Proliferative signaling by ERBB proteins and RAF/MEK/ERK effectors in polycystic kidney disease

A primary pathological feature of polycystic kidney disease (PKD) is the hyperproliferation of epithelial cells in renal tubules, resulting in formation of fluid-filled cysts. The proliferative aspects of the two major forms of PKD—autosomal dominant PKD (ADPKD), which arises from mutations in the polycystins PKD1 and PKD2, and autosomal recessive PKD (ARPKD), which arises from mutations in PKHD1—has encouraged investigation into protein components of the core cell proliferative machinery as potential drivers of PKD pathogenesis. In this review, we examine the role of signaling by ERBB proteins and their effectors, with a primary focus on ADPKD. The ERBB family of receptor tyrosine kinases (EGFR/ERBB1, HER2/ERBB2, ERBB3, and ERBB4) are activated by extracellular ligands, inducing multiple pro-growth signaling cascades; among these, activation of signaling through the RAS GTPase, and the RAF, MEK1/2, and ERK1/2 kinases enhance cell proliferation and restrict apoptosis during renal tubuloepithelial cyst formation. Characteristics of PKD include overexpression and mislocalization of the ERBB receptors and ligands, leading to enhanced activation and increased activity of downstream signaling proteins. The altered regulation of ERBBs and their effectors in PKD is influenced by enhanced activity of SRC kinase, which is promoted by the loss of cytoplasmic Ca²⁺ and an increase in cAMP-dependent PKA kinase activity that stimulates CFTR, driving the secretory phenotype of ADPKD. We discuss the interplay between ERBB/SRC signaling, and polycystins and their depending signaling, with emphasis on thes changes that affect cell proliferation in cyst expansion, as well as the inflammation-associated fibrogenesis, which characterizes progressive disease. We summarize the current progress of preclinical and clinical trials directed at inhibiting this signaling axis, and discuss potential future strategies that may be productive for controlling PKD.     

PRRT2 Mutations in Paroxysmal Kinesigenic Dyskinesia with Infantile Convulsions in a Taiwanese Cohort

Background

Mutations in the PRRT2 gene have recently been identified in patients with familial paroxysmal kinesigenic dyskinesia with infantile convulsions (PKD/IC) and patients with sporadic PKD/IC from several ethnic groups. To extend these recent genetic reports, we investigated the frequency and identities of PRRT2 mutations in a cohort of Taiwanese patients with PKD/IC.

Methodology and Principal Findings

We screened all 3 coding exons of PRRT2 for mutations in 28 Taiwanese patients with PKD/IC. Among them, 13 had familial PKD/IC and 15 were apparently sporadic cases. In total, 7 disparate mutations were identified in 13 patients, including 8 familial cases and 5 apparently sporadic cases. The mutations were not present in 500 healthy controls. Four mutations were novel. One patient had a missense mutation and all other patients carried PRRT2 mutations putatively resulting in a protein truncation. Haplotype analysis revealed that 5 of the 7 patients with the PRRT2 p.R217Pfs*8 mutation shared the same haplotype linked to the mutation.

Conclusions and Significance

PRRT2 mutations account for 61.5% (8 out of 13) of familial PKD/IC and 33.3% (5 out of 15) of apparently sporadic PKD/IC in the Taiwanese cohort. Most patients with the PRRT2 p.R217Pfs*8 mutation in Taiwan likely descend from a single common ancestor. This study expands the spectrum of PKD/IC-associated PRRT2 mutations, highlights the pathogenic role of PRRT2 mutations in PKD/IC, and suggests genetic heterogeneity within idiopathic PKD.     

Molecular basis of Parkinsons’s disease linked to LRRK2 mutations

Parkinson’s disease (PD) is a common neurodegenerative disorder whose symptoms are consistent with death of dopaminergic neurons in the substantia nigra of the brain. The pathogenesis of PD involves several factors, such as α-synuclein aggregation, oxidative stress, mitochondrial dysfunction, and activation of apoptosis, but the exact molecular mechanism of neurodegeneration remains obscure. PD is usually sporadic, while rare monogenic forms have been identified and described in the past 15 years. Familial Parkinson’s disease is most commonly associated with mutations of the leucine repeat-rich kinase 2 gene (LRRK2). The mechanism of the disease due to LRRK2 mutations is unknown. The signaling cascades regulated by LRRK2 are difficult to study because the physiological substrates of the enzyme are unidentified. The G2019S substitution has been found to be the most common LRRK2 mutation, facilitating a search for patients with LRRK2-associated PD in various populations. The review considers the effects of LRRK2 mutations on protein and, in particular, α-synuclein aggregation, cytoskeletal dynamics, the inflammatory response, and the induction of apoptosis as revealed in both in vitro experiments and studies in PD patients. Investigation of rare hereditary PD forms with known etiology provides for a better understanding of the mechanism of neurodegeneration in more common sporadic PD forms.     

Dopamine Receptor and Gα(olf) Expression in DYT1 Dystonia Mouse Models during Postnatal Development

Background

DYT1 dystonia is a heritable, early-onset generalized movement disorder caused by a GAG deletion (ΔGAG) in the DYT1 gene. Neuroimaging studies and studies using mouse models suggest that DYT1 dystonia is associated with dopamine imbalance. However, whether dopamine imbalance is key to DYT1 or other forms of dystonia continues to be debated.

Methodology/Principal Findings

We used Dyt1 knock out (Dyt1 KO), Dyt1 ΔGAG knock-in (Dyt1 KI), and transgenic mice carrying one copy of the human DYT1 wild type allele (DYT1 hWT) or human ΔGAG mutant allele (DYT1 hMT). D1R, D2R, and Gα(olf) protein expression was analyzed by western blot in the frontal cortex, caudate-putamen and ventral midbrain in young adult (postnatal day 60; P60) male mice from all four lines; and in the frontal cortex and caudate putamen in juvenile (postnatal day 14; P14) male mice from the Dyt1 KI and KO lines. Dopamine receptor and Gα(olf) protein expression were significantly decreased in multiple brain regions of Dyt1 KI and Dyt1 KO mice and not significantly altered in the DYT1 hMT or DYT1 hWT mice at P60. The only significant change at P14 was a decrease in D1R expression in the caudate-putamen of the Dyt1 KO mice.

Conclusion/Significance

We found significant decreases in key proteins in the dopaminergic system in multiple brain regions of Dyt1 KO and Dyt1 KI mouse lines at P60. Deletion of one copy of the Dyt1 gene (KO mice) produced the most pronounced effects. These data offer evidence that impaired dopamine receptor signaling may be an early and significant contributor to DYT1 dystonia pathophysiology.     

Defective pre-mRNA splicing in PKD1 due to presumed missense and synonymous mutations causing autosomal dominant polycystic disease

Autosomal dominant polycystic kidney disease is the most common human monogenic disorder and is caused by mutations in the PKD1 or PKD2 genes. Most patients with the disease present mutations in PKD1, and a considerable number of these alterations are single base substitutions within the coding sequence that are usually predicted to lead to missense or synonymous mutations. There is growing evidence that some of these mutations can be detrimental by affecting the pre-mRNA splicing process. The aim of our study was to test PKD1 mutations, described as missense or synonymous in the literature or databases, for their effects on exon inclusion. Bioinformatics tools were used to select mutations with a potential effect on pre-mRNA splicing. Mutations were experimentally tested using minigene assays. Exons and adjacent intronic sequences were PCR-amplified and cloned in the splicing reporter minigene, and selected mutations were introduced by site-directed mutagenesis. Minigenes were transfected into kidney derived cell lines. RNA from cultured cells was analyzed by RT-PCR and DNA sequencing. Analysis of thirty-three PKD1 exonic mutations revealed three mutations that induce splicing defects. The substitution c.11156G > A, previously predicted as missense mutation p.R3719Q, abolished the donor splice site of intron 38 and resulted in the incorporation of exon 38 with 117 bp of intron 38 and skipping of exon 39. Two synonymous variants, c.327A > T (p.G109G) and c.11257C > A (p.R3753R), generated strong donor splice sites within exons 3 and 39 respectively, resulting in incorporation of incomplete exons. These three nucleotide substitutions represent the first PKD1 exonic mutations that induce aberrant mRNAs. Our results strengthen the importance to evaluate the consequences of presumed missense and synonymous mutations at the mRNA level.     

A high-resolution structure of the EF-hand domain of human polycystin-2

Autosomal dominant polycystic kidney disease (ADPKD) affects over 1:1000 of the worldwide population and is caused by mutations in two genes, PKD1 and PKD2. PKD2 encodes a 968-amino acid membrane spanning protein, Polycystin-2 (PC-2), which is a member of the TRP ion channel family. The C-terminal cytoplasmic tail contains an EF-hand motif followed by a short coiled-coil domain. We have determined the structure of the EF-hand region of PC-2 using NMR spectroscopy. The use of different boundaries, compared with those used in previous studies, have enabled us to determine a high resolution structure and show that the EF hand motif forms a standard calcium-binding pocket. The affinity of this pocket for calcium has been measured and mutants that both decrease and increase its affinity for the metal ion have been created.     

No genetic association between attention-deficit/hyperactivity disorder (ADHD) and Parkinson’s disease in nine ADHD candidate SNPs

Attention-deficit/hyperactivity disorder (ADHD) and Parkinson’s disease (PD) involve pathological changes in brain structures such as the basal ganglia, which are essential for the control of motor and cognitive behavior and impulsivity. The cause of ADHD and PD remains unknown, but there is increasing evidence that both seem to result from a complicated interplay of genetic and environmental factors affecting numerous cellular processes and brain regions. To explore the possibility of common genetic pathways within the respective pathophysiologies, nine ADHD candidate single nucleotide polymorphisms (SNPs) in seven genes were tested for association with PD in 5333 cases and 12,019 healthy controls: one variant, respectively, in the genes coding for synaptosomal-associated protein 25 k (SNAP25), the dopamine (DA) transporter (SLC6A3; DAT1), DA receptor D4 (DRD4), serotonin receptor 1B (HTR1B), tryptophan hydroxylase 2 (TPH2), the norepinephrine transporter SLC6A2 and three SNPs in cadherin 13 (CDH13). Information was extracted from a recent meta-analysis of five genome-wide association studies, in which 7,689,524 SNPs in European samples were successfully imputed. No significant association was observed after correction for multiple testing. Therefore, it is reasonable to conclude that candidate variants implicated in the pathogenesis of ADHD do not play a substantial role in PD.     

Mutations in HPCA Cause Autosomal-Recessive Primary Isolated Dystonia

Reports of primary isolated dystonia inherited in an autosomal-recessive (AR) manner, often lumped together as “DYT2 dystonia,” have appeared in the scientific literature for several decades, but no genetic cause has been identified to date. Using a combination of homozygosity mapping and whole-exome sequencing in a consanguineous kindred affected by AR isolated dystonia, we identified homozygous mutations in HPCA, a gene encoding a neuronal calcium sensor protein found almost exclusively in the brain and at particularly high levels in the striatum, as the cause of disease in this family. Subsequently, compound-heterozygous mutations in HPCA were also identified in a second independent kindred affected by AR isolated dystonia. Functional studies suggest that hippocalcin might play a role in regulating voltage-dependent calcium channels. The identification of mutations in HPCA as a cause of AR primary isolated dystonia paves the way for further studies to assess whether “DYT2 dystonia” is a genetically homogeneous condition or not.     

Zystennieren – eine Übersicht

Cystic kidney diseases are a clinically and genetically heterogeneous group of disorders, representing one of the most frequent genetic conditions with a prevalence of about 1 in 1000. The most important forms include autosomal dominant polycystic kidney disease (ADPKD) caused by mutations in the PKD1 and PKD2 genes and the autosomal recessive polycystic kidney disease (ARPKD) caused by mutations in the PKHD1 gene. The proteins encoded by the involved genes are summarized as cystoproteins. On the cellular level, the majority of these cystoproteins co-localize in primary cilia, the basal body or the centrosome of renal epithelial cells. Inherited polycystic kidney diseases belong to the increasing number of reported ciliopathies which include many syndromic forms, e.g. Bardet-Biedl syndrome, Meckel syndrome and Joubert syndrome. Identifying the genetic defect can help establish the correct diagnosis, define the clinical prognosis and forms the basis for genetic counselling. In addition to establishing a clinical, ultrasonographic and morphological picture of the underlying kidney disease, the algorithm of genetic diagnosis should take the presence of further organ dysfunction or malformation as well as family history into consideration.     

Polycystins as components of large multiprotein complexes of polycystin interactors

Naturally occurring mutations in two separate genes, PKD1 and PKD2, are responsible for the vast majority of all cases of autosomal dominant polycystic kidney disease (ADPKD), one of the most common genetic diseases affecting 1 in 1000 Americans. The hallmark of ADPKD is the development of epithelial cysts in the kidney, liver, and pancreas. PKD1 encodes a large plasma membrane protein (PKD1, PC1, or Polycystin-1) with a long extracellular domain and has been speculated to function as an atypical G protein coupled receptor. PKD2 encodes an ion channel of the Transient Receptor Potential superfamily (TRPP2, PKD2, PC2, or Polycystin-2). Despite the identification of these genes more than 20 years ago, the molecular function of their encoded proteins and the mechanism(s) by which mutations in PKD1 and PKD2 cause ADPKD remain elusive. Genetic, biochemical, and functional evidence suggests they form a multiprotein complex present in multiple locations in the cell, including the plasma membrane, endoplasmic reticulum, and the primary cilium. Over the years, numerous interacting proteins have been identified using directed and unbiased approaches, and shown to modulate function, cellular localization, and protein stability and turnover of Polycystins. Delineation of the molecular composition of the Polycystin complex can have a significant impact on understanding their cellular function in health and disease states and on the identification of more specific and effective therapeutic targets.     

Divergent function of polycystin 1 and polycystin 2 in cell size regulation

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 or PKD2, the genes encoding polycystin 1 (PC1) and polycystin 2 (PC2), respectively. PC1 and PC2 localize to the primary cilium and form a protein complex, which is thought to regulate signaling events. PKD1 mutations are associated with a stronger phenotype than PKD2, suggesting the existence of PC1 specific functions in renal tubular cells. However, the evidence for diverging molecular functions is scant. The bending of cilia by fluid flow induces a reduction in cell size through a mechanism that involves the kinase LKB1 but not PC2. Here, using different in vitro approaches, we show that contrary to PC2, PC1 regulates cell size under flow and thus phenocopies the loss of cilia. PC1 is required to couple mechanical deflection of cilia to mTOR in tubular cells. This study pinpoints divergent functions of the polycystins in renal tubular cells that may be relevant to disease severity in ADPKD.     

Substitution p.A350V in Na+/Mg2+ Exchanger SLC41A1, Potentially Associated with Parkinson's Disease,Is a Gain-of-Function Mutation

Parkinson''s disease (PD) is a complex multifactorial ailment predetermined by the interplay of various environmental and genetic factors. Systemic and intracellular magnesium (Mg) deficiency has long been suspected to contribute to the development and progress of PD and other neurodegenerative diseases. However, the molecular background is unknown. Interestingly, gene SLC41A1 located in the novel PD locus PARK16 has recently been identified as being a Na⁺/Mg²⁺ exchanger (NME, Mg²⁺ efflux system), a key component of cellular magnesium homeostasis. Here, we demonstrate that the substitution p.A350V potentially associated with PD is a gain-of-function mutation that enhances a core function of SLC41A1, namely Na⁺-dependent Mg²⁺ efflux by 69±10% under our experimental conditions (10-minute incubation in high-Na⁺ (145 mM) and completely Mg²⁺-free medium). The increased efflux capacity is accompanied by an insensitivity of mutant NME to cAMP stimulation suggesting disturbed hormonal regulation and leads to a reduced proliferation rate in p.A350V compared with wt cells. We hypothesize that enhanced Mg²⁺-efflux conducted by SLC41A1 variant p.A350V might result, in the long-term, in chronic intracellular Mg²⁺-deficiency, a condition that is found in various brain regions of PD patients and that exacerbates processes triggering neuronal damage.     

Novel SLC20A2 mutations identified in southern Chinese patients with idiopathic basal ganglia calcification

Idiopathic basal ganglia calcification (IBGC) is a rare neuropsychiatric disorder characterized by bilateral and symmetric cerebral calcifications. Recently, SLC20A2 was identified as a causative gene for familial IBGC, and three mutations were reported in a northern Chinese population. Here, we aimed to explore the mutation spectrum of SLC20A2 in a southern Chinese population. Sanger sequencing was employed to screen mutations within SLC20A2 in two IBGC families and 14 sporadic IBGC cases from a southern Han Chinese population. Four novel mutations (c.82G > A p.D28N, c.185T > C p.L62P, c.1470_1478delGCAGGTCCT p.Q491_L493del and c.935-1G > A) were identified in two families and two sporadic cases, respectively; none were detected in 200 unrelated controls. No mutation was found in the remaining 12 patients. Different mutations may result in varied phenotypes, including brain calcification and clinical manifestations. Our study supports the hypothesis that SLC20A2 is a causative gene of IBGC and expands the mutation spectrum of SLC20A2, which facilitates the understanding of the genotype–phenotype correlation of IBGC.     

Polycystic kidney disease: novel insights into polycystin function

Autosomal dominant polycystic kidney disease (ADPKD) is a life-threatening monogenic disease caused by mutations in PKD1 and PKD2 that encode polycystin 1 (PC1) and polycystin 2 (PC2). PC1/2 localize to cilia of renal epithelial cells, and their function is believed to embody an inhibitory activity that suppresses the cilia-dependent cyst activation (CDCA) signal. Consequently, PC deficiency results in activation of CDCA and stimulates cyst growth. Recently, re-expression of PCs in established cysts has been shown to reverse PKD. Thus, the mode of action of PCs resembles a ‘counterbalance in cruise control’ to maintain lumen diameter within a designated range. Herein we review recent studies that point to novel arenas for future PC research with therapeutic potential for ADPKD.     

Mutations in the GABA Transporter SLC6A1 Cause Epilepsy with Myoclonic-Atonic Seizures

GAT-1, encoded by SLC6A1, is one of the major gamma-aminobutyric acid (GABA) transporters in the brain and is responsible for re-uptake of GABA from the synapse. In this study, targeted resequencing of 644 individuals with epileptic encephalopathies led to the identification of six SLC6A1 mutations in seven individuals, all of whom have epilepsy with myoclonic-atonic seizures (MAE). We describe two truncations and four missense alterations, all of which most likely lead to loss of function of GAT-1 and thus reduced GABA re-uptake from the synapse. These individuals share many of the electrophysiological properties of Gat1-deficient mice, including spontaneous spike-wave discharges. Overall, pathogenic mutations occurred in 6/160 individuals with MAE, accounting for ∼4% of unsolved MAE cases.     

Elevated CSF N-acetylaspartylglutamate suggests specific molecular diagnostic abnormalities in patients with white matter diseases

Background: In order to identify biomarkers useful for the diagnosis of genetic white matter disorders we compared the metabolic profile of patients with leukodystrophies with a hypomyelinating or a non-hypomyelinating MRI pattern. Methods: We used a non-a priori method of in vitro ¹H-NMR spectroscopy on CSF samples of 74 patients with leukodystrophies. Results: We found an elevation of CSF N-acetylaspartylglutamate (NAAG) in patients with Pelizaeus–Merzbacher disease (PMD)—PLP1 gene, Pelizaeus–Merzbacher-like disease—GJC2 gene and Canavan disease—ASPA gene. In the PMD group, NAAG was significantly elevated in the CSF of all patients with PLP1 duplication (19/19) but was strictly normal in 6 out of 7 patients with PLP1 point mutations. Additionally, we previously reported increased CSF NAAG in patients with SLC17A5 mutations. Conclusions: Elevated CSF NAAG is a biomarker that suggests specific molecular diagnostic abnormalities in patients with white matter diseases. Our findings also point to unique pathological functions of the overexpressed PLP in PMD patients with duplication of this gene.     

Genetik des essenziellen Tremors

Essential tremor (ET) is—with a lifetime prevalence of approximately 1?%—one of the commonest movement disorders. ET patients predominantly suffer from a postural and kinetic tremor of the arms which might severely impair fine motor skills. The question whether additional symptoms such as mild cognitive deficits and depression, which occur in some patients, belong to the clinical picture of ET is a matter of debate. More than 50?% of all ET patients have a positive family history. In many families ET segregates in a manner compatible with autosomal dominant transmission. Recently mutations in the fused in sarcoma (FUS) gene have been identified as one potential cause of monogenic ET. In the majority of patients ET is genetically complex. Twin studies suggest a very high heritability. Two relatively small genome-wide association studies identified risk variants in the LINGO1 gene which plays a role in neuroregeneration and in the SLC1A2 gene which encodes the most important glutamate reuptake transporter of the brain.     

SLC45A2 mutation frequency in Oculocutaneous Albinism Italian patients doesn't differ from other European studies

Background

Oculocutaneous Albinism (OCA) is a heterogeneous group of inherited diseases involving hair, skin and eyes. To date, six forms are recognized on the effects of different melanogenesis genes.OCA4 is caused by mutations in SLC45A2 showing a heterogeneous phenotype ranging from white hair, blue irides and nystagmus to brown/black hair, brown irides and no nystagmus. The high clinic variety often leads to misdiagnosis.Our aim is to contribute to OCA4 diagnosis defining SLC45A2 genetic variants in Italian patients with OCA without any TYR, OCA2 and TYRP1 gene defects.

Materials and methods

After the clinical diagnosis of OCA, all patients received genetic counseling and genetic test. Automatic sequencing of TYR, OCA2, and TYRP1 genes was performed on DNA of 117 albino patients. Multiplex Ligation-dependent Probe Amplification (MLPA) was carried out on TYR and OCA2 genes to increase the mutation rate. SLC45A2 gene sequencing was then executed in the patients with a single mutation in one of the TYR, OCA2, TYRP1 genes and in the patients, which resulted negative at the screening of these genes.

Results

SLC45A2 gene analysis was performed in 41 patients and gene alterations were found in 5 patients. Four previously reported SLC45A2 mutations were found: p.G100S, p.W202C, p.A511E and c.986delC, and three novel variants were identified: p.M265L, p.H94D, and c.1156+1G>A. All the alterations have been detected in the group of patients without mutations in the other OCA genes.

Conclusions

Three new variants were identified in OCA4 gene; the analysis allowed the classification of a patient previously misdiagnosed as OA1 because of skin and hair pigmentation presence. The molecular defects in SLC45A2 gene represent the 3.4% in this cohort of Italian patients, similar to other Caucasian populations; our data differ from those previously published by an Italian researcher group, obtained on a smaller cohort of patients.     

Post-translational modifications of the polycystin proteins

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited cause of kidney failure and affects up to 12 million people worldwide. Germline mutations in two genes, PKD1 or PKD2, account for almost all patients with ADPKD. The ADPKD proteins, polycystin-1 (PC1) and polycystin-2 (PC2), are regulated by post-translational modifications (PTM), with phosphorylation, glycosylation and proteolytic cleavage being the best described changes. A few PTMs have been shown to regulate polycystin trafficking, signalling, localisation or stability and thus their physiological function. A key challenge for the future will be to elucidate the functional significance of all the individual PTMs reported to date. Finally, it is possible that site-specific mutations that disrupt PTM could contribute to cystogenesis although in the majority of cases, confirmatory evidence is awaited.