Pediatric Surgery Firearm Injury Collaborative Symposium.

Injury from a firearm is now the leading cause of death of children and youth under age 19 in the United States (U.S.) [1] and the incidence of these deaths continues to increase each year [2]. For every death from firearm violence, there are several young people who have been injured by a bullet but not killed. As pediatric surgeons, we are on the front lines of treating these young patients. We have the unforgettable memories of delivering the horrible news to parents in "quiet rooms." [3]. As these injuries fall within our scope of practice, it is incumbent on us as professionals to work to prevent these injuries, apply best practices and work for the best pathways to recovery for our patients who do survive. There is a diverse community of pediatric surgeons tackling this public health problem in a variety of ways [4]. In a pre-meeting symposium at the APSA 2023 Annual meeting, we brought together a community of pediatric surgeons working on this critical area. The following summarizes the presentations of the symposium, with topics including Risk Factors, Injury Prevention, Treatment, Public Initiatives, and National Collaborative Efforts. TYPE OF STUDY: Review Article, Proceedings of a Symposium. LEVEL OF EVIDENCE: 1 through 4 all presented.

Neonatal peri-procedural extracorporeal membrane oxygenation in the management of tracheo-esophageal fistula/esophageal atresia and left pulmonary agenesis.

Congenital tracheo-esophageal fistula/esophageal atresia (TEF/EA) with concomitant pulmonary agenesis is exceedingly rare and has a high mortality rate. While there are several reported cases of successful repair, all but one patient had right-sided pulmonary agenesis. In the case of left-sided pulmonary agenesis, the patient had incomplete agenesis and underwent repair through a left thoracotomy. We present the first successful repair of TEF/EA with complete left-sided pulmonary agenesis. This patient also underwent elective pre-operative veno-venous extracorporeal membrane oxygenation (ECMO) and subsequent repair of the TEF/EA. We discuss the management, anesthesia risks, and role of periprocedural ECMO in pediatric patients who are high anesthetic risk.

Ultrasound assessment of abdominal adhesions in neonates: data over dogma for re-operative timing.

INTRODUCTION: Neonatal abdominal reoperation is difficult and can be complicated by abdominal adhesions. Identifying patients who could safely undergo early reoperation would save TPN and central line days, decrease associated infection and liver injury, and NICU and hospital length of stay. We sought to determine if ultrasound (US) could accurately assess the location and severity of adhesions in neonates as an objective dynamic marker capable of informing reoperation timing. METHODS: After IRB approval, we conducted a prospective observational study including neonates undergoing abdominal operations. Patients received surgeon-performed US approximately every 2 weeks until reoperation or discharge. Adhesions were assessed in five zones: right upper quadrant (RUQ), right lower quadrant (RLQ), left upper quadrant (LUQ), left lower quadrant (LLQ) and peri-incision (INC). RESULTS: Over a 6-month study period, 16 neonates were enrolled. Median gestational age was 34 weeks at birth and median weight 2.2 kg. 6 underwent reoperation within initial NICU admission. At time of operation US correctly identified the absence or presence and severity of adhesions in: RUQ (3/3); RLQ (6/6); LUQ (4/5); LLQ (6/6); and INC (5/5). CONCLUSION: US can identify location and severity of post-operative adhesions in neonates, potentially identifying patients who can safely undergo reoperation earlier than predetermined wait periods. LEVEL OF EVIDENCE: IV.

In utero adenine base editing corrects multi-organ pathology in a lethal lysosomal storage disease.

In utero base editing has the potential to correct disease-causing mutations before the onset of pathology. Mucopolysaccharidosis type I (MPS-IH, Hurler syndrome) is a lysosomal storage disease (LSD) affecting multiple organs, often leading to early postnatal cardiopulmonary demise. We assessed in utero adeno-associated virus serotype 9 (AAV9) delivery of an adenine base editor (ABE) targeting the Idua G→A (W392X) mutation in the MPS-IH mouse, corresponding to the common IDUA G→A (W402X) mutation in MPS-IH patients. Here we show efficient long-term W392X correction in hepatocytes and cardiomyocytes and low-level editing in the brain. In utero editing was associated with improved survival and amelioration of metabolic, musculoskeletal, and cardiac disease. This proof-of-concept study demonstrates the possibility of efficiently performing therapeutic base editing in multiple organs before birth via a clinically relevant delivery mechanism, highlighting the potential of this approach for MPS-IH and other genetic diseases.

Effect of Feeding Strategies on the Development of Fulminant Necrotizing Enterocolitis.

INTRODUCTION: Necrotizing enterocolitis (NEC) causes significant neonatal morbidity. A subset of infants experience precipitous decline and death from fulminant-NEC (F-NEC). We sought to determine the effect of feeding practices on the development of this more virulent form of NEC. MATERIALS AND METHODS: Premature neonates developing Bell's stage II or III NEC between May 2011 and June 2017 were reviewed. Infants were stratified as having NEC or F-NEC, defined as NEC-totalis or NEC causing rapid decline and death within 72 hours. Risk factors extracted included demographics, gestational age, and weight at NEC diagnosis. Feeding data extracted included age at first feed, caloric density, type of feed (breast milk or formula), and whether full volume feeds were reached. Univariate analysis and multivariate analysis were performed. RESULTS: A total of 98 patients were identified, of which 80 were included. In total, 57 patients had NEC and 23 had F-NEC. Reaching full volume feeds was associated with F-NEC on both univariate and multivariate analysis (37.9 vs. 4.5%; odds ratio: 67, 95% confidence interval: 6.606-2041, p = 0.003). Infants developing F-NEC achieved full feeds earlier (22.5 vs. 19.8 days, p = 0.025) on univariate but not multivariate analysis. There was no difference in the rates of NEC and F-NEC among infants receiving breast milk (standard or fortified) or formula (standard or increased caloric density; p = 0.235). CONCLUSION: Among premature neonates with NEC, reaching full volume feedings was associated with a nearly 70-fold increased risk of F-NEC. Assuming it was possible to predict an infant's development of NEC, alternative feeding regimens might reduce the risk of F-NEC in this population.

A Rabbit Model for Optimization of Amniotic Fluid Components in the EXTrauterine Environment for Newborn Development (EXTEND) System.

In this model article, we present a protocol for continuous amniotic fluid exchange in rabbits using a novel system to test the effects of growth factor-deficient, artificial amniotic fluid on bowel development. BACKGROUND: Ideally, the EXTrauterine Environment for Neonatal Development (EXTEND) will provide physiologic support to the extreme premature infant. An important component of that environment is the amniotic fluid. Thus, we developed an animal model to study the growth factors found within amniotic fluid and inform design of a synthetic fluid to optimize fetal development. METHODS: We designed a model of amniotic fluid exchange within the pregnant rabbit, continuously removing the natural fluid from around 2 fetuses per doe and replacing it with a physiologic electrolyte solution during the final 100 h of gestation. Two fetuses from the contralateral uterine horn were used as sham-operated controls. Thirty-eight fetuses were analyzed, 19 in each group. We analyzed the fetal growth and bowel development. RESULTS: Ultrasound after 100 h of exchange showed equivalent fluid volumes, p = 0.63. Cultures were negative for bacterial colonization. Final fluid protein concentrations were 11.6% that of control fluid (mean 1,451 ± 224.2 vs. 12,491 ± 849.2 µg/mL). There was no significant difference in fetal growth, with experimental weights 91.4% of control weights, p = 0.07. Fetal bowel weights (90.1%, p = 0.16) and lengths (94.2%, p = 0.49) were also not significantly less compared to controls. There was no significant difference in villous height or crypt depth measurements between the groups, and absorptive capacity of the bowel was not different between groups, p = 0.44. CONCLUSION: This animal model allows for manipulation of the components of amniotic fluid. Marked reduction of natural amniotic fluid proteins during gestation does not appear to significantly impair fetal growth or bowel development. Further work with this model will assess the importance of amniotic fluid components for normal development to inform design of a synthetic fluid for use during EXTEND.

Evaluation of the Use of Low-Dose 4-Factor Prothrombin Complex Concentrate in the Reversal of Direct Oral Anticoagulants in Bleeding Patients.

OBJECTIVE: This study investigated the percentage of patients who achieved hemostasis with 4-factor prothrombin complex concentrate (4-factor PCC) 35 U/kg. The primary end point was to determine the effect of 4-factor PCC 35 U/kg on bleeding progression, assessed using computed tomography. METHODS: This was a retrospective, observational, single-center study conducted in patients with a major bleed admitted to a level 1 trauma center from May 1, 2013, to June 15, 2015, who received 4-factor PCC 35 U/kg for reversal of a direct factor Xa inhibitor taken prior to admission. RESULTS: Thirty-three patients were included in the study, with 31 patients in the final analysis. The mean (standard deviation) age was 73 (14.8) years; 54.5% of patients were female. Of the 33 patients, 13 presented with a traumatic brain injury, 9 with an aneurysmal subarachnoid hemorrhage, 8 with an intracerebral hemorrhage, 1 with a gastrointestinal bleed, 1 with a hematoma with active extravasation, and 1 with an intra-abdominal bleed. The most frequently used direct factor Xa inhibitor was rivaroxaban (81.8%). Overall, 83.8% of patients achieved hemostasis with 4-factor PCC 35 U/kg. Progression of hemorrhage was observed in 4 patients on repeat computed tomography scan and 1 patient had continued surgical bleeding. No thromboembolic events were reported. CONCLUSIONS: Low-dose, 4-factor PCC 35 U/kg appeared to produce hemostasis in a majority of the patients. This may be an effective dosing regimen for anticoagulant reversal of factor Xa inhibitors in clinically bleeding patients.

In utero gene editing for monogenic lung disease.

Monogenic lung diseases that are caused by mutations in surfactant genes of the pulmonary epithelium are marked by perinatal lethal respiratory failure or chronic diffuse parenchymal lung disease with few therapeutic options. Using a CRISPR fluorescent reporter system, we demonstrate that precisely timed in utero intra-amniotic delivery of CRISPR-Cas9 gene editing reagents during fetal development results in targeted and specific gene editing in fetal lungs. Pulmonary epithelial cells are predominantly targeted in this approach, with alveolar type 1, alveolar type 2, and airway secretory cells exhibiting high and persistent gene editing. We then used this in utero technique to evaluate a therapeutic approach to reduce the severity of the lethal interstitial lung disease observed in a mouse model of the human SFTPCI73T mutation. Embryonic expression of SftpcI73T alleles is characterized by severe diffuse parenchymal lung damage and rapid demise of mutant mice at birth. After in utero CRISPR-Cas9-mediated inactivation of the mutant SftpcI73T gene, fetuses and postnatal mice showed improved lung morphology and increased survival. These proof-of-concept studies demonstrate that in utero gene editing is a promising approach for treatment and rescue of monogenic lung diseases that are lethal at birth.

Premature Lambs Exhibit Normal Mitochondrial Respiration after Long-Term Extrauterine Support.

BACKGROUND: In an effort to mitigate the major morbidities and mortality associated with extreme prematurity, we have developed an EXTrauterine Environment for Neonatal Development (EXTEND) designed to provide physiologic support of extremely premature infants. OBJECTIVES: We have previously shown that long-term, physiologic support of premature fetal lambs is possible with EXTEND, but in this study, we sought to demonstrate bioenergetic equipoise at the tissue level. METHODS: Four premature fetal lambs were delivered by hysterotomy at gestational ages (GA) of 105-107 days (term ∼145 days), cannulated via the umbilical vessels, and transitioned to support on EXTEND for 3-4 weeks. Five control fetuses were age-matched to the GA of experimental fetuses at the time of study end (128-134 days GA) and immediately sacrificed after hysterotomy. Mitochondria were isolated from the heart, liver, kidney, and skeletal muscle of fetuses at the time of sacrifice, and oxygen consumption rates (OCRs) were measured. RESULTS: There were no differences in basal mitochondrial OCR between EXTEND and control fetuses for heart, kidney, or skeletal muscle. For liver, the basal OCR was higher in EXTEND fetuses compared to controls. There were no differences in physiologic maximal OCR or reserve capacity for any tissue analyzed. CONCLUSIONS: Fetal lambs supported by EXTEND demonstrate physiologic mitochondrial function as evidenced by adequate basal and physiologic maximal cellular respiration as well as preserved reserve capacity.

Comparison of Clevidipine and Nicardipine for Acute Blood Pressure Reduction in Patients With Stroke.

OBJECTIVE: The aim of this study was to determine whether clevidipine (CLEV) achieved faster blood pressure control compared to nicardipine (NIC) in patients presenting with either an acute ischemic stroke (AIS) or a spontaneous intracerebral hemorrhage (ICH). METHODS: This was a retrospective, observational, cohort study conducted in patients with AIS or ICH admitted to the emergency department of a Comprehensive Stroke Center from November 2011 to June 2013 who received CLEV or NIC continuous infusion for acute blood pressure management. RESULTS: The study included 210 patients: 70 in the CLEV group and 140 in the NIC group. There was no difference in mean time (standard deviation [SD]) from initiation of the infusion to goal systolic blood pressure (SBP), CLEV: 50 (83) minutes versus NIC: 74 (103) minutes, P = .101. Comparison of the 2 agents within diagnosis showed no difference. Hypotension developed in 5 (7.1%) CLEV patients versus 14 (10%) NIC patients (P = .003). There was no difference in the percentage change at 2 hours; CLEV: -20% (16%) versus NIC: -16% (16%), P = .058. Mean (SD) time to alteplase administration from admission was 56 (22) minutes in the CLEV group versus 59 (25) minutes in the NIC group (P = .684). CONCLUSIONS: There was no difference in the mean time from initiation of the infusion to the SBP goal between agents or in the secondary outcomes. Due to the lack of differences observed, each agent should be considered based on the patient care needs of the institution.

In utero CRISPR-mediated therapeutic editing of metabolic genes.

In utero gene editing has the potential to prenatally treat genetic diseases that result in significant morbidity and mortality before or shortly after birth. We assessed the viral vector-mediated delivery of CRISPR-Cas9 or base editor 3 in utero, seeking therapeutic modification of Pcsk9 or Hpd in wild-type mice or the murine model of hereditary tyrosinemia type 1, respectively. We observed long-term postnatal persistence of edited cells in both models, with reduction of plasma PCSK9 and cholesterol levels following in utero Pcsk9 targeting and rescue of the lethal phenotype of hereditary tyrosinemia type 1 following in utero Hpd targeting. The results of this proof-of-concept work demonstrate the possibility of efficiently performing gene editing before birth, pointing to a potential new therapeutic approach for selected congenital genetic disorders.

Pre-Existing Maternal Antibodies Cause Rapid Prenatal Rejection of Allotransplants in the Mouse Model of In Utero Hematopoietic Cell Transplantation.

In utero hematopoietic cell transplantation (IUHCT) is a nonmyeloablative nonimmunosuppressive alternative to postnatal hematopoietic stem cell transplantation for the treatment of congenital hemoglobinopathies. Anti-HLA donor-specific Abs (DSA) are associated with a high incidence of graft rejection following postnatal hematopoietic stem cell transplantation. We determine if DSA present in the mother can similarly cause graft rejection in the fetus following IUHCT. Ten million C57BL/6 (B6, H2kb) bone marrow cells were transplanted in utero into gestational day 14 BALB/c (H2kd) fetuses. The pregnant BALB/c dams carrying these fetuses either had been previously sensitized to B6 Ag or were injected on gestational days 13-15 with serum from B6-sensitized BALB/c females. Maternal-fetal Ab transmission, Ab opsonization of donor cells, chimerism, and frequency of macrochimeric engraftment (chimerism >1%) were assessed by flow cytometry. Maternal IgG was transmitted to the fetus and rapidly opsonized donor cells following IUHCT. Donor cell rejection was observed as early as 4 h after IUHCT in B6-sensitized dams and 24 h after IUHCT in dams injected with B6-sensitized serum. Efficient opsonization was strongly correlated with decreased chimerism. No IUHCT recipients born to B6-sensitized dams or dams injected with B6-sensitized serum demonstrated macrochimeric engraftment at birth compared with 100% of IUHCT recipients born to naive dams or dams injected with naive serum (p < 0.001). In summary, maternal donor-specific IgG causes rapid, complete graft rejection in the fetus following IUHCT. When a third-party donor must be used for clinical IUHCT, the maternal serum should be screened for DSA to optimize the chance for successful engraftment.

Safety and Efficacy of High-Dose Unfractionated Heparin for Prevention of Venous Thromboembolism in Overweight and Obese Patients.

STUDY OBJECTIVE: To determine the safety and efficacy of high-dose subcutaneous unfractionated heparin (UFH) for prevention of venous thromboembolism (VTE) in overweight and obese patients. DESIGN: Single-center retrospective observational cohort study. SETTING: Large academic tertiary care medical center. PATIENTS: A total of 1335 adults who weighed more than 100 kg on admission and received either subcutaneous UFH 7500 units every 8 hours (751 patients [high-dose group]) or 5000 units every 8 hours (584 patients [low-dose group]) for VTE prophylaxis during their hospitalization between January 1, 2013, and August 31, 2014. MEASUREMENTS AND MAIN RESULTS: The incidences of VTE and bleeding complications were assessed in each group. Each group was further divided into four groups based on their body mass index (BMI): overweight (BMI 25-29.9 kg/m(2) ), obese class I (BMI 30-34.9 kg/m(2) ), obese class II (BMI 35-39.9 kg/m(2) ), and obese class III (BMI ≥ 40 kg/m(2) ). The incidence of VTE was similar for patients in the high-dose group versus those in the low-dose group for all BMI categories. Bleeding complications were significantly higher for patients in the high-dose group. The proportion of patients with at least a 2-g/dl hemoglobin drop from admission was higher in patients in the high-dose groups in obese classes II and III: obese class II, 46 (30%) of 152 patients in the high-dose group versus 30 (18%) of 171 patients in the low-dose group (p<0.01); obese class III, 109 (25%) of 432 patients in the high-dose group versus 31 (12%) of 249 patients in the low-dose group (p<0.01). In addition, the proportion of patients who received at least 2 units of packed red blood cell transfusion was significantly higher in patients in the high-dose group who were in obese class III: 47 (11%) of 432 in the high-dose group versus 13 (5%) of 249 in the low-dose group (p<0.01). CONCLUSION: Administering a higher dose of heparin to patients weighing more than 100 kg may not impart additional efficacy in reducing the incidence of VTE. However, it may increase the risk for bleeding.

Parental timing of allergenic food introduction in urban and suburban populations.

BACKGROUND: Recommendations on timing for introduction of allergenic foods in an infant diet have changed twice during the past decade. How families with different demographic characteristics implement the change has not been studied in the United States. OBJECTIVE: To compare the age of introduction of allergenic foods between an urban Medicaid-based population and a suburban private insurance-based population in Cincinnati, Ohio. METHODS: Two hundred parent surveys were distributed at well-child checkups between 4 and 36 months of age. Data were analyzed using distribution mapping to determine the difference in the age of introduction of infant formula, infant solids, whole cow's milk, eggs, peanut, and fish. Random forest analysis was used to determine the most important factors affecting the age of introduction for both populations. RESULTS: There was no statistically significant difference in the age of infant solid introduction, but urban populations introduced allergenic foods earlier than suburban populations, with a statistically significant difference in the age of introduction of infant formula, whole cow's milk, eggs, peanut, and fish. The most important factor for the timing of all food introductions was the recommended age of introduction from health care professionals. CONCLUSION: There is a difference between urban and suburban populations in the timing of introduction of allergenic foods but not in other infant solid foods. The reliance on physician recommendation for both populations supports the need for education and guidance to health care professionals on up-to-date guidance and recommendations.

Gain of Function Mutations of PIK3CD as a Cause of Primary Sclerosing Cholangitis.

Gain of function (GOF) mutation in the p110δ catalytic subunit of the phosphatidylinositol-3-OH kinase (PIK3CD) is the cause of a primary immunodeficiency (PID) characterized by recurrent sinopulmonary infections and lymphoproliferation. We describe a family of two adults and three children with GOF mutation in PIK3CD, all with recurrent sinopulmonary infections and varied infectious and non-infectious complications. The two adults have Primary Sclerosing Cholangitis (PSC) without evidence of Cryptosporidium parvum infection and have required liver transplantation. PSC is a novel phenotype of GOF mutation in PIK3CD.

Identification of novel peptide substrates for protein farnesyltransferase reveals two substrate classes with distinct sequence selectivities.

Prenylation is a posttranslational modification essential for the proper localization and function of many proteins. Farnesylation, the attachment of a 15-carbon farnesyl group near the C-terminus of protein substrates, is catalyzed by protein farnesyltransferase (FTase). Farnesylation has received significant interest as a target for pharmaceutical development, and farnesyltransferase inhibitors are in clinical trials as cancer therapeutics. However, as the total complement of prenylated proteins is unknown, the FTase substrates responsible for farnesyltransferase inhibitor efficacy are not yet understood. Identifying novel prenylated proteins within the human proteome constitutes an important step towards understanding prenylation-dependent cellular processes. Based on sequence preferences for FTase derived from analysis of known farnesylated proteins, we selected and screened a library of small peptides representing the C-termini of 213 human proteins for activity with FTase. We identified 77 novel FTase substrates that exhibit multiple-turnover (MTO) reactivity within this library; our library also contained 85 peptides that can be farnesylated by FTase only under single-turnover (STO) conditions. Based on these results, a second library was designed that yielded an additional 29 novel MTO FTase substrates and 45 STO substrates. The two classes of substrates exhibit different specificity requirements. Efficient MTO reactivity correlates with the presence of a nonpolar amino acid at the a(2) position and a Phe, Met, or Gln at the terminal X residue, consistent with the proposed Ca(1)a(2)X sequence model. In contrast, the sequences of the STO substrates vary significantly more at both the a(2) and the X residues and are not well described by current farnesylation algorithms. These results improve the definition of prenyltransferase substrate specificity, test the efficacy of substrate algorithms, and provide valuable information about therapeutic targets. Finally, these data illuminate the potential for in vivo regulation of prenylation through modulation of STO versus MTO peptide reactivity with FTase.

Cessation of renal morphogenesis in mice.

The kidney develops by cycles of ureteric bud branching and nephron formation. The cycles begin and are sustained by reciprocal inductive interactions and feedback between ureteric bud tips and the surrounding mesenchyme. Understanding how the cycles end is important because it controls nephron number. During the period when nephrogenesis ends in mice, we examined the morphology, gene expression, and function of the domains that control branching and nephrogenesis. We found that the nephrogenic mesenchyme, which is required for continued branching, was gone by the third postnatal day. This was associated with an accelerated rate of new nephron formation in the absence of apoptosis. At the same time, the tips of the ureteric bud branches lost the typical appearance of an ampulla and lost Wnt11 expression, consistent with the absence of the capping mesenchyme. Surprisingly, expression of Wnt9b, a gene necessary for mesenchyme induction, continued. We then tested the postnatal day three bud branch tip and showed that it maintained its ability both to promote survival of metanephric mesenchyme and to induce nephrogenesis in culture. These results suggest that the sequence of events leading to disruption of the cycle of branching morphogenesis and nephrogenesis began with the loss of mesenchyme that resulted from its conversion into nephrons.

Laser capture-microarray analysis of Lim1 mutant kidney development.

The Lim1 gene has essential functions during several stages of kidney development. In particular, a tissue-specific knockout in the early metanephric mesenchyme results in the formation of the earliest nephron precursor, the renal vesicle, but failure of this structure to progress to the next stage, the comma-shaped body. To better understand the molecular nature of this developmental arrest, we used a laser capture microdissection-microarray strategy to examine the perturbed gene expression pattern of the mutant renal vesicles. Among the genes found differently expressed were Chrdl2, an inhibitor of BMP signaling, the proapoptotic factor Bmf, as well as myob5, an atypical myosin that modulates chemokine signaling, and pdgfrl, which is important in epithelial folding. Of particular interest, the microarray data indicated that the Dkk1 gene, which encodes an inhibitor of Wnt signaling, was downregulated ninefold in mutants. This was confirmed by in situ hybridizations. It is interesting to note that Lim1 and Dkk1 mutant mice have striking similarities in phenoytpe. These results suggest that the Dkk1 gene might be a key downstream effector of Lim1 function.

Pygo1 and Pygo2 roles in Wnt signaling in mammalian kidney development.

BACKGROUND: The pygopus gene of Drosophila encodes an essential component of the Armadillo (beta-catenin) transcription factor complex of canonical Wnt signaling. To better understand the functions of Pygopus-mediated canonical Wnt signaling in kidney development, targeted mutations were made in the two mammalian orthologs, Pygo1 and Pygo2. RESULTS: Each mutation deleted >80% of the coding sequence, including the critical PHD domain, and almost certainly resulted in null function. Pygo2 homozygous mutants, with rare exception, died shortly after birth, with a phenotype including lens agenesis, growth retardation, altered kidney development, and in some cases exencephaly and cleft palate. Pygo1 homozygous mutants, however, were viable and fertile, with no detectable developmental defects. Double Pygo1/Pygo2 homozygous mutants showed no apparent synergy in phenotype severity. The BAT-gal transgene reporter of canonical Wnt signaling showed reduced levels of expression in Pygo1-/-/Pygo2-/- mutants, with tissue-specific variation in degree of diminution. The Pygo1 and Pygo2 genes both showed widespread expression in the developing kidney, with raised levels in the stromal cell compartment. Confocal analysis of the double mutant kidneys showed disturbance of both the ureteric bud and metanephric mesenchyme-derived compartments. Branching morphogenesis of the ureteric bud was altered, with expanded tips and reduced tip density, probably contributing to the smaller size of the mutant kidney. In addition, there was an expansion of the zone of condensed mesenchyme capping the ureteric bud. Nephron formation, however, proceeded normally. Microarray analysis showed changed expression of several genes, including Cxcl13, Slc5a2, Klk5, Ren2 and Timeless, which represent candidate Wnt targets in kidney development. CONCLUSION: The mammalian Pygopus genes are required for normal branching morphogenesis of the ureteric bud during kidney development. Nevertheless, the relatively mild phenotype observed in the kidney, as well as other organ systems, indicates a striking evolutionary divergence of Pygopus function between mammals and Drosophila. In mammals, the Pygo1/Pygo2 genes are not absolutely required for canonical Wnt signaling in most developing systems, but rather function as quantitative transducers, or modulators, of Wnt signal intensity.