CAKUT refer to a diverse group of kidney and urinary tract anomalies due to defects in kidney embryonic development. Genetic and phenotype heterogeneity and incomplete genetic penetrance are factors that challenge the genetic diagnosis of the disease (Nicolau et al. 2015). Therefore, the pathogenesis of CAKUT can be multifactorial and associated with epigenetic and environmental factors.
Our research group performed previous studies with the same group of patients. Miranda et al. showed that PAX2 gene polymorphisms were associated with VUR in our sample, but not with UPJO and MCDK (Miranda et al. 2014a). In another study, an association between AT2R gene and UJPO was detected, suggesting that AT2R was involved in the pathogenesis of CAKUT and that phenotype expression would vary according to the genetic origin of the disease (Miranda et al. 2014b). The current study was performed in order to evaluate whether an association of I/D polymorphisms of ACE gene and CAKUT would occur or if the polymorphisms would be associated with a specific phenotype of CAKUT in Brazillian population.
ACE is a RAS component that converts Ang I into Ang II. Ang II is a potent vasoconstrictor that exerts an essential role in blood pressure control, by promoting vasoconstriction and by releasing Aldosterone, which increases the reabsorption of sodium and water from urine (Simões e Silva et al. 2020; Santos-Junior et al. 2014). As mentioned, ACE gene has insertion and deletion polymorphisms on intron 16, which are suggested to be associated with some clinical outcomes related to blood pressure, atherosclerosis, diabetic nephropathy, muscle performance, Alzheimer disease and longevity (Sayed-Tabatabaei et al. 2006). Previous studies had shown that D alleles are related to greater serum activity of ACE (Rigat et al. 1990), however the functionality of this polymorphism is not well elucidated (Sayed-Tabatabaei et al. 2006). Our main hypothesis for this study was that ACE I/D polymorphisms could influence ACE serum activity, changing the concentrations of Ang I and of Ang II.
The metanephros are the definitive kidney of mammals. Its embryogenesis is resulted from reciprocal interactions between the ureteric bud (UB) and the metanephric mesenchyme. UBs will develop into the collecting ducts, pelvis and the ureteric epithelia, while the metanephric mesenchyme is responsible for ureteric branching and nephrogenesis, by differentiation of mesenchymal cells into epithelial cells of the nephron (Serqueira and Gomez 2004). In this regard, an intact RAS is required for normal kidney development (Guron and Friberg 2000). Ang II promotes UB cell branching in collagen gel cultures in vitro, induces UB morphogenesis in intact whole embryonic metanephros grown ex vivo, stimulates tyrosine phosphorylation of the epidermal growth factor receptor (EGFR) in UB cells and promotes directional migration of UB cells (Yosypiv, 2008; Song et al. 2011). Additionally, Zhang et al. (2004) showed that Ang II is involved in kidney embryogenesis by increasing paired homeobox 2 gene (PAX-2) expression in fetal kidney cells via the AT2R, by downregulating or desensitizing this receptor. According to Song et al. (2010), AT2R-deficient mice present abnormal ureteric bud (UB) budding, increased incidence of double ureters and vesicoureteral reflux. In contrast, this association was not found in a study with 66 japanese boys with CAKUT and healthy controls, as they showed no evidence for AT2R gene derangement in human urinary tract anomalies (Hiraoka et al. 2001).
For this analysis, we hypothesized that greater occurrence of DD genotypes and D allele frequency would cause an elevation of Ang II levels by increasing serum activity of ACE. Greater prevalence of II genotype and I allele frequency would have the opposite effect, reducing Ang II levels (Rigat et al. 1990). However, our study did not find any association between ACE I/D polymorphisms and CAKUT in general or CAKUT specific phenotypes. Genotype distribution and allele frequency were very similar in both healthy controls and CAKUT patients. When it comes to specific phenotypes of CAKUT, UPJO and MCDK phenotypes were slightly associated with DD genotypes, but this trend did not achieve statistical significance. Children with VUR had genotypes and alleles frequencies very similar to healthy controls.
Literature data is controversial. A meta-analysis showed that D allele and DD genotype are risk factors for the VUR susceptibility/renal scar in Turkish children (Zhou et al. 2012). In contrast, a bulgarian study performed by Kostadinova and colleagues (2017) found no significant association between ACE I/D genotype and CAKUT phenotypes, supporting our findings. Additionally, this bulgarian research group also showed that the highest serum ACE level encoded by ACE I/D polymorphisms was associated with higher risk of severe hydronephrosis in Bulgarian patients with obstructive uropathies (Kostadinova et al. 2017). Unfortunately, these findings were not analysed in our study.
This study has several limitations. First of all, CAKUT is defined as a complex multifactorial disease and may have several variables that contribute to the presented phenotype (Nicolau et al. 2015). Second, as mentioned, the functionality of ACE I/D polymorphisms are not well elucidated. It is suspected that a polymorphism on intron 18, rather than on intron 16, is more likely to be functional on ACE serum activity (Sayed-Tabatabaei et al. 2006). Nonetheless, intron 16 is still used as a marker due to tight relationship with ACE levels. Finally, an genetic-epidemiological approach would provide more information by including environmental factors associated with CAKUT.
Genetic, epigenetic and environmental factors might be involved in the pathogenesis of CAKUT (Nicolau et al. 2015). We did not find any association between ACE I/D polymorphisms and CAKUT in Brazilian children. DD genotypes were slightly increased in patients with UPJO and MCDK, but did not achieve statistical significance. However, this study still contributes to the understanding of CAKUT origins in the Brazilian population. In the future, we believe that further studies with genetic-epidemiological approaches would contribute to the investigation of CAKUT.