PAX2 mutations in oligomeganephronia

Reactive oxygen species and nuclear factor-kappa B pathway mediate high glucose-induced Pax-2 gene expression in mouse embryonic mesenchymal epithelial cells and kidney explants

Diabetic mellitus confers a major risk of congenital malformations, and is associated with diabetic embryopathy, affecting multiple organs including the kidney. The DNA paired box-2 (Pax-2) gene is essential in nephrogenesis. We investigated whether high glucose alters Pax-2 gene expression and aimed to delineate its underlying mechanism(s) of action using both in vitro (mouse embryonic mesenchymal epithelial cells (MK4) and ex vivo (kidney explant from Hoxb7-green florescent protein (GFP) mice) approaches. Pax-2 gene expression was determined by reverse transcriptase-polymerase chain reaction, Western blotting, and immunofluorescent staining. A fusion gene containing the full-length 5'-flanking region of the human Pax-2 promoter linked to a luciferase reporter gene, pGL-2/hPax-2, was transfected into MK4 cells with or without dominant negative IkappaBalpha (DN IkappaBalpha) cotransfection. Fusion gene expression level was quantified by cellular luciferase activity. Reactive oxygen species (ROS) generation was measured by lucigenin assay. Embryonic kidneys from Hoxb7-GFP mice were cultured ex vivo. High D(+) glucose (25 mM), compared to normal glucose (5 mM), specifically induced Pax-2 gene expression in MK4 cells and kidney explants. High glucose-induced Pax-2 gene expression is mediated, at least in part, via ROS generation and activation of the nuclear factor kappa B signaling pathway, but not via protein kinase C, p38 mitogen-activated protein kinase (MAPK), and p44/42 MAPK signaling.

Prevalence of mutations in renal developmental genes in children with renal hypodysplasia: results of the ESCAPE study

Renal hypodysplasia (RHD) is characterized by a reduced nephron number, small kidney size, and disorganized renal tissue. A hereditary basis has been established for a subset of affected patients, suggesting a major role of developmental genes that are involved in early kidney organogenesis. Gene mutations that have dominant inheritance and cause RHD, urinary tract anomalies, and defined extrarenal symptoms have been identified in TCF2 (renal cysts and diabetes syndrome), PAX2 (renal-coloboma syndrome), EYA1 and SIX1 (branchio-oto-renal syndrome), and SALL1 (Townes-Brocks syndrome). For estimation of the prevalence of these events, an unselected cohort of 99 unrelated patients with RHD that was associated with chronic renal insufficiency were screened for mutations in TCF2, PAX2, EYA1, SIX1, and SALL1. Mutations or variants in the genes of interest were detected in 17 (17%) unrelated families: One mutation, two variants, and four deletions of TCF2 in eight unrelated patients; four different PAX2 mutations in six families; one EYA1 mutation and one deletion in two patients with branchio-oto-renal syndrome; and one SALL1 mutation in a patient with isolated RHD. Of a total of 27 patients with renal cysts, six (22%) carried a mutation in TCF2. It is interesting that a SIX1 sequence variant was identified in two siblings with renal-coloboma syndrome as a result of a PAX2 mutation, suggesting an oligogenic inheritance. Careful clinical reevaluation that focused on discrete extrarenal symptoms and thorough family analysis revealed syndrome-specific features in nine of the 17 patients. In conclusion, 15% of patients with RHD show mutations in TCF2 or PAX2, whereas abnormalities in EYA1, SALL1, and SIX1 are less frequent.

Neuronal apoptosis inhibitory protein is expressed in developing kidney and is regulated by PAX2

During fetal kidney development, the extent of ureteric bud (UB) branching will determine final nephron endowment for life. Nephron number varies widely among normal humans and those who are born at the low end of the nephron number spectrum may be at risk for essential hypertension in adulthood. Little is known about how nephron number is set. However, we previously showed that the transcription factor, Pax2, suppresses apoptosis in UB cells during kidney development and optimizes branching morphogenesis. Here, we report that PAX2 directly binds to a specific recognition motif in the human neuronal apoptosis inhibitory protein (NAIP) gene promoter. NAIP is an endogenous inhibitor of apoptosis, inactivating caspase-3 and caspase-7 in neuronal tissues. PAX2 activates NAIP gene transcription (7-fold) in vitro and NAIP transcript level is increased fourfold in HEK293 cells stably transfected with PAX2. We show that Naip is expressed in embryonic day 15 (E15) fetal kidney tissue (RT-PCR) and NAIP protein is demonstrated by immunohistochemistry in E15 mouse kidney collecting ducts and P1 proximal tubules. Naip mRNA is significantly reduced (50%) in heterozygous Pax2 mutant mice. Finally, we show that an antisense Naip1 cDNA transfected into murine collecting duct cells doubles caspase-3/7 activity induced by Baxalpha. These observations suggest that the powerful effects of PAX2 on renal branching morphogenesis and final nephron number may be mediated by activation of Naip which then suppresses apoptosis in UB cells.

Suppression of ureteric bud apoptosis rescues nephron endowment and adult renal function in Pax2 mutant mice

The molecular mechanisms that set congenital nephron number are unknown. However, humans with modest suboptimal nephron number may be at increased risk for essential hypertension, and those with more severe nephron deficits at birth may develop progressive renal insufficiency. A model of branching morphogenesis during fetal kidney development in which the extent of ureteric bud arborization is dependent on suppression of programmed cell death has been proposed. This study shows that the increased apoptosis and reduced ureteric bud branching of heterozygous Pax2 mutant mice is associated with 40% decrease in nephron number at birth. This leads to postnatal glomerular hypertrophy and long-term renal insufficiency in the absence of glomerulosclerosis. To determine whether restoration of antiapoptotic factors alone is sufficient to rescue the nephron deficit in these mice, a BCL2 transgene that is under the control of the PAX2 promoter was targeted to the ureteric bud. The transgene suppressed programmed cell death in the ureteric bud lineage, increased nephron number to 90% of that of wild-type littermates at birth, and normalized renal function at 1 yr. These observations lend strong support to the hypothesis that factors that control ureteric bud apoptosis are powerful determinants of congenital nephron endowment.

Molecular mechanisms of oestrogen and SERMs in endometrial carcinogenesis

Endometrial cancer is the most common gynaecological cancer, and is associated with endometrial hyperplasia, unopposed oestrogen exposure and adjuvant therapy for breast cancer using selective oestrogen-receptor modulators (SERMs), particularly tamoxifen. Oestrogen and SERMs are thought to be involved in endometrial carcinogenesis through their effects on transcriptional regulation. Ultimately, oestrogen and SERMs affect the transduction of cellular signalling pathways that govern cell growth and proliferation, through downstream effectors such as PAX2 (paired box 2).

Adult hypertension and kidney disease: the role of fetal programming

Hypertension (HTN) and chronic kidney disease are highly prevalent diseases that tend to occur more frequently among disadvantaged populations, in whom prenatal care also tends to be poor. More and more evidence is emerging highlighting the important role of fetal programming in the development of adult disease, suggesting a possible common pathophysiologic denominator in the development of these disorders. Epidemiologic evidence accumulated over the past 2 decades has demonstrated an association between low birth weight and subsequent adult HTN, diabetes, and cardiovascular disease. More recently, a similar association has been found with chronic kidney disease. Animal studies and indirect evidence from human studies support the hypothesis that low birth weight, as a marker of adverse intrauterine circumstances, is associated with a congenital deficit in nephron number. The precise mechanism of the reduction in nephron number has not been established, but several hypotheses have been put forward, including changes in DNA methylation, increased apoptosis in the developing kidney, alterations in renal renin-angiotensin system activity, and increased fetal glucocorticoid exposure. A reduction in nephron number is associated with compensatory glomerular hypertrophy and an increased susceptibility to renal disease progression. HTN in low birth weight individuals also appears to be mediated in part through a reduction in nephron number. Increased awareness of the implications of low birth weight and inadequate prenatal care should lead to public health policies that may have long-term benefits in curbing the epidemics of HTN, diabetes, and kidney disease in generations to come.

Renal hypoplasia: lessons from Pax2

The functions of Pax2 during renal development are many. It organizes caudal descent of the nephric duct, emergence of the ureteric bud, branching morphogenesis, and sustained arborization of the collecting system. In this review, we use lessons from the study of Pax2 as organizing principles to focus on the developmental processes which, if disrupted, might lead to renal hypoplasia in humans. We consider the problem of renal hypoplasia as a continuum, ranging from renal agenesis to subtle congenital nephron deficits. Early failure in the first two developmental stages (e.g. homozygous inactivation of Pax2) should preclude formation of metanephric kidneys and cause bilateral renal agenesis, incompatible with life. Interference with the later stages affects the extent of branching morphogenesis (e.g. heterozygous Pax2 mutations). Although the resulting nephron deficits are compatible with life, they may be moderately severe and account for up to 40% of the children in dialysis and transplant units around the world. Finally, the effect of Pax2 on apoptosis in the branching ureteric bud seems to imply a quantitative process which is finely tuned. Modest changes in this program could account for subtle nephron deficits in "normal" humans and increased risk of hypertension or susceptibility to acquired renal disease later in life.

Interaction of the LMX1B and PAX2 gene products suggests possible molecular basis of differential phenotypes in Nail-Patella syndrome

The LMX1B gene, encoding a protein involved in limb, kidney and eye development, is mutated in patients affected by Nail-Patella syndrome. Inter- and intrafamilial variability is common in this disorder for skeletal abnormalities, presence and severity of nephropathy and ocular anomalies. Phenotypic variability might depend on interactions of the LMX1B causative gene with other genes during development of both kidney and eye, which might act as modifier genes. Results are presented on the interaction between LMX1B and PAX2 proteins, obtained by both direct yeast two-hybrid assay and coimmunoprecipitation. Such interaction provides support to further studies on pathways underlying important developmental processes.

Multicystic dysplastic kidney and variable phenotype in a family with a novel deletion mutation of PAX2

The renal coloboma syndrome (OMIM 120330) is caused by mutations in the PAX2 gene. Typical findings in these patients include renal hypoplasia, renal insufficiency, vesicoureteric reflux, and optic disc coloboma. A family with a novel heterozygous 10-bp deletion in exon 2 of the PAX2 gene leading to a truncating mutation and variable phenotype across three generations is reported. The first presentation of multicystic dysplastic kidney in this syndrome is reported. The possibility that abnormal PAX2 protein in this case may cause a dominant negative effect also is discussed. The finding of multicystic dysplastic kidney in renal coloboma syndrome could suggest that PAX2 may play a role in early ureteric obstruction and subsequent renal maldevelopment.

Glomerular hyperfiltration and hypertrophy in the rat hypoplastic kidney as a model of oligomeganephronic disease

BACKGROUND

Rat male hypogonadisim (hgn/hgn) is accompanied by bilateral hypoplastic kidney (HPK). The HPK contains a reduced number of nephrons that progress to chronic renal failure. In this study, we describe the renal pathophysiology in adult HPK rats as a potential model of oligomeganephronic disease.

METHODS

Urine and blood samples were collected from adult male HPK rats and phenotypically normal littermates at 70 days of age for measurements of urea-nitrogen and creatinine clearances (Cun and Ccr). Glomerular number (GN) and glomerular projective area were determined using the maceration method. Blood pressure was measured. Urinary protein and renal histology were examined. Urinary albumin concentration was determined at early postnatal ages.

RESULTS

Renal weight was significantly smaller in adult HPK males than in normal males. Polyuria and polydipsia were observed in HPK rats. Ccr and Cun were low in HPK rats compared with those in normal rats. The HPK contained only 20% of the nephrons present in normal kidneys. The Cun and Ccr divided by GN (average values of single nephron Cun and Ccr) of HPK rats were about two and four times greater than normal levels, respectively. This hyperfiltration was not accompanied by systemic hypertension, but was associated with marked glomerular hypertrophy and glomerulosclerosis, which were observed mainly in the inner cortex. A considerable heterogeneity of glomerular size was found in HPK and most glomeruli of surface nephrons retained normal size and histology. A remarkable leakage of albumin into urine was found at 35 and 70 days of age.

CONCLUSIONS

The HPK rat is a useful model for studying the pathophysiology of oligomeganephronic disease as well as glomerular hyperfiltration and hypertrophy induced by severe congenital reduction of nephrons.

Angiotensin II stimulates Pax-2 in rat kidney proximal tubular cells: impact on proliferation and apoptosis

BACKGROUND

The intrarenal renin-angiotensin system (RAS) is intimately involved in the tubular cell proliferation, apoptosis and regeneration that occur following renal injury. Though tubular angiotensin II (Ang II) type 2 receptors (AT2R) decrease greatly after birth, their number increases after injury. Notably, during recovery from injury, renal tubular cells display a relatively immature phenotype expressing genes that are involved in nephron development, for example, the paired homeobox-2 gene (Pax-2). The present investigation hypothesized that AT2R activation would stimulate Pax-2 gene expression in immortalized rat renal proximal tubular cells (IRPTC), as we have found in fetal cells.

METHODS

Pax-2 gene expression in IRPTC was evaluated by immunofluorescence, Western blot, reverse transcription-polymerase chain reaction (RT-PCR) with or without Ang II treatment; apoptosis and proliferation were analyzed by terminal transferase-mediated deoxyuridine triphosphate (dUTP) nick end-labeling (TUNEL) assay and bromodeoxyuridine (BrdU) incorporation in stable IRPTC transformants with Pax-2 sense and antisense orientation, respectively.

RESULTS

Ang II up-regulated Pax-2 gene expression via AT2R in IRPTC. The stimulatory effect of both Ang II on Pax-2 gene expression was blocked by PD123319 (AT2R inhibitor), AG 490 (specific Janus kinase 2 (JAK2) inhibitor) and genistein (tyrosine kinase inhibitor), but not by losartan (AT1R inhibitor). Stable transfection of sense Pax-2 cDNA increased, whereas antisense Pax-2 cDNA down-regulated Pax-2 expression.

CONCLUSION

Our studies suggest that Ang II stimulates Pax-2 gene expression in IRPTC via AT2R and the JAK2/signal transducers and activators of transcription (STAT) signaling transduction pathway, which may be important in renal repair following injury. Cells lacking Pax-2 gene expression appear to be prone toward apoptosis rather than proliferation.

Congenital disorders of the optic nerve: excavations and hypoplasia

The principal congenital abnormalities of the optic disc that can significantly impair visual function are excavation of the optic disc and optic nerve hypoplasia. The excavated optic disc abnormalities comprise optic disc coloboma, morning glory syndrome, and peripapillary staphyloma. Optic nerve hypoplasia manifests as a small optic nerve, which may or may not be accompanied by a peripapillary ring (the double ring sign). In addition, the optic disc cupping, which occurs as a sequel to some cases of periventricular leucomalacia, can arguably be classified as a type of optic nerve hypoplasia. All of these conditions can be unilateral or bilateral and can impair visual function mildly or severely. It is essential that children with poor vision due to any of these conditions are managed by treating refractive errors, giving occlusion therapy in selected cases, and optimising the conditions at home and at school in an attempt to ensure that impaired vision does not impede development or education.

Ureteric Bud Apoptosis and Renal Hypoplasia in Transgenic PAX2-Bax Fetal Mice Mimics the Renal-Coloboma Syndrome

ABSTRACT. In humans, PAX2 haploinsufficiency causes renal-coloboma syndrome (RCS) involving eye abnormalities, renal hypoplasia, and renal failure in early life. The authors previously showed that heterozygous mutant Pax2 mice have smaller kidneys with fewer nephrons, associated with elevated apoptosis in the ureteric bud (UB). However, PAX2 may have a variety of developmental functions such as effects on cell fate and differentiation. To determine whether apoptosis alone is sufficient to cause a UB branching deficit, the authors targeted a pro-apoptotic gene (Baxα) to the embryonic kidney under the control of human PAX2 regulatory elements. The exogenous PAX2 promoter directed Baxα gene expression specifically to the developing kidney UB, eye, and mid/hindbrain. At E15.5 PAX2Promoter-Baxα fetal mice exhibited renal hypoplasia, elevated UB apoptosis, and retinal defects, mimicking the phenotype observed in RCS. The kidneys of E15.5 PAX2Promoter-Baxα fetal mice were 55% smaller than those of wild-type fetal mice, and they contained 70% of the normal level of UB branching. The data indicate that loss of Pax2 anti-apoptotic activity is sufficient to account for the reduced UB branching observed in RCS and suggest that elevated UB apoptosis may be a key process responsible for renal hypoplasia. The authors propose a morphogenic unit model in which cell survival influences the rate of UB branching and determines final nephron endowment. E-mail: meccles@otago.ac.nz

The molecular control of renal branching morphogenesis: current knowledge and emerging insights

Mammalian kidney development requires the formation of a patterned, branched network of collecting ducts, a process termed renal branching morphogenesis. Disruption of renal branching morphogenesis during human kidney development results in renal dysplasia, the major cause of renal failure in young children. Genetic evidence, combined with in vitro data, have implicated transcription factors, secreted growth factors, and cell surface signaling peptides as critical regulators of renal branching morphogenesis. This review discusses the current knowledge regarding the regulation of renal branching morphogenesis in vivo provided by the analysis of genetic mutations in mice and humans which disrupt collecting duct system development. In addition, in vivo and in vitro evidence regarding the functions of several other gene families are considered, rendering new insight into emerging regulatory roles for these molecules in renal branching morphogenesis.