Functional analysis of BMP4 mutations identified in pediatric CAKUT patients

Dominant Stickler Syndrome

The Stickler syndromes are a group of genetic connective tissue disorders associated with an increased risk of rhegmatogenous retinal detachment, deafness, cleft palate, and premature arthritis. This review article focuses on the molecular genetics of the autosomal dominant forms of the disease. Pathogenic variants in COL2A1 causing Stickler syndrome usually result in haploinsufficiency of the protein, whereas pathogenic variants of type XI collagen more usually exert dominant negative effects. The severity of the disease phenotype is thus dependent on the location and nature of the mutation, as well as the normal developmental role of the respective protein.

Btg2 mutation induces renal injury and impairs blood pressure control in female rats

Hypertension (HTN) is a complex disease influenced by heritable genetic elements and environmental interactions. Dietary salt is among the most influential modifiable factors contributing to increased blood pressure (BP). It is well established that men and women develop BP impairment in different patterns and a recent emphasis has been placed on identifying mechanisms leading to the differences observed between the sexes in HTN development. The current work reported here builds on an extensive genetic mapping experiment which sought to identify genetic determinants of salt sensitive (SS) HTN using the Dahl SS rat. BTG anti-proliferation factor 2 (Btg2) was previously identified by our group as a candidate gene contributing to SS HTN in female rats. In the current study, Btg2 was mutated using TALEN targeted gene disruption on the SSBN congenic rat background. The Btg2 mutated rats exhibited impaired BP and proteinuria responses to a high salt diet compared to wild type rats. Differences in body weight, mutant pup viability, skeletal morphology, and adult nephron density suggest a potential role for Btg2 in developmental signaling pathways. Subsequent cell cycle gene expression assessment provides several additional signaling pathways that Btg2 may function through during salt handling in the kidney. The expression analysis also identified several potential upstream targets that can be explored to further isolate therapeutic approaches for SS HTN.

Targeting optimal PD management in children: what have we learned from the IPPN registry?

National and international registries have great potential for providing data that describe disease burden, treatments, and outcomes especially in rare diseases. In the setting of pediatric end-stage renal disease (ESRD), the available data are limited to highly developed countries, whereas the lack of data from emerging economies blurs the global perspective. In order to improve the pediatric dialysis care worldwide, provide global benchmarking of pediatric dialysis outcome, and assign useful tools and management algorithms based on evidence-based medicine, the International Pediatric Peritoneal Dialysis Network (IPPN) was established in 2007. In recent years, the Registry has provided comprehensive data on relevant clinical issues in pediatric peritoneal dialysis patients including nutritional status, growth, cardiovascular disease, anemia management, mineral and bone disorders, preservation of residual kidney function, access-related complications, and impact of associated comorbidities. A unique feature of the registry is the ability to compare practices and outcomes between countries and world regions. In the current review, we describe study design and collection methods, summarize the core IPPN findings based on its 12-year experience and 13 publications, and discuss the future perspective.

Dental and Craniofacial Characteristics in Patients With 14Q22.1-Q22.2 Deletion: A Case Series

This case series is a follow-up report focusing on dental and facial characteristics in patients with a rare microdeletion in chromosome 14q22.1-q22.2. Usually, these patients have severe ocular, brain, and digital abnormalities. However, this case series shows that clinical presentation can be mild. Four relatives spanning 3 generations were diagnosed with a familial autosomal dominant 2.79 Mb microdeletion in chromosome 14q22.1-q22.2. Genetic screening was done by the Bacterial Artificial Chromosome array-comparative genome hybridization and was confirmed by the fluorescence in situ hybridization technique. Dental and craniofacial data were collected from medical files, clinical examinations, clinical photos, panoramic and cephalometric radiographs, and dental casts. Written informed consent for scientific use was obtained for all family members. No larger syndrome could be identified. All cases had similar facial red flag characteristics, consisting of a long face with retrognathia and open mouth relation, associated oral clefts in varying degrees, depressed nasal bridge, delayed tooth development, hypertelorism, and low-set angular ears. The dental casts showed a distal molar occlusion and a lack of space in the dental arches. Developmental delay was noted together with limb defects such as poly- and syndactyly. Microphthalmia and hearing loss were present in the most severe cases. This rare congenital disorder, associated with facial dysmorphia, oral clefts, and tooth agenesis, can remain undiagnosed until adulthood. A family history of short stature, developmental delay, poly- or syndactyly, and micropthalmia are suggestive features. Similar reports help to raise awareness among dental practitioners, leading to an early genetic diagnosis.

Mouse Models of Congenital Kidney Anomalies

Congenital anomalies of the kidney and urinary tract (CAKUT) are common birth defects, which cause the majority of chronic kidney diseases in children. CAKUT covers a wide range of malformations that derive from deficiencies in embryonic kidney and lower urinary tract development, including renal aplasia, hypodysplasia, hypoplasia, ectopia, and different forms of ureter abnormalities. The majority of the genetic causes of CAKUT remain unknown. Research on mutant mice has identified multiple genes that critically regulate renal differentiation. The data generated from this research have served as an excellent resource to identify the genetic bases of human kidney defects and have led to significantly improved diagnostics. Furthermore, genetic data from human CAKUT studies have also revealed novel genes regulating kidney differentiation.

BMP4 mutations in tooth agenesis and low bone mass

OBJECTIVE

To identify an uncommon genetic cause of tooth agenesis (TA) by utilizing whole exome sequencing (WES) and targeted Sanger sequencing in a cohort of 120 patients with isolated TA.

DESIGN

One deleterious mutation in the gene encoding bone morphogenetic protein 4 (BMP4) was identified in 6 unrelated patients with TA by WES. After that, the coding exons of BMP4 were examined in 114 TA patients using Sanger sequencing. Dual-energy X-ray absorptiometry (DEXA) was used to measure the bone mineral density of patients who carried a BMP4 mutation. Finally, preliminary functional studies of two BMP4 mutants were performed.

RESULTS

We detected 3 novel missense mutations (c.58 G > A: p.Gly20Ser, c.326 G > T: p.Arg109Leu and c.614 T > C: p.Val205Ala) and 1 reported mutation in the BMP4 gene among 120 TA probands. The previously reported BMP4 mutation (c.751C > T: p.His251Tyr) was associated with urethra and eye anomalies. By extending the pedigrees, we determined that the tooth phenotypes had an autosomal dominant inheritance pattern, as individuals carrying a BMP4 mutation exhibit different types of dental anomalies. Interestingly, we observed that patients harboring a BMP4 mutation manifested early onset osteopenia or osteoporosis. Further in vitro functional assays demonstrated that two BMP4 mutants resulted in a decreased activation of Smad signaling. Therefore, a loss-of-function in BMP4 may contribute to the clinical phenotypes seen in this study.

CONCLUSIONS

We identified 4 mutations in the BMP4 gene in 120 TA patients. To our knowledge, this is the first study to describe human skeletal diseases associated with BMP4 mutations.

Kongenitale Anomalien der Nieren und ableitenden Harnwege (CA KUT)

Der Begriff CAKUT (Congenital Anomalies of the Kidney and Urinary Tract) bezeichnet diverse angeborene Fehlbildungen der Nieren und ableitenden Harnwege. Da alle CAKUT-Phänotypen zusammengenommen etwa 15–30 % aller pränatal diagnostizierten Fehlbildungen ausmachen und etwa 40 % der Fälle mit terminalem Nierenversagen bei Kindern und Jugendlichen verursachen, sind diese Anomalien epidemiologisch hochrelevant. Die Diagnosestellung erfolgt mit radiologischen Verfahren, insbesondere mit Ultraschall, wobei bei vielen Patienten eine Kombination verschiedener CAKUT-Phänotypen nachgewiesen wird. CAKUT tritt zu etwa 85 % sporadisch auf, zu etwa 15 % familiär. Das Vererbungsmuster ist häufig dominant, kann aber auch rezessiv sein. CAKUT kann isoliert auftreten, aber auch als Teil einer syndromalen Erkrankung. Variable Expressivität und inkomplette Penetranz sind bei CAKUT häufig. CAKUT ist genetisch sehr heterogen. Im Mausmodell wurden bislang über 180 CAKUT-assoziierte Gene beschrieben. Da Mutationen in den etwa 50 bisher bekannten humanen CAKUT-Genen nur ca. 20 % der CAKUT-Fälle erklären und sich verschiedene chromosomale Aberrationen wie Mikrodeletionen in weiteren ca. 15 % der Patienten insbesondere mit syndromalen CAKUT finden, sind exom-/genomweite Screeningverfahren für die Aufklärung genetischer CAKUT-Ursachen besonders geeignet. Bei sporadischen Fällen ist eine Trio-basierte Analyse der Exome/Genome von Patienten-Eltern-Trios zur Identifizierung von De-novo-Aberrationen und biallelischen Varianten vielversprechend. Eine Abklärung der genetischen Ursache ist für die Präzisierung von Wiederholungsrisiken sowie eine gezielte Untersuchung von CAKUT-Patienten im Hinblick auf extrarenale Phänotypen von klinischer Bedeutung.

Genetics of Congenital Anomalies of the Kidney and Urinary Tract: The Current State of Play

Congenital anomalies of the kidney and urinary tract (CAKUT) are the most frequent form of malformation at birth and represent the cause of 40–50% of pediatric and 7% of adult end-stage renal disease worldwide. The pathogenesis of CAKUT is based on the disturbance of normal nephrogenesis, secondary to environmental and genetic causes. Often CAKUT is the first clinical manifestation of a complex systemic disease, so an early molecular diagnosis can help the physician identify other subtle clinical manifestations, significantly affecting the management and prognosis of patients. The number of sporadic CAKUT cases explained by highly penetrant mutations in a single gene may have been overestimated over the years and a genetic diagnosis is missed in most cases, hence the importance of identifying new genetic approaches which can help unraveling the vast majority of unexplained CAKUT cases. The aim of our review is to clarify the current state of play and the future perspectives of the genetic bases of CAKUT.

Nephron number, hypertension, and CKD: physiological and genetic insight from humans and animal models

The kidneys play a vital role in the excretion of waste products and the regulation of electrolytes, maintenance of acid-base balance, regulation of blood pressure, and production of several hormones. Any alteration in the structure of the nephron (basic functional unit of the kidney) can have a major impact on the kidney's ability to work efficiently. Progressive decline in kidney function can lead to serious illness and ultimately death if not treated by dialysis or transplantation. While there have been numerous studies that implicate lower nephron numbers as being an important factor in influencing susceptibility to developing hypertension and chronic kidney disease, a direct association has been difficult to establish because of three main limitations: 1) the large variation in nephron number observed in the human population; 2) no established reliable noninvasive methods to determine nephron complement; and 3) to date, nephron measurements have been done after death, which doesn't adequately account for potential loss of nephrons with age or disease. In this review, we will provide an overview of kidney structure/function, discuss the current literature for both humans and other species linking nephron deficiency and cardio-renal complications, as well as describe the major molecular signaling factors involved in nephrogenesis that modulate variation in nephron number. As more detailed knowledge about the molecular determinants of nephron development and the role of nephron endowment in the cardio-renal system is obtained, it will hopefully provide clinicians the ability to accurately identify people at risk to develop CKD/hypertension and lead to a shift in patient care from disease treatment to prevention.

Dysfunctional miRNA-Mediated Regulation in Chromophobe Renal Cell Carcinoma

Past research on pathogenesis of a complex disease suggests that differentially expressed message RNAs (mRNAs) can be noted as biomarkers of a disease. However, significant miRNA-mediated regulation change might also be more deep underlying cause of a disease. In this study, a miRNA-mediated regulation module is defined based on GO terms (Gene Ontology terms) from which dysfunctional modules are identified as the suspected cause of a disease. A miRNA-mediated regulation module contains mRNAs annotated to a GO term and MicroRNAs (miRNAs) which regulate the mRNAs. Based on the miRNA-mediated regulation coefficients estimated from the expression profiles of the mRNA and the miRNAs, a SW (single regulation-weight) value is then designed to evaluate the miRNA-mediated regulation change of an mRNA, and the modules with significantly differential SW values are thus identified as dysfunctional modules. The approach is applied to Chromophobe renal cell carcinoma and it identifies 70 dysfunctional miRNA-mediated regulation modules from initial 4381 modules. The identified dysfunctional modules are detected to be comprehensive reflection of chromophobe renal cell carcinoma. The proposed approach suggests that accumulated alteration in miRNA-mediated regulation might cause functional alterations, which further cause a disease. Moreover, this approach can also be used to identify diffentially miRNA-mediated regulated mRNAs showing more comprehensive underlying association with a disease than differentially expressed mRNAs.

Genetic, environmental, and epigenetic factors involved in CAKUT

Congenital anomalies of the kidney and urinary tract (CAKUT) refer to a spectrum of structural renal malformations and are the leading cause of end-stage renal disease in children. The genetic diagnosis of CAKUT has proven to be challenging due to genetic and phenotypic heterogeneity and incomplete genetic penetrance. Monogenic causes of CAKUT have been identified using different approaches, including single gene screening, and gene panel and whole exome sequencing. The majority of the identified mutations, however, lack substantial evidence to support a pathogenic role in CAKUT. Copy number variants or single nucleotide variants that are associated with CAKUT have also been identified. Numerous studies support the influence of epigenetic and environmental factors on kidney development and the natural history of CAKUT, suggesting that the pathogenesis of this syndrome is multifactorial. In this Review we describe the current knowledge regarding the genetic susceptibility underlying CAKUT and the approaches used to investigate the genetic basis of CAKUT. We outline the associated environmental risk factors and epigenetic influences on CAKUT and discuss the challenges and strategies used to fully address the involvement and interplay of these factors in the pathogenesis of the disease.

Rare variants in BMP2 and BMP4 found in otosclerosis patients reduce Smad signaling

HYPOTHESIS

Genetic variation in BMP2 and BMP4 found in otosclerosis patients result in altered Smad signaling.

BACKGROUND

Otosclerosis is a common form of adult-onset conductive hearing loss resulting from abnormal bone remodeling of the bony labyrinth that surrounds the inner ear. Both genetic and environmental factors are implicated in the disease, yet very little is known about its pathogenesis. The evidence for a genetic component has been established through family-based linkage and population-based association studies. Previously, members of the TGF-β superfamily of genes have been associated with otosclerosis.

METHODS

Sequencing of BMP2 and BMP4 coding regions was performed to identify common and rare variation in German otosclerosis patients compared with controls. Functional analyses of rare variation in the patient cohort were conducted by exposing an osteosarcoma cell line to conditioned media containing either wild type or variant forms of BMP2 or BMP4 and analyzing Smad1/5/8 phosphorylation.

RESULTS

Although no significant association with common variation in these 2 genes was detected, there were 8 singleton variants identified in the German population. Of the 4 coding variants found solely in otosclerosis patients, two--BMP4(N150K) and BMP2(K357-R396del)--were found to decrease Smad1/5/8 signaling.

CONCLUSION

Rare variants in BMP2 and BMP4 are not a major genetic component in the otosclerosis population. However, those with functional affect showed decreased Smad signaling. Further analysis of Smad signaling molecules should be performed to determine if these pathways in combination are a major contributor to otosclerosis, which could lead to additional treatment options for otosclerosis patients.

Bone Morphogenetic Protein (BMP) signaling in development and human diseases

Bone Morphogenetic Proteins (BMPs) are a group of signaling molecules that belongs to the Transforming Growth Factor-β (TGF-β) superfamily of proteins. Initially discovered for their ability to induce bone formation, BMPs are now known to play crucial roles in all organ systems. BMPs are important in embryogenesis and development, and also in maintenance of adult tissue homeostasis. Mouse knockout models of various components of the BMP signaling pathway result in embryonic lethality or marked defects, highlighting the essential functions of BMPs. In this review, we first outline the basic aspects of BMP signaling and then focus on genetically manipulated mouse knockout models that have helped elucidate the role of BMPs in development. A significant portion of this review is devoted to the prominent human pathologies associated with dysregulated BMP signaling.

Simultaneous rather than ordered cleavage of two sites within the BMP4 prodomain leads to loss of ligand in mice

ProBMP4 is generated as a latent precursor that is sequentially cleaved at two sites within the prodomain to generate an active ligand. An initial cleavage occurs adjacent to the ligand domain, which generates a non-covalently associated prodomain/ligand complex that is subsequently dissociated by cleavage at an upstream site. An outstanding question is whether the two sites need to be cleaved sequentially and in the correct order to achieve proper control of BMP4 signaling during development. In the current studies, we demonstrate that mice carrying a knock-in point mutation that causes simultaneous rather than sequential cleavage of both prodomain sites show loss of BMP4 function and die during mid-embryogenesis. Levels of mature BMP4 are severely reduced in mutants, although levels of precursor and cleaved prodomain are unchanged compared with wild type. Our biochemical analysis supports a model in which the transient prodomain/ligand complex that forms during sequential cleavage plays an essential role in prodomain-mediated stabilization of the mature ligand until it can acquire protection from degradation by other means. By contrast, simultaneous cleavage causes premature release of the ligand from the prodomain, leading to destabilization of the ligand and loss of signaling in vivo.

Single-gene causes of congenital anomalies of the kidney and urinary tract (CAKUT) in humans

Congenital anomalies of the kidney and urinary tract (CAKUT) cover a wide range of structural malformations that result from defects in the morphogenesis of the kidney and/or urinary tract. These anomalies account for about 40-50 % of children with chronic kidney disease worldwide. Knowledge from genetically modified mouse models suggests that single gene mutations in renal developmental genes may lead to CAKUT in humans. However, until recently, only a handful of CAKUT-causing genes were reported, most of them in familial syndromic cases. Recent findings suggest that CAKUT may arise from mutations in a multitude of different single gene causes. We focus here on single-gene causes of CAKUT and their developmental origin. Currently, more than 20 monogenic CAKUT-causing genes have been identified. High-throughput sequencing techniques make it likely that additional CAKUT-causing genes will be identified in the near future.

MicroRNAs: potential regulators of renal development genes that contribute to CAKUT

Congenital anomalies of the kidney and urinary tract (CAKUT) are the leading cause of childhood chronic kidney disease (CKD). While mutations in several renal development genes have been identified as causes for CAKUT, most cases have not yet been linked to known mutations. Furthermore, the genotype-phenotype correlation is variable, suggesting that there might be additional factors that have an impact on the severity of CAKUT. MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the post-transcriptional level, and are involved in many developmental processes. Although little is known about the function of specific miRNAs in kidney development, several have recently been shown to regulate the expression of, and/or are regulated by, crucial renal development genes present in other organ systems. In this review, we discuss how miRNA regulation of common developmental signaling pathways may be applicable to renal development. We focus on genes that are known to contribute to CAKUT in humans, for which miRNA interactions in other contexts have been identified, with miRNAs that are present in the kidney. We hypothesize that miRNA-mediated processes might play a role in kidney development through similar mechanisms, and speculate that genotypic variations in these small RNAs or their targets could be associated with CAKUT.

Study of the association between the BMP4 gene and congenital anomalies of the kidney and urinary tract

OBJECTIVE

To determine the frequency of different phenotypes for congenital anomalies of the kidney and urinary tract (CAKUT) in a Brazilian sample, and to evaluate the association between the CAKUT phenotypes and the BMP4 gene.

METHODS

In this study, 457 Brazilian individuals were analyzed in an attempt to establish the association between the BMP4 gene and the CAKUT diagnosis. A case-control sample was genotyped for three BMP4 gene polymorphisms.

RESULTS

Association data was established with CAKUT sample as a whole and with the three most important CAKUT phenotypes: multicystic dysplastic kidney disease (MDK), ureteropelvic junction obstruction (UPJO) and vesicoureteral reflux (VUR). When the sample was segregated in these three phenotypes, associations between the BMP4 gene were observed with UPJO and with MDK. Conversely, VUR was not associated to the polymorphisms of the BMP4 gene.

CONCLUSIONS

The present data suggest that Brazilian individuals with polymorphisms of the BMP4 gene have a higher risk to develop CAKUT, especially the malformations related to nephrogenesis and initial branching such as MDK and UPJO. Conversely, VUR appeared not to be related to BMP4 gene.

Comorbidities in chronic pediatric peritoneal dialysis patients: a report of the International Pediatric Peritoneal Dialysis Network

BACKGROUND, OBJECTIVES, AND METHODS: Hospitalization and mortality rates in pediatric dialysis patients remain unacceptably high. Although studies have associated the presence of comorbidities with an increased risk for death in a relatively small number of pediatric dialysis patients, no large-scale study had set out to describe the comorbidities seen in pediatric dialysis patients or to evaluate the impact of those comorbidities on outcomes beyond the newborn period. In the present study, we evaluated the prevalence of comorbidities in a large international cohort of pediatric chronic peritoneal dialysis (CPD) patients from the International Pediatric Peritoneal Dialysis Network registry and began to assess potential associations between those comorbidities and hospitalization rates and mortality.

RESULTS

Information on comorbidities was available for 1830 patients 0 - 19 years of age at dialysis initiation. Median age at dialysis initiation was 9.1 years [interquartile range (IQR): 10.9], median follow-up for calculation of hospitalization rates was 15.2 months (range: 0.2 - 80.9 months), and total follow-up time in the registry was 2095 patient-years. At least 1 comorbidity had been reported for 602 of the patients (32.9%), with 283 (15.5%) having cognitive impairment; 230 (12.6%), motor impairment; 167 (9.1%), cardiac abnormality; 76 (4.2%), pulmonary abnormality; 212 (11.6%), ocular abnormality; and 101 (5.5%), hearing impairment. Of the 150 patients (8.2%) that had a defined syndrome, 85% had at least 1 nonrenal comorbidity, and 64% had multiple comorbidities. The presence of at least 1 comorbidity was associated with a higher hospitalization rate [hospital days per 100 observation days: 1.7 (IQR: 5.8) vs 1.2 (IQR: 3.9), p = 0.001] and decreased patient survival (4-year survival rate: 73% vs 90%, p < 0.0001).

CONCLUSIONS

Nearly one third of pediatric CPD patients in a large international cohort had at least 1 comorbidity, and multiple comorbidities were frequently reported among patients with a defined syndrome. Preliminary analysis suggests an association between comorbidity and poor outcome in those patients. As this powerful international registry matures, further multivariate analyses will be important to more clearly define the impact of comorbidities on hospitalization rates and mortality in pediatric CPD patients.

Functional evaluation of a novel tooth agenesis-associated bone morphogenetic protein 4 prodomain mutation

The detection of gene mutations in patients with congenitally missing teeth is not very complicated; however, proving causality is often quite difficult. Here, we report the detection of a substitution mutation, A42P, within the prodomain of bone morphogenetic protein 4 (BMP4) in a small family with tooth agenesis and describe a functional alteration that may be responsible for the tooth phenotype. As BMP4 is essential for the development of teeth and also for many other organs, it would be of considerable interest to find a BMP4 mutation that is associated only with tooth agenesis. Our in vitro investigations revealed that the A42P mutation neither affected processing and secretion of BMP4 nor altered functional properties, such as the induction of alkaline phosphatase or signaling through Smad1/5/8 phosphorylation by the mature BMP4 ligand. However, immunofluorescence staining revealed that the prodomains of BMP4 which harbor the A42P substitution form fibrillar structures around transfected cells in culture and that this fibrillar network is significantly decreased when mutant prodomains are expressed. Our finding suggests that in vivo, BMP4 prodomain behavior might also be altered by the mutation and could influence storage or transport of mature BMP4 in the extracellular matrix of the developing tooth.

Deletion of fibroblast growth factor receptor 2 from the peri-wolffian duct stroma leads to ureteric induction abnormalities and vesicoureteral reflux

Purpose

Pax3cre-mediated deletion of fibroblast growth factor receptor 2 (Fgfr2) broadly in renal and urinary tract mesenchyme led to ureteric bud (UB) induction defects and vesicoureteral reflux (VUR), although the mechanisms were unclear. Here, we investigated whether Fgfr2 acts specifically in peri-Wolffian duct stroma (ST) to regulate UB induction and development of VUR and the mechanisms of Fgfr2 activity.

Methods

We conditionally deleted Fgfr2 in ST (Fgfr2^ST−/−) using Tbx18cre mice. To look for ureteric bud induction defects in young embryos, we assessed length and apoptosis of common nephric ducts (CNDs). We performed 3D reconstructions and histological analyses of urinary tracts of embryos and postnatal mice and cystograms in postnatal mice to test for VUR. We performed in situ hybridization and real-time PCR in young embryos to determine mechanisms underlying UB induction defects.

Results

We confirmed that Fgfr2 is expressed in ST and that Fgfr2 was efficiently deleted in this tissue in Fgfr2^ST−/− mice at embryonic day (E) 10.5. E11.5 Fgfr2^ST−/− mice had randomized UB induction sites with approximately 1/3 arising too high and 1/3 too low from the Wolffian duct; however, apoptosis was unaltered in E12.5 mutant CNDs. While ureters were histologically normal, E15.5 Fgfr2^ST−/− mice exhibit improper ureteral insertion sites into the bladder, consistent with the ureteric induction defects. While ureter and bladder histology appeared normal, postnatal day (P) 1 mutants had high rates of VUR versus controls (75% versus 3%, p = 0.001) and occasionally other defects including renal hypoplasia and duplex systems. P1 mutant mice also had improper ureteral bladder insertion sites and shortened intravesicular tunnel lengths that correlated with VUR. E10.5 Fgfr2^ST−/− mice had decreases in Bmp4 mRNA in stromal tissues, suggesting a mechanism underlying the ureteric induction and VUR phenotypes.

Conclusion

Mutations in FGFR2 could possibly cause VUR in humans.

Double homozygous missense mutations in DACH1 and BMP4 in a patient with bilateral cystic renal dysplasia

Renal hypodysplasia (RHD) is characterized by small and/or disorganized kidneys following abnormal organogenesis. Mutations in several genes have been identified recently to be associated with RHD in humans, including BMP4, a member of the transforming growth factor (TGF)-β family of growth factors. DACH1 has been proposed as a candidate gene for RHD because of its involvement in the EYA-SIX-DACH network of renal developmental genes. Here, we present a patient with renal dysplasia carrying homozygous missense mutations in both BMP4 (p.N150K) and DACH1 (p.R684C). The genotype-phenotype correlation in the family hints at an oligogenic mode of inheritance of the disease in this kindred. Functional analyses of the identified DACH1 mutation in HEK293T cells demonstrated enhanced suppression of the TGF-β pathway suggesting that both mutations could act synergistically in the development of the phenotype in this patient. This finding provides a model for RHD as an oligo-/polygenic disorder and supports a role for DACH1 in the development of RHD in humans.

The many faces of RET dysfunction in kidney

Signaling pathways that are activated upon interaction of glial cell-line derived neurotrophic factor (Gdnf), its coreceptor Gfra1, and receptor tyrosine kinase Ret are critical for kidney development and ureter maturation. Outside the kidney, this pathway is implicated in a number of congenital diseases including Hirschsprung disease (intestinal aganglionosis, HSCR) and hereditary cancer syndromes (MEN 2). Total lack of Gdnf, Gfra1 or Ret in mice results in perinatal lethality due to bilateral renal agenesis or aplasia. In humans, RET mutations have been identified in a spectrum of congenital malformations involving the RET axis including isolated HSCR, isolated congenital anomalies of kidney or urinary tract (CAKUT), or CAKUT and HSCR together. The molecular basis for these pleiotropic effects of RET has just begun to be unraveled. In an effort to delineate the pathogenetic mechanisms that underlie these congenital malformations, we and others have characterized Ret's role in early kidney and urinary system development. Here we present a brief overview of the "many faces" of Ret dysfunction in kidney with particular emphasis on Ret's signaling specificity and intergenic interactions that confer normal urinary system development.

The hedgehog signal induced modulation of bone morphogenetic protein signaling: an essential signaling relay for urinary tract morphogenesis

BACKGROUND

Congenital diseases of the urinary tract are frequently observed in infants. Such diseases present a number of developmental anomalies such as hydroureter and hydronephrosis. Although some genetically-modified mouse models of growth factor signaling genes reproduce urinary phenotypes, the pathogenic mechanisms remain obscure. Previous studies suggest that a portion of the cells in the external genitalia and bladder are derived from peri-cloacal mesenchymal cells that receive Hedgehog (Hh) signaling in the early developmental stages. We hypothesized that defects in such progenitor cells, which give rise to urinary tract tissues, may be a cause of such diseases.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate the pathogenic mechanisms of upper urinary tract malformations, we analyzed a series of Sonic hedgehog (Shh) deficient mice. Shh(-/-) displayed hydroureter and hydronephrosis phenotypes and reduced expression of several developmental markers. In addition, we suggested that Shh modulation at an early embryonic stage is responsible for such phenotypes by analyzing the Shh conditional mutants. Tissue contribution assays of Hh-responsive cells revealed that peri-cloacal mesenchymal cells, which received Hh signal secreted from cloacal epithelium, could contribute to the ureteral mesenchyme. Gain- and loss-of-functional mutants for Hh signaling revealed a correlation between Hh signaling and Bone morphogenetic protein (Bmp) signaling. Finally, a conditional ablation of Bmp receptor type IA (BmprIA) gene was examined in Hh-responsive cell lineages. This system thus made it possible to analyze the primary functions of the growth factor signaling relay. The defective Hh-to-Bmp signaling relay resulted in severe urinary tract phenotypes with a decrease in the number of Hh-responsive cells.

CONCLUSIONS/SIGNIFICANCE

This study identified the essential embryonic stages for the pathogenesis of urinary tract phenotypes. These results suggested that Hh-responsive mesenchymal Bmp signaling maintains the population of peri-cloacal mesenchyme cells, which is essential for the development of the ureter and the upper urinary tract.

Mammalian kidney development: principles, progress, and projections

The mammalian kidney is a vital organ with considerable cellular complexity and functional diversity. Kidney development is notable for requiring distinct but coincident tubulogenic processes involving reciprocal inductive signals between mesenchymal and epithelial progenitor compartments. Key molecular pathways mediating these interactions have been identified. Further, advances in the analysis of gene expression and gene activity, coupled with a detailed knowledge of cell origins, are enhancing our understanding of kidney morphogenesis and unraveling the normal processes of postnatal repair and identifying disease-causing mechanisms. This article focuses on recent insights into central regulatory processes governing organ assembly and renal disease, and predicts future directions for the field.

Novel genetic aspects of congenital anomalies of kidney and urinary tract

PURPOSE OF REVIEW

Congenital anomalies of the kidney and urinary tract (CAKUT) are among the most frequent organ malformations. They are a relevant cause of chronic renal failure in children. Apart from isolated forms of CAKUT, more than 500 syndromes have been described that are characterized by combined defects of the kidney and other organ systems. Familial aggregation of renal malformations in approximately 10% of patients suggests that genetic events might be involved. Modifying effects due to missense mutations in additional developmental genes seem to enhance the phenotypic variability in affected families. In these families, genetic counseling can be difficult. In contrast, in patients with defined autosomal dominant disease, genetic counseling is of high clinical relevance, also with respect to additional extrarenal symptoms.

RECENT FINDINGS

Due to the development of numerous genetic knock-out mouse models, the identification of specific renal developmental genes and the application of novel sequencing techniques of the human genome, our understanding of kidney organogenesis has largely improved during very recent years.

SUMMARY

This review will focus on important genetic factors that influence nephrogenesis and highlight important human disorders that are associated with anomalies of kidneys, proximal and distal urinary tract.

Mutation screening of BMP4 and Id2 genes in Chinese patients with congenital ureteropelvic junction obstruction

Ureteropelvic junction obstruction (UPJO) is the most common congenital anomaly of the urinary tract. Evidence has shown that BMP4 and Id2 play crucial roles in nephrogenesis, alterations of which may cause ureteral developmental anomalies. Here, we directly sequenced the coding sequences in BMP4 and Id2 genes of 108 unrelated Chinese patients with ureteropelvic junction stenosis. One missense mutation c.485G> A (p.R162Q) in BMP4 and two synonymous mutations (c.1167T> C in BMP4 and c.108A> G in Id2) were detected in three cases. None of these variations were present in the 150 normal controls. Comparative amino acid sequence alignments of BMP4 in humans and other vertebrate orthologs show that p.R162 located to a highly conserved amino acid residue. Moreover, computational analysis predicted that R162Q probably infect the function of BMP4 protein.

CONCLUSION

The mutation c.485G> A in BMP4 might be one of the causes of human UPJO. Further functional studies are required to validate the association between this variation and UPJO. Otherwise, Id2 mutations do not seem to be involved in this disease.

Genetics of congenital anomalies of the kidney and urinary tract

Congenital anomalies of the kidney and urinary tract (CAKUT) occur in 1 in 500 births and are a major cause of morbidity in children. Notably, CAKUT account for the most cases of pediatric end-stage renal disease and predispose the individual to hypertension and cardiovascular disease throughout life. Although some forms of CAKUT are a part of a syndrome or are associated with a positive family history, most cases of renal system anomalies are sporadic and isolated to the urinary tract. Broad phenotypic spectrum of CAKUT and variability in genotype-phenotype correlation indicate that pathogenesis of CAKUT is a complex process that depends on interplay of many factors. This review focuses on the genetic mechanisms (single-gene mutations, modifier genes) leading to renal system anomalies in humans and discusses emerging insights into the role of epigenetics, in utero environmental factors, and micro-RNAs (miRNAs) in the pathogenesis of CAKUT. Common gene networks that function in defined temporospatial fashion to orchestrate renal system morphogenesis are highlighted. Derangements in cellular, molecular, and morphogenetic mechanisms that direct normal renal system development are emphasized as a major cause of CAKUT. Integrated understanding of how morphogenetic process disruptions are linked to CAKUT will enable improved diagnosis, treatment, and prevention of congenital renal system anomalies and their consequences.

Congenital anomalies of kidney and urinary tract

Congenital anomalies of the kidney and urinary tract anatomy (CAKUT) are common in children and represent approximately 30% of all prenatally diagnosed malformations. CAKUT is phenotypically variable and can affect the kidney(s) alone and/or the lower urinary tract. The spectrum includes more common anomalies such as vesicoureteral reflux and, rarely, more severe malformations such as bilateral renal agenesis. In young children, congenital anomalies are the leading cause of kidney failure and for kidney transplantation or dialysis. CAKUT can also lead to significant renal problems in adulthood and may present itself with hypertension and/or proteinuria. Congenital renal anomalies can be sporadic or familial, syndromic (also affecting nonrenal or non-urinary tract tissues), or nonsyndromic. Genetic causes have been identified for the syndromic forms and have shed some light into the molecular mechanisms of kidney development in human beings. The genetic causes for the more common nonsyndromic forms of CAKUT are unknown. The role of prenatal interventions and postnatal therapies as well as the benefits of screening affected individuals and their family members are not clear.

Hydroureternephrosis due to loss of Sox9-regulated smooth muscle cell differentiation of the ureteric mesenchyme

Congenital ureter anomalies, including hydroureter, affect up to 1% of the newborn children. Despite the prevalence of these developmental abnormalities in young children, the underlying molecular causes are only poorly understood. Here, we show that the high mobility group domain transcription factor Sox9 plays an important role in ureter development in the mouse. Transient Sox9 expression was detected in the undifferentiated ureteric mesenchyme and inactivation of Sox9 in this domain resulted in strong proximal hydroureter formation due to functional obstruction. Loss of Sox9 did not affect condensation, proliferation and apoptosis of the undifferentiated mesenchyme, but perturbed cyto-differentiation into smooth muscle cells (SMCs). Expression of genes encoding extracellular matrix (ECM) components was strongly reduced, suggesting that deficiency in ECM composition and/or signaling may underlie the observed defects. Prolonged expression of Sox9 in the ureteric mesenchyme led to increased deposition of ECM components and SMC dispersal. Furthermore, Sox9 genetically interacts with the T-box transcription factor 18 gene (Tbx18) during ureter development at two levels--as a downstream mediator of Tbx18 function and in a converging pathway. Together, our results argue that obstructive uropathies in campomelic dysplasia patients that are heterozygous for mutations in and around SOX9 arise from a primary requirement of Sox9 in the development of the ureteric mesenchyme.

Growth and body composition in very young SGA children

Infants with a very low birth weight are at risk of a reduced number of nephrons predisposing to kidney disorder, hypertension, and metabolic syndrome. Approximately 3% of infants are born small for gestational age (SGA), defined as birth weight and/or length at least 2 SD below the mean for gestational age (GA), independently of whether these children are born prematurely or at term. About 10% of these children do not show postnatal catch-up growth and remain of short stature during childhood. Most of these infants are not growth hormone (GH)-deficient, but may have GH resistance. Although GH-resistant, the majority of patients benefit from GH therapy, normalize height during childhood, maintain a normal growth velocity during puberty, and attain a normal adult height. To date, GH has been shown to be safe and no significant adverse effects have been demonstrated. Children with congenital chronic kidney disease (CKD) are born with subnormal birth weight and length and about 25% are born SGA. Shortness and need for GH treatment is highly correlated with weight at birth and gestational age. Primary renal disorders modify the response to GH treatment. Analysis of whether SGA is an additional risk factor for CKD regarding the development of hypertension, metabolic syndrome and cardiovascular complications is required.