Renal-Hepatic-Pancreatic Dysplasia 2

A number sign (#) is used with this entry because of evidence that renal-hepatic-pancreatic dysplasia-2 (RHPD2) is caused by homozygous or compound heterozygous mutation in the NEK8 gene (609799) on chromosome 17q11.

Description

RHPD2 is an autosomal recessive multisystemic disorder with severe abnormalities apparent in utero and often resulting in fetal death or death in infancy. The main organs affected include the kidney, liver, and pancreas, although other abnormalities, including cardiac, skeletal, and lung defects, may also be present. Affected individuals often have situs inversus. The disorder results from a defect in ciliogenesis and ciliary function, as well as in cell proliferation and epithelial morphogenesis; thus, the clinical manifestations are highly variable (summary by Grampa et al., 2016).

For a discussion of genetic heterogeneity of renal-hepatic-pancreatic dysplasia, see RHPD1 (208540).

Clinical Features

Frank et al. (2013) reported 3 fetuses from a consanguineous pedigree who presented in utero with enlarged organs and cystic/dysplastic changes in the kidney, liver, and pancreas. Two of the affected pregnancies were terminated at 21 and 22 weeks' gestation, and the third was terminated at 18 weeks' gestation after intrauterine fetal demise. Additional features found in at least 1 of the fetuses included severe heart defects, such as truncus arteriosus and unseptated atrium and ventricle, hypoplastic lungs with lobulation defects, asplenia, uterine agenesis, and shortened legs due to bowed femurs. One patient had a right-sided stomach, consistent with heterotaxy or isomerism. Two patients had Potter sequence associated with oligo-/anhydramnios. Histologic examination of tissues from 2 of the fetuses showed cystic changes in the liver, kidneys, and pancreas. Liver samples showed hepatic fibrosis with ductal plate malformation, and the pancreas showed rudimentary lobulation and reduced acinar units as well as complete absence of the islets of Langerhans. The kidneys consisted of predominantly immature mesenchyme with no zonal partition between cortical and medullary areas, and the glomeruli were immature and decreased in number.

Rajagopalan et al. (2016) reported 2 brothers with severe congenital defects, including complex cardiac defects, enlarged liver with bile duct hypoplasia, and enlarged hyperechogenic cystic kidneys. The patients died at 4 months and at 15 days. Neither had pancreatic involvement.

Al-Hamed et al. (2016) reported a stillborn fetus, conceived of consanguineous Saudi Arabian parents (family FT-36), with RHPD2 confirmed by genetic analysis. In addition to enlarged cystic kidneys, the fetus had cerebellar vermis aplasia, dilated cisterna magna, and bowed femurs; the pregnancy showed oligo-/anhydramnios.

Grampa et al. (2016) reported 5 unrelated patients with variable manifestations of RHPD2. Three died in utero, 1 died at age 3 days, and 1 died at age 2 months. All had severe renal involvement associated with extrarenal defects. Two sibs (family 2) had previously been reported by Alessandri et al. (2009). There was an apparent genotype/phenotype correlation: 2 with total NEK8 loss-of-function mutations had enlarged cystic kidneys and pancreas associated with proliferative cystic biliary ducts, characteristic of RHPD2, whereas the 3 patients carrying missense or an in-frame deletion had asymmetric dysplastic/hypodysplastic kidneys or renal agenesis with loss of differentiation, cortical interstitial fibrosis, dilated tubules and cartilage nodules, cysts, and paucity of hepatic bile ducts. Additional features in these patients included situs inversus in 4 patients, cardiomegaly in 3, narrow thorax and short bowed femurs in 2, and brain defects such as corpus callosum or vermis agenesis in 2. One female fetus had absence of the uterus and vagina.

Inheritance

The transmission pattern of RHPD2 in the family reported by Frank et al. (2013) was consistent with autosomal recessive inheritance.

Mapping

By genomewide linkage analysis of a consanguineous family with renal-hepatic-pancreatic dysplasia, Frank et al. (2013) found linkage to a 6.3-Mb region on chromosome 17q11.2-q12 (maximum lod score of 3.6).

Molecular Genetics

In 3 fetuses from a consanguineous pedigree with renal-hepatic-pancreatic dysplasia-2, Frank et al. (2013) identified a homozygous nonsense mutation in the NEK8 gene (R599X; 609799.0002). The mutation was found by homozygosity mapping and candidate gene analysis. Cultured fibroblasts derived from the fetuses showed decreased expression of polycystic kidney disease genes PKD1 (601313) and PKD2 (173910) and increased expression of the MYC (190080) oncogene, providing potential explanations for the observed renal phenotype. Patient cells showed downregulation of the TAZ (WWTR1; 607392)/Hippo signaling pathway. The findings suggested that NEK8 is essential for organ development and that the complete loss of NEK8 perturbs multiple pathways important in early embryonic development. Sequencing of the NEK8 gene in 288 patients with cystic kidney disease and/or other ciliopathies did not identify any pathogenic mutations.

In a stillborn fetus, conceived of consanguineous Saudi Arabian parents (family FT-36), with RHPD2, Al-Hamed et al. (2016) identified a homozygous nonsense mutation in the NEK8 gene (W467X; 609799.0003). The mutation was confirmed by Sanger sequencing; the unaffected parents were heterozygous for the mutation. The family was 1 of 44 Saudi Arabian families with evidence of antenatal cystic kidney disease and ciliopathy phenotypes who underwent targeted exon sequencing of a renal gene panel; this was the only mutation identified in the NEK8 gene. Functional studies of the variant and studies of patient cells were not performed.

In 2 sibs with RHPD2, Rajagopalan et al. (2016) identified compound heterozygous mutations in the NEK8 gene (609799.0004 and 609799.0005). The mutations were found by exome sequencing and confirmed by Sanger sequencing. Functional studies of the variants and studies of patient cells were not performed.

In 5 unrelated patients with variable manifestations of RHPD2, Grampa et al. (2016) identified homozygous or compound heterozygous mutations in the NEK8 gene (see, e.g., 609799.0006-609799.0011). The mutations were found by exon-enriched targeting of 1,222 genes associated with cilia structure and function in large cohorts of patients and fetuses with renal abnormalities. There were 8 novel mutations, including 4 missense mutations, 1 in-frame deletion, 1 splice site mutation, and 2 nonsense mutations. Studies of patient-derived cells showed that missense mutant proteins affecting the RCC1 domain were absent from the cilia and instead accumulated abnormally in the Golgi compartment, whereas nonsense mutations resulted in a loss of protein expression. A missense mutation affecting the kinase domain localized properly about half of the time. Functional analyses of NEK8 mutations in patient fibroblasts and medullary cells showed that the mutations differentially affected ciliogenesis, cell proliferation, apoptosis, DNA damage response, and epithelial morphogenesis. NEK8 missense and loss-of-function mutations differentially affected the regulation of the main Hippo signaling effector, YAP (606608), as well as the expression of its target genes in patient fibroblasts and in renal cells. Missense mutations exacerbated some of the defects due to NEK8 loss of function, suggesting that they likely had gain-of-function effects. Missense mutations led to more prominent defects in ciliogenesis with hypodysplasia and loss of differentiation, whereas truncated mutations led to a more proliferative cystic phenotype. YAP imbalance was also observed in enlarged renal epithelial cells with Nek8 alterations and in cystic kidneys of Jck mice. These changes were associated with increased YAP activity, and treatment with the YAP blocker verteporfin partially rescued the phenotype. The findings demonstrated that NEK8 mutations can cause major organ developmental defects due to altered ciliogenesis and cell differentiation/proliferation through deregulation of the Hippo pathway.

Animal Model

A mutation in the Nek8 gene causes juvenile cystic kidney disease (jck) in mice (Liu et al., 2002). Manning et al. (2013) found that Nek8-null mice died shortly after birth and showed a variety of laterality defects, including randomization of left-right asymmetry and structural cardiac abnormalities. Mutant embryos showed intact nodal cilia, but there was aberrant expression and localization of several genes in the left- right axis-determining pathway. Ciliogenesis in renal cells was also normal in Nek8-null embryos. A comparison of jck, jck/Nek8- and Nek8 -/- mouse kidneys showed that the compound heterozygotes had less severe cystic disease than jck homozygotes, suggesting that the jck allele results in a gain of function. Studies in cultured renal cells lacking Nek8 showed a reduced calcium response to flow, suggesting that Pkd2 function may be compromised in the absence of Nek8.

Grampa et al. (2016) found that morpholino knockdown of the nek8 gene in zebrafish embryos resulted in a classic ciliopathy-related phenotype including curved body axis, laterality defects and pronephric cysts. These defects could be rescued by human wildtype NEK8, but not by mutant NEK8. However, overexpression of wildtype human NEK8 also resulted in increased frequency of shortened dorsally curved embryos, laterality defects, and pronephros abnormalities. Some of these abnormalities were exacerbated by NEK8 missense mutations, suggesting a possible gain-of-function effect conferred by missense mutations. These changes were associated with increased YAP (606608) activity, and treatment with the YAP blocker verteporfin partially rescued the phenotype. Grampa et al. (2016) found that Yap expression was increased in the nuclei of kidneys of 5-week-old Jck mice, suggesting that Yap activity was abnormally upregulated around the time that they developed polycystic kidneys.