Nephrogenesis in extremely low birthweight infants and nephropathogenesis in relation to biomarkers and research to long distance prognosis

The aims of this PhD–project were to a) determine screening criteria for HNF1B analysis in patients with congenital anomalies of kidney and urinary tract (CAKUT), b) generate an infrastructure to facilitate longitudinal follow–up of a birth cohort of former extremely low birth weight (ELBW) neonates and subsequent data pooling with other cohorts and c) to use this infrastructure to study the consequences of preterm birth and investigate risk factors or biomarkers to quantify problems in young adolescence (chapter 2).

Early development of the kidney implies interaction of several pathways to create the kidney. Failure of these complex interactions may lead to CAKUT and/or disrupted nephrogenesis. The etiology of CAKUT is heterogeneous and involves genetic, epigenetic and environmental factors. Moreover, there is extensive phenotypic variability, even within patients carrying the same mutation. In this PhD–project, we focused on screening criteria for genetic HNF1B analysis, as one of the most frequent examples of a monogenetic cause of CAKUT. We investigated the incidence of HNF1B gene abnormalities in CAKUT patients based on literature–based screening criteria in a prospective cohort of CAKUT cases. Based on our observations, we propose to restrict HNF1B genetic analysis to patients having bilateral renal anomalies, and in particular renal cysts from unknown origin and/or in combination with hypomagnesemia (chapter 3)

Awareness of renal and also cardiovascular implications (i.e. hypertension) of CAKUT and its confounders may improve long–term outcome. CAKUT and low birth weight/preterm birth have a low nephron endowment in common (i.e. oligonephronia). Since nephrogenesis requires 32–36 weeks of gestation, nephrogenesis is still on–going and partly hindered at the time of birth in extreme premature infants. According to the Brenner hypothesis, children at the lower end of the nephron endowment spectrum, i.e. children with low birth weight (growth restriction in term infants, preterm or both), have the highest risk for developing additional problems (i.e. ‘secondary hits’) like accelerated nephron loss and hypertension. In this PhD–project, we investigated the renal and cardiovascular consequences or biomarkers of prematurity in a very specific subgroup of former prematurely born children (ELBW, i.e. <1000 g) during childhood and compared findings with a control group of term born children of equal sex and age. We showed that assessment of cardiovascular and renal health in childhood in a clinical research setting is feasible and well tolerated (chapter 4). The impact of preterm birth is obviously not limited to the renal development. Whereas Brenner focused on the lower nephron endowment in low birth weight (term) infants, Barker suggested the broader ‘Developmental Origins of Health and Disease (DOHaD) concept’ or the Barker hypothesis and described the fetal programming in case of intra–uterine growth restriction on different organ systems with adult consequences like hypertension, diabetes and stroke. Circumstances during sensitive periods of development in perinatal life may permanently shape programming and subsequently affect future health and disease risks.

Preterm birth affects normal development and has the potential to induce developmental changes. In this PhD–project, the extended DOHaD concept is applied to a cohort of ELBW infants, in which the phenomena of growth restriction is common either before or after birth (i.e. ‘extra–uterine growth restriction’) and in which the preterm born child does not reach its genetic potential of growth. We found that ELBW neonates displayed extra–uterine growth restriction up to at least 24 months. Moreover, growth patterns beyond neonatal stay were associated with body composition in young adolescence (chapter 5). Improved neurocognitive performance at 24 months was associated with catch–up growth between 9 and 24 months. In young adolescence (~11 years), ELBW children consistently performed worse on neurocognitive testing. We studied neonatal creatinemia trends and linked these trends to neurocognitive outcome at the age of two years (chapter 6), but these trends were not associated with long–term renal function (chapter 8). The estimated glomerular filtration rate (eGFR, cystatin C) was 1 SD lower in former ELBW neonates, but still mainly in the normal range (chapter 7 and 8). We found no evidence that a potential nephrotoxic drug (ibuprofen) used in premature infants to close a patent ductus arteriosus has an impact on the long–term estimated glomerular filtration rate and renal length (chapter 8). Intriguingly and despite an extensive search for risk factors (perinatal characteristics, post discharge growth, postnatal creatinine trends), we could not identify any association with renal outcome in young adolescence in these cases.

Interestingly, plasma renin activity (PRA) is suppressed in these children and in the multivariate model including sodium load, PRA correlated with systolic blood pressure. Moreover, we observed that ELBW children have a higher risk (5 to 9–fold) to have pre–hypertension or hypertension respectively (chapter 7). We performed a pilot study to derive neonatal proximal tubular epithelial cells (nPTEC) to use as model to study pharmacology (chapter 9). We hypothesized that cells from neonates of different gestational ages (i.e. before or after completion of nephrogenesis), display ontogenetic changes during development. However, our results suggest that although kidneys did not complete nephrogenesis in neonates before 34 weeks of gestation, cultured renal proximal tubular cells display similar characteristics in comparison to those obtained from samples collected after completion of nephrogenesis.

We propose the hypothesis that an impaired microcirculation underlying adverse health outcomes as prematurely born children age is a common mechanism. The retina offers a unique opportunity to investigate the retinal microcirculation as a mirror of the microcirculation in other organs. Analyses showed significant arteriolar narrowing, impaired arteriolar branching and more tortuous arterioles in cases (Chapter 10). ELBW young adolescents display retinal vascular maldevelopment. Since this includes arteriolar narrowing, this is a pattern similar to an increased cardiovascular risk profile described for other populations.

In conclusion, we found that a poor start to life casts its shadow into young adolescence in which changes are already visible. Young adolescence might be the ideal time window for interventions and education to promote healthy aging and referral to adult specialists. Linking these outcome variables in adolescence with perinatal characteristics may guide future perinatal clinical management. Finally, the PREMATCH infrastructure can be used to assess microvascular outcome for other patient categories like cases with CAKUT or to pool data with other cohorts to further explore the phenotype variability, its causes and its consequences throughout life (i.e. ‘healthy aging’).