAB, and SEC61B, are also linked to ADPLD [32]. Using in vitro experiments, they also indicated that similar to PRKCSH and SEC63, ALG8, GANAB, and SEC61B are related to protein biogenesis pathway in the endoplasmic reticulum and loss of function mutation of each one of these genes results in defective maturation and trafficking of PC1 (Table 1 and Fig. 1) [32].

A recent study has shown that heterozygous mutations of the low-density lipoprotein receptor-related protein 5 (LRP5) gene, particularly p.R1188W variant, can lead to ADPLD; however, another study reported that some variants of LRP5, such as rs724159825, can also lead to ADPKD [65, 66]. Mechanistically, LRP5 mutations were shown to impair canonical signaling of Wnt3α and promote the expression of several proliferative genes such as adenomatous polyposis coli (APC), glycogen synthase kinase 3β (GSK3β), and leucine-rich repeat-containing G-protein-coupled receptor 5 (LGR5), transcription factor v-myc avian myelocytomatosis viral oncogene homolog (c-Myc), and cyclin D1 (Table 1 and Fig. 1) [66].

Currently, genetic screening is not widely used to confirm ADPKD, ARPKD, and ADPLD as their imaging characteristics and clinical presentations are distinct and there are few differential diagnoses [67]. On the other hand, already known disease-causing genetic mutations include a wide spectrum and still do not explain a considerable proportion of cases, particularly in ADPLD [68]. In addition, it has been shown that the affected gene or the type of mutation cannot significantly alter the phenotype of PLD [67]. Therefore, current guidelines do not recommend routine genetic testing for PLD [67].

However, genetic testing is not necessary to confirm ADPKD, ARPKD, and ADPLD or enough to rule out these diseases; it may help categorize patients and potentially identify those eligible for future modalities of genetic intervention. Furthermore, a recent study reported that genetic confirmation can predict the risk of hospitalization in both isolated and non-isolated PLD [69]. Specifically, the study indicated that mutation carriers were significantly younger when waitlisting for liver transplantation and first hospitalization compared to patients without genetic diagnosis; however, current imaging classifications could not differentiate between severe and moderate courses [69].

Genetic testing can also be helpful when patients come with atypical presentations, which mimic other diseases and make diagnosis complex for clinicians [68]. In addition, genetic testing is the last resort when patients present with clinical symptoms or complications, but their cyst number in imaging still does not satisfy the diagnostic criteria for ADPLD or ADPKD [68]. On the other hand, with recent findings and future advances toward the pharmacological and genetic interventions for ADPLD, ADPKD, and ARPKD, genetic testing can allow early diagnosis and management of these diseases. Early diagnosis and management can considerably improve patients’ outcome and prevent serious complications [68]. Therefore, future studies may define new applications for genetic testing of PLD.

Pkd1 and Pkd2 mutations have been linked to deregulated activation of proliferative signaling pathways. Indeed, decreased intracellular Ca2+ concentration following impaired function of PC2 is believed to be responsible for activating proliferative pathways 70. Intracellular Ca2+ depletion can activate adenylyl cyclase 5, which in turn upregulates intracellular cyclic adenosine monophosphate (cAMP) levels [70]. Increased cAMP can subsequently overactivate protein kinase A (PKA)/Ras/extracellular signal-regulated kinases (ERK)/hypoxia-inducible factor α (HIF-α) pathway, promote vascular endothelial growth factor A (VEGF-A) expression, and enhance angiogenesis for cholangiocyte proliferation [71, 72]. Consistently, adenylyl cyclase 5 inhibition and knockout both significantly reduced hepatic cystogenesis in Pkd knockout mice [70]. Likewise, VEGF receptor inhibition was shown to inhibit liver cyst growth in pkd2 (WS25/ −) mice [73], and serum levels of VEGF were positively correlated with total cyst volume but negatively correlated with creatinine clearance in patients with ADPKD [74]. Moreover, PKA inhibition in liver cyst epithelial cells decreased VEGF expression and ERK1/2 activation [71]. ERK inhibition also reduced the proliferation of liver cyst epithelial cells [71].

Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway is also aberrantly activated in ADPKD and contributes to epithelial cell proliferation [75, 76]. It was shown that JAK2 expression strongly increases in ADPKD and JAK2 blockade reduces cyst growth. JAK2 is a key kinase that most likely contributes to cyst growth by activating STAT as a transcription factor [77].

Similar to the JAK/STAT signaling pathway, dysregulated mechanistic target of rapamycin (mTOR), Wnt, and Hippo signaling pathways have also been implicated in the pathogenesis of ADPKD. It was shown that the mTOR pathway is abnormally activated in cyst-forming epithelial cells in patients with ADPKD and in the mice model of ADPKD [78]. Rapamycin, an mTOR inhibitor, was shown to effectively suppress cystogenesis in two mouse models of PKD. Moreover, treatment with rapamycin markedly decreased native polycystic kidney size in patients with ADPKD who received kidney transplants [78].

Similarly, it has been indicated the lack of PC2 can overactivate the Wnt/β-catenin pathway in murine embryonic fibroblasts, renal epithelia, and isolated collecting duct cells [79]. In addition, inhibition of the Wnt/β-catenin pathway prevented renal cyst formation and prolonged survival in a mice model of ADPKD [79]. Similarly, non-canonical Wnt/planar cell polarity (PCP) pathway has been implicated in the proliferative response after Pkhd1 mutation in ARPKD [80]. Wnt can also bind to the extracellular domain of PC1, thereby inducing PC2-dependent Ca2+ influx in epithelial cells [81]. Pathogenic mutations in Pkd1 and Pkd2 were shown to abrogate PC1-PC2 complex formation, reduce cell surface localization of PC1, and hinder PC2 activation by Wnt molecule 81. Besides, mutations in several PLD-causing genes, such as LRP5, Sec63, and Pkhd1, were shown to impair Wnt signaling pathway, which makes it interesting for further investigation [66, 80, 82].

Previously, it has been reported that overactivation of Hippo/Yes-associated protein (YAP) and their transcriptional target four-jointed (Fjx1) is a major driver of cystogenesis in ADPKD [83]. Consistently, it was shown that simultaneous knockout of Fjx1 decelerates renal fibrosis, alleviates renal inflammation, and preserves renal function in mice with Pkd1 deletion; however, Fjx1 knockout did not markedly inhibit cyst formation [84].

As PC1-PC2 complex deficiency leads to decreased intracellular Ca2+ concentration, activation of transient receptor potential vanilloid (Trpv4), a calcium-entry channel in cholangiocytes, has been proposed as a therapeutic option 86. In-vitro experiments showed that Trpv4 activation increases intracellular Ca2+ concentration and decreases cholangiocyte proliferation and cyst growth in 3-dimensional culture [85]. In vivo, Trpv4 activation significantly reduced renal cystic area and non-significantly reduced liver cysts [85]. Similarly, it was found that Trpv4 activation downregulates cAMP levels and decelerates the progression of ARPKD in rats [86].

Using tissues from patients with ADPLD and in vivo and in vitro experiments, it was shown that i