The Groucho-associated phosphatase PPM1B displaces Pax transactivation domain interacting protein (PTIP) to switch the transcription factor Pax2 from a transcriptional activator to a repressor

Pax protein depletion in proximal tubules triggers conserved mechanisms of resistance to acute ischemic kidney injury preventing transition to chronic kidney disease

Acute kidney injury (AKI) is a common condition that lacks effective treatments. In part, this shortcoming is due to an incomplete understanding of the genetic mechanisms that control pathogenesis and recovery. Identifying the molecular and genetic regulators unique to nephron segments that dictate vulnerability to injury and regenerative potential could lead to new therapeutic targets to treat ischemic kidney injury. Pax2 and Pax8 are homologous transcription factors with overlapping functions that are critical for kidney development and are re-activated in AKI. Here, we examined the role of Pax2 and Pax8 in recovery from ischemic AKI and found them upregulated after severe AKI and correlated with chronic injury. Surprisingly, proximal-tubule-selective deletion of Pax2 and Pax8 resulted in a less severe chronic injury phenotype. This effect was mediated by protection against the acute insult, similar to pre-conditioning. Prior to injury, Pax2 and Pax8 mutant mice develop a unique subpopulation of proximal tubule cells in the S3 segment that displayed features usually seen only in acute or chronic injury. The expression signature of these cells was strongly enriched with genes associated with other mechanisms of protection against ischemic AKI including caloric restriction, hypoxic pre-conditioning, and female sex. Thus, our results identified a novel role for Pax2 and Pax8 in mature proximal tubules that regulates critical genes and pathways involved in both the injury response and protection from ischemic AKI.

Pax Protein Depletion in Proximal Tubules Triggers Conserved Mechanisms of Resistance to Acute Ischemic Kidney Injury and Prevents Transition to Chronic Kidney Disease

Acute kidney injury (AKI) is a common condition that lacks effective treatments. In part this shortcoming is due to an incomplete understanding of the genetic mechanisms that control pathogenesis and recovery. Pax2 and Pax8 are homologous transcription factors with overlapping functions that are critical for kidney development and are re-activated in AKI. In this report, we examined the role of Pax2 and Pax8 in recovery from ischemic AKI. We found that Pax2 and Pax8 are upregulated after severe AKI and correlate with chronic injury. Surprisingly, we then discovered that proximal-tubule-selective deletion of Pax2 and Pax8 resulted in a less severe chronic injury phenotype. This effect was mediated by protection against the acute insult, similar to preconditioning. Prior to injury, Pax2 and Pax8 mutant mice develop a unique subpopulation of S3 proximal tubule cells that display features usually seen only in acute or chronic injury. The expression signature of these cells was strongly enriched with genes associated with other mechanisms of protection against ischemic AKI including caloric restriction, hypoxic preconditioning, and female sex. Taken together, our results identify a novel role for Pax2 and Pax8 in mature proximal tubules that regulates critical genes and pathways involved in both injury response and protection from ischemic AKI.

TRANSLATIONAL STATEMENT

Identifying the molecular and genetic regulators unique to the nephron that dictate vulnerability to injury and regenerative potential could lead to new therapeutic targets to treat ischemic kidney injury. Pax2 and Pax8 are two homologous nephron-specific transcription factors that are critical for kidney development and physiology. Here we report that proximal-tubule-selective depletion of Pax2 and Pax8 protects against both acute and chronic injury and induces an expression profile in the S3 proximal tubule with common features shared among diverse conditions that protect against ischemia. These findings highlight a new role for Pax proteins as potential therapeutic targets to treat AKI.

Hnf1b renal expression directed by a distal enhancer responsive to Pax8

Xenopus provides a simple and efficient model system to study nephrogenesis and explore the mechanisms causing renal developmental defects in human. Hnf1b (hepatocyte nuclear factor 1 homeobox b), a gene whose mutations are the most commonly identified genetic cause of developmental kidney disease, is required for the acquisition of a proximo-intermediate nephron segment in Xenopus as well as in mouse. Genetic networks involved in Hnf1b expression during kidney development remain poorly understood. We decided to explore the transcriptional regulation of Hnf1b in the developing Xenopus pronephros and mammalian renal cells. Using phylogenetic footprinting, we identified an evolutionary conserved sequence (CNS1) located several kilobases (kb) upstream the Hnf1b transcription start and harboring epigenomic marks characteristics of a distal enhancer in embryonic and adult renal cells in mammals. By means of functional expression assays in Xenopus and mammalian renal cell lines we showed that CNS1 displays enhancer activity in renal tissue. Using CRISPR/cas9 editing in Xenopus tropicalis, we demonstrated the in vivo functional relevance of CNS1 in driving hnf1b expression in the pronephros. We further showed the importance of Pax8-CNS1 interaction for CNS1 enhancer activity allowing us to conclude that Hnf1b is a direct target of Pax8. Our work identified for the first time a Hnf1b renal specific enhancer and may open important perspectives into the diagnosis for congenital kidney anomalies in human, as well as modeling HNF1B-related diseases.

Identification of Pax protein inhibitors that suppress target gene expression and cancer cell proliferation

The Pax family of developmental control genes are frequently deregulated in human disease. In the kidney, Pax2 is expressed in developing nephrons but not in adult proximal and distal tubules, whereas polycystic kidney epithelia or renal cell carcinoma continues to express high levels. Pax2 reduction in mice or cell culture can slow proliferation of cystic epithelial cells or renal cancer cells. Thus, inhibition of Pax activity may be a viable, cell-type-specific therapy. We designed an unbiased, cell-based, high-throughput screen that identified triazolo pyrimidine derivatives that attenuate Pax transactivation ability. We show that BG-1 inhibits Pax2-positive cancer cell growth and target gene expression but has little effect on Pax2-negative cells. Chromatin immunoprecipitation suggests that these inhibitors prevent Pax protein interactions with the histone H3K4 methylation complex at Pax target genes in renal cells. Thus, these compounds may provide structural scaffolds for kidney-specific inhibitors with therapeutic potential.

The role of PP5 and PP2C in cardiac health and disease

Protein phosphatases are important, for example, as functional antagonists of β-adrenergic stimulation of the mammalian heart. While β-adrenergic stimulations increase the phosphorylation state of regulatory proteins and therefore force of contraction in the heart, these phosphorylations are reversed and thus force is reduced by the activity of protein phosphatases. In this context the role of PP5 and PP2C is starting to unravel. They do not belong to the same family of phosphatases with regard to sequence homology, many similarities with regard to location, activation by lipids and putative substrates have been worked out over the years. We also suggest which pathways for regulation of PP5 and/or PP2C described in other tissues and not yet in the heart might be useful to look for in cardiac tissue. Both phosphatases might play a role in signal transduction of sarcolemmal receptors in the heart. Expression of PP5 and PP2C can be increased by extracellular stimuli in the heart. Because PP5 is overexpressed in failing animal and human hearts, and because overexpression of PP5 or PP2C leads to cardiac hypertrophy and KO of PP5 leads to cardiac hypotrophy, one might argue for a role of PP5 and PP2C in heart failure. Because PP5 and PP2C can reduce, at least in vitro, the phosphorylation state of proteins thought to be relevant for cardiac arrhythmias, a role of these phosphatases for cardiac arrhythmias is also probable. Thus, PP5 and PP2C might be druggable targets to treat important cardiac diseases like heart failure, cardiac hypertrophy and cardiac arrhythmias.

Identification of candidate PAX2-regulated genes implicated in human kidney development

PAX2 is a transcription factor essential for kidney development and the main causative gene for renal coloboma syndrome (RCS). The mechanisms of PAX2 action during kidney development have been evaluated in mice but not in humans. This is a critical gap in knowledge since important differences have been reported in kidney development in the two species. In the present study, we hypothesized that key human PAX2-dependent kidney development genes are differentially expressed in nephron progenitor cells from induced pluripotent stem cells (iPSCs) in patients with RCS relative to healthy individuals. Cap analysis of gene expression revealed 189 candidate promoters and 71 candidate enhancers that were differentially activated by PAX2 in this system in three patients with RCS with PAX2 mutations. By comparing this list with the list of candidate Pax2-regulated mouse kidney development genes obtained from the Functional Annotation of the Mouse/Mammalian (FANTOM) database, we prioritized 17 genes. Furthermore, we ranked three genes-PBX1, POSTN, and ITGA9-as the top candidates based on closely aligned expression kinetics with PAX2 in the iPSC culture system and susceptibility to suppression by a Pax2 inhibitor in cultured mouse embryonic kidney explants. Identification of these genes may provide important information to clarify the pathogenesis of RCS, human kidney development, and kidney regeneration.

Epigenetic regulation of arginine vasopressin receptor 2 expression by PAX2 and Pax transcription interacting protein

Renal arginine vasopressin receptor 2 (AVPR2) plays a crucial role in osmoregulation. Engagement of ligand with AVPR2 results in aquaporin 2 movement to the apical membrane and water reabsorption from the urinary filtrate. Despite this essential role, little is known about transcriptional regulation of Avpr2. Here, we identify novel roles for PAX2, a transcription factor crucial for kidney development, and its adaptor protein, Pax transcription interacting protein (PTIP), for epigenetic regulation of Avpr2 and thus body water balance. Chromatin immunoprecipitation (ChIP) from murine inner medulla cells (IMCD-3) identified the minimal DNA-binding region of PAX2 on the Avpr2 promoter. Regulation of Avpr2 by PAX2 was confirmed using a heterologous DNA expression system. PAX2 recruits the adaptor protein PTIP and its associated histone methyltransferase (HMT) complex to Avpr2 promoter, imposing epigenetic marks on this region and throughout the coding sequence that modulate Avpr2 gene transcription. Reduction of PAX2 or PTIP protein levels by siRNA prevented histone lysine methylation and expression of Avpr2. ChIP using mouse or human kidneys determined that PAX2 is highly enriched in the AVPR2 promoter alongside PTIP and HMT proteins, leading to high levels of histone H3 lysine trimethylation within the promoter and throughout the gene. In conclusion, PAX2 provides locus specificity for PTIP, allowing the HMT complex to impart epigenetic changes at the Avpr2 locus and regulate Avpr2 transcription. These finding have major implications for understanding regulation of body water balance.NEW & NOTEWORTHY The transcription factor PAX2 plays an indispensable role in kidney development. In the adult kidney, we identified the first described protein this protein regulates. PAX2 and its interacting partner Pax transcription interacting protein recruit a histone methyltransferase complex to the promoter and epigentically regulate the expression of arginine vasopressin receptor 2, a protein that plays a crucial role in osmoregulation in the distal tubule.

Identification of HN252 as a potent inhibitor of protein phosphatase PPM1B

Protein phosphatase 1B (PPM1B), a member of metal-dependent protein serine/threonine phosphatase family, is involved in the regulation of several signalling pathways. However, our understanding of its substrate interaction and physiological functions is still largely limited. There is no reported PPM1B inhibitor to date. In this study, we identified HN252, a p-terphenyl derivative, as a potent PPM1B inhibitor (K_i  = 0.52 ± 0.06 µM). HN252 binding to PPM1B displayed remarkable and specific inhibition of PPM1B in both in vitro and ex vivo. With the aid of this small molecular inhibitor, we identified 30 proteins' serine/threonine phosphorylation as potential substrates of PPM1B, 5 of which were demonstrated by immunoprecipitation, including one known (CDK2) and 4 novel ones (AKT1, HSP90B, β-catenin and BRCA1). Furthermore, GO and KEGG analysis of dramatically phosphorylated proteins by PPM1B inhibition indicated that PPM1B plays roles in the regulation of multiple cellular processes and signalling pathways, such as gene transcription, inflammatory regulation, ageing and tumorigenesis. Our work provides novel insights into further investigation of molecular mechanisms of PPM1B.

Nuclear receptor corepressors in intellectual disability and autism

Autism spectrum disorder (ASD) is characterized by neurocognitive dysfunctions, such as impaired social interaction and language learning. Gene-environment interactions have a pivotal role in ASD pathogenesis. Nuclear receptor corepressors (NCORs) are transcription co-regulators physically associated with histone deacetylases (HDACs) and many known players in ASD etiology such as transducin β-like 1 X-linked receptor 1 and methyl-CpG binding protein 2. The epigenome-modifying NCOR complex is sensitive to many ASD risk factors, including HDAC inhibitor valproic acid and a variety of endocrine factors, xenobiotic chemicals, or metabolites that can directly bind to multiple nuclear receptors. Here, we review recent studies of NCORs in neurocognition using animal models and human genetics approaches. We discuss functional interplays between NCORs and other known players in ASD etiology. It is conceivable that the NCOR complex may bridge the in utero environmental risk factors of ASD with epigenetic remodeling and can serve as a converging point for many gene-environment interactions in the pathogenesis of ASD and intellectual disability.

CRISPR-Cas9-Mediated Correction of the G189R-PAX2 Mutation in Induced Pluripotent Stem Cells from a Patient with Focal Segmental Glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is defined by focal (involving few glomeruli) and segmental sclerosis of the glomerular tuft that manifests with nephrotic syndrome. Mutations in genes involved in the maintenance of structure and function of podocytes have been found in a minority of these patients. A family with adult-onset autosomal dominant FSGS was recently found to carry a new germline missense heterozygous mutation (p.G189R) in the octapeptide domain of the transcription factor PAX2. Here, we efficiently corrected this point mutation in patient-derived induced pluripotent stem cells (iPSCs) by means of CRISPR-Cas9-based homology-directed repair. The iPSC lines were differentiated into podocytes, which were tested for their motility. Editing the PAX2 p.G189R mutation restored podocyte motility, which was altered in podocytes derived from patient iPSCs.

Infection Reveals a Modification of SIRT2 Critical for Chromatin Association

Sirtuin 2 is a nicotinamide-adenine-dinucleotide-dependent deacetylase that regulates cell processes such as carcinogenesis, cell cycle, DNA damage, and infection. Subcellular localization of SIRT2 is crucial for its function but is poorly understood. Infection with the bacterial pathogen Listeria monocytogenes, which relocalizes SIRT2 from the cytoplasm to the chromatin, provides an ideal stimulus for the molecular study of this process. In this report, we provide a map of SIRT2 post-translational modification sites and focus on serine 25 phosphorylation. We show that infection specifically induces dephosphorylation of S25, an event essential for SIRT2 chromatin association. Furthermore, we identify a nuclear complex formed by the phosphatases PPM1A and PPM1B, with SIRT2 essential for controlling H3K18 deacetylation and SIRT2-mediated gene repression during infection and necessary for a productive Listeria infection. This study reveals a molecular mechanism regulating SIRT2 function and localization, paving the way for understanding other SIRT2-regulated cellular processes.

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Infection induces modification of the host deacetylase SIRT2 to target it to chromatin

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Dephosphorylation of serine 25 on SIRT2 controls its subcellular localization

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PPM1A and PPM1B form a nuclear complex to dephosphorylate serine 25

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The phosphorylation state of serine 25 is crucial for Listeria infection

A heterozygous deficiency in protein phosphatase Ppm1b results in an altered ovulation number in mice

Ppm1b, a metal‑dependent serine/threonine protein phosphatase, catalyzes the dephosphorylation of a variety of phosphorylated proteins. Ppm1b‑/‑ mouse embryos die at the fertilized oocyte stage, whereas Ppm1b+/‑ mice with a C57BL/6 background exhibit no phenotypic abnormalities. Because the C57BL/6 strain produces a limited number of pups, in an attempt to produce Ppm1b‑/‑ mice, congenic Ppm1b+/‑ mice with an ICR background were established, which are more fertile and gave birth to more pups. As a result, however, no Ppm1b‑/‑ offspring were obtained when pairs of Ppm1b+/‑ ICR mice were bred again. Ppm1b+/‑ male and female ICR mice were analyzed from the viewpoint of fecundity. The Ppm1b haploinsufficiency had no effect on testicular weight or the number of sperm in male mice. Despite the fact that the levels of Ppm1b protein in the ovaries of sexually mature Ppm1b+/‑ mice were decreased compared with those of Ppm1b+/+ mice, there appeared to be no significant difference in the histological appearance of the ovaries, litter sizes or plasma progesterone levels at the estrous stage. When superovulation was induced by stimulation using a hormone treatment, the number of ovulated oocytes were the same for Ppm1b+/‑ and Ppm1b+/+ mice at 4 weeks of age when the estrous cycle did not proceed, however, the number of ovulated oocytes was lower in sexually mature Ppm1b+/‑ mice at 11 weeks of age compared with Ppm1b+/+ mice in the first and the second superovulation cycles. These collective results suggest that follicle development is excessive in Ppm1b+/‑ mice, and that this leads to a partial depletion of matured follicles and a corresponding decrease in the number of ovulated oocytes.

Transcription factor Zbtb38 downregulates the expression of anti-inflammatory IL1r2 in mouse model of rheumatoid arthritis

DNA methylation is a decisive regulator of gene expression. Differentially methylated promoters were described in rheumatoid arthritis (RA), but we do not know how these epimutations can trigger a proinflammatory cytokine milieu. B cell-focused DNA methylome studies identified a group of genes that had undergone disease-associated changes in a murine model of RA. An arthritis-specific epimutation (hypomethylation) was detected in the promoter region of the Zbtb38 gene, which encodes a transcriptional repressor. Gene expression studies revealed that hypomethylation of the Zbtb38 promoter was accompanied by disease-specific repressor expression, and two anti-inflammatory factors interleukin 1 receptor 2 gene (IL1r2) and interleukin-1 receptor antagonist (IL1rn) were among the downregulated genes. We hypothesized that Zbtb38 repressor could induce downregulated expression of these anti-inflammatory genes and that this could significantly contribute to arthritis pathogenesis. Our studies demonstrate that Zbtb38 forms a molecular bridge between an arthritis-associated epimutation (DNA hypomethylation in Zbtb38 promoter) and transcriptional silencing of the IL1r2 gene in B cells. In this way, disease-associated DNA hypomethylation can support autoimmune arthritis by interfering with an anti-inflammatory pathway.

Epigenetic regulation of renal development

Developmental changes in cell fate are tightly regulated by cell-type specific transcription factors. Chromatin reorganization during organismal development ensures dynamic access of developmental regulators to their cognate DNA sequences. Thus, understanding the epigenomic states of promoters and enhancers is of key importance. Recent years have witnessed significant advances in our knowledge of the transcriptional mechanisms of kidney development. Emerging evidence suggests that histone deacetylation by class I HDACs and H3 methylation on lysines 4, 27 and 79 play important roles in regulation of early and late gene expression in the developing kidney. Equally exciting is the realization that nephrogenesis genes in mesenchymal nephron progenitors harbor bivalent chromatin domains which resolve upon differentiation implicating chromatin bivalency in developmental control of gene expression. Here, we review current knowledge of the epigenomic states of nephric cells and current techniques used to study the dynamic chromatin states. These technological advances will provide an unprecedented view of the enhancer landscape during cell fate commitment and help in defining the complex transcriptional networks governing kidney development and disease.

Collaborative repressive action of the antagonistic ETS transcription factors Pointed and Yan fine-tunes gene expression to confer robustness in Drosophila

The acquisition of cellular identity during development depends on precise spatiotemporal regulation of gene expression, with combinatorial interactions between transcription factors, accessory proteins and the basal transcription machinery together translating complex signaling inputs into appropriate gene expression outputs. The opposing repressive and activating inputs of the Drosophila ETS family transcription factors Yan and Pointed orchestrate numerous cell fate transitions downstream of receptor tyrosine kinase signaling, providing one of the premier systems for studying this process. Current models describe the differentiative transition as a switch from Yan-mediated repression to Pointed-mediated activation of common target genes. We describe here a new layer of regulation whereby Yan and Pointed co-occupy regulatory elements to repress gene expression in a coordinated manner, with Pointed being unexpectedly required for the genome-wide occupancy of both Yan and the co-repressor Groucho. Using even skipped as a test-case, synergistic genetic interactions between Pointed, Groucho, Yan and components of the RNA polymerase II pausing machinery suggest that Pointed integrates multiple scales of repressive regulation to confer robustness. We speculate that this mechanism may be used broadly to fine-tune the expression of many genes crucial for development.

Are Pax proteins potential therapeutic targets in kidney disease and cancer?

Pax genes encode developmental regulators that are expressed in a variety of tissues and control critical events in morphogenesis. In the kidney, Pax2 and Pax8 are expressed in embryonic development and in specific renal diseases associated with aberrant epithelial cell proliferation. Prior genetic and cell biological studies suggest that reducing the activity of Pax proteins in renal cancer or in polycystic kidney disease can slow the progression of these conditions. The Pax proteins may be critical for providing tissue and locus specificity to recruit epigenetic modifiers that control gene expression and chromatin structure. Although they are nuclear, targeting Pax proteins to inhibit function may be feasible with small molecules. Such inhibition of Pax protein function may provide novel therapies for subsets of renal disorders that are tissue- and cell type-specific and avoid systemic effects on non-Pax-expressing cells and tissues. Given the paucity of effective treatments for renal cancer and cystic disease, the Pax family of proteins represents new pharmaceutical targets that merit exploration and further development.

Repression of Interstitial Identity in Nephron Progenitor Cells by Pax2 Establishes the Nephron-Interstitium Boundary during Kidney Development

The kidney contains the functional units, the nephrons, surrounded by the renal interstitium. Previously we discovered that, once Six2-expressing nephron progenitor cells and Foxd1-expressing renal interstitial progenitor cells form at the onset of kidney development, descendant cells from these populations contribute exclusively to the main body of nephrons and renal interstitial tissues, respectively, indicating a lineage boundary between the nephron and renal interstitial compartments. Currently it is unclear how lineages are regulated during kidney organogenesis. We demonstrate that nephron progenitor cells lacking Pax2 fail to differentiate into nephron cells but can switch fates into renal interstitium-like cell types. These data suggest that Pax2 function maintains nephron progenitor cells by repressing a renal interstitial cell program. Thus, the lineage boundary between the nephron and renal interstitial compartments is maintained by the Pax2 activity in nephron progenitor cells during kidney organogenesis.

Inhibition of Pax2 Transcription Activation with a Small Molecule that Targets the DNA Binding Domain

The Pax gene family encodes DNA binding transcription factors that control critical steps in embryonic development and differentiation of specific cell lineages. Often, Pax proteins are re-expressed or ectopically expressed in cancer and other diseases of abnormal proliferation, making them attractive targets for tissue specific inhibition by small molecules. In this report, we used a homology model of the Pax2 paired domain and a virtual screen to identify small molecules that can inhibit binding of the paired domain to DNA and Pax2 mediated transcription activation. Candidates from the virtual screen were then confirmed in a cell based Pax2 transactivation assay. Subsequently, we tested analogs of these hits to identify a single compound that effectively blocked Pax2 activity and DNA binding with a K_d of 1.35-1.5 μM. The compound, termed EG1, was used to inhibit embryonic kidney development, a process directly dependent on Pax2 activity. Furthermore, we show that EG1 can inhibit proliferation of Pax2 positive renal and ovarian cancer cell lines but has little effect on Pax2 negative cancer cells. These data confirm that small molecules targeting the DNA binding paired domain can be identified and may be good lead compounds for developing tissue and cell-type specific anticancer therapies.

Master regulators in development: Views from the Drosophila retinal determination and mammalian pluripotency gene networks

Among the mechanisms that steer cells to their correct fate during development, master regulatory networks are unique in their sufficiency to trigger a developmental program outside of its normal context. In this review we discuss the key features that underlie master regulatory potency during normal and ectopic development, focusing on two examples, the retinal determination gene network (RDGN) that directs eye development in the fruit fly and the pluripotency gene network (PGN) that maintains cell fate competency in the early mammalian embryo. In addition to the hierarchical transcriptional activation, extensive positive transcriptional feedback, and cooperative protein-protein interactions that enable master regulators to override competing cellular programs, recent evidence suggests that network topology must also be dynamic, with extensive rewiring of the interactions and feedback loops required to navigate the correct sequence of developmental transitions to reach a final fate. By synthesizing the in vivo evidence provided by the RDGN with the extensive mechanistic insight gleaned from the PGN, we highlight the unique regulatory capabilities that continual reorganization into new hierarchies confers on master control networks. We suggest that deeper understanding of such dynamics should be a priority, as accurate spatiotemporal remodeling of network topology will undoubtedly be essential for successful stem cell based therapeutic efforts.

Association of PAX2 and Other Gene Mutations with the Clinical Manifestations of Renal Coloboma Syndrome

BACKGROUND

Renal coloboma syndrome (RCS) is characterized by renal anomalies and optic nerve colobomas. PAX2 mutations contribute to RCS. However, approximately half of the patients with RCS have no mutation in PAX2 gene.

METHODS

To investigate the incidence and effects of mutations of PAX2 and 25 candidate genes, patient genes were screened using next-generation sequence analysis, and candidate mutations were confirmed using Sanger sequencing. The correlation between mutations and clinical manifestation was evaluated.

RESULT

Thirty patients, including 26 patients (two families of five and two, 19 sporadic cases) with RCS, and 4 optic nerve coloboma only control cases were evaluated in the present study. Six PAX2 mutations in 21 probands [28%; two in family cohorts (n = 5 and n = 2) and in 4 out of 19 patients with sporadic disease] including four novel mutations were confirmed using Sanger sequencing. Moreover, four other sequence variants (CHD7, SALL4, KIF26B, and SIX4) were also confirmed, including a potentially pathogenic novel KIF26B mutation. Kidney function and proteinuria were more severe in patients with PAX2 mutations than in those without the mutation. Moreover, the coloboma score was significantly higher in patients with PAX2 gene mutations. Three out of five patients with PAX2 mutations had focal segmental glomerulosclerosis (FSGS) diagnosed from kidney biopsies.

CONCLUSION

The results of this study identify several new mutations of PAX2, and sequence variants in four additional genes, including a novel potentially pathogenic mutation in KIF26B, which may play a role in the pathogenesis of RCS.

Pax factors in transcription and epigenetic remodelling

The nine Pax transcription factors that constitute the mammalian family of paired domain (PD) factors play key roles in many developmental processes. As DNA binding transcription factors, they exhibit tremendous variability and complexity in their DNA recognition patterns. This is ascribed to the presence of multiple DNA binding structural domains, namely helix-turn-helix (HTH) domains. The PD contains two HTH subdomains and four of the nine Pax factors have an additional HTH domain, the homeodomain (HD). We now review these diverse DNA binding modalities together with their properties as transcriptional activators and repressors. The action of Pax factors on gene expression is also exerted through recruitment of chromatin remodelling complexes that introduce either activating or repressive chromatin marks. Interestingly, the recent demonstration that Pax7 has pioneer activity, the unique property to "open" chromatin, further underlines the mechanistic versatility and the developmental importance of these factors.