A novel missense variant in the ATPase domain of ATP8A2 and review of phenotypic variability of ATP8A2 -related disorders caused by missense changes.

ATPase, class 1, type 8A, member 2 (ATP8A2) is a P4-ATPase with a critical role in phospholipid translocation across the plasma membrane. Pathogenic variants in ATP8A2 are known to cause cerebellar ataxia, mental retardation, and disequilibrium syndrome 4 (CAMRQ4) which is often associated with encephalopathy, global developmental delay, and severe motor deficits. Here, we present a family with two siblings presenting with global developmental delay, intellectual disability, spasticity, ataxia, nystagmus, and thin corpus callosum. Whole exome sequencing revealed a homozygous missense variant in the nucleotide binding domain of ATP8A2 (p.Leu538Pro) that results in near complete loss of protein expression. This is in line with other missense variants in the same domain leading to protein misfolding and loss of ATPase function. In addition, by performing diffusion-weighted imaging, we identified bilateral hyperintensities in the posterior limbs of the internal capsule suggesting possible microstructural changes in axon tracts that had not been appreciated before and could contribute to the sensorimotor deficits in these individuals.

Functional and in silico analysis of ATP8A2 and other P4-ATPase variants associated with human genetic diseases.

P4-ATPases flip lipids from the exoplasmic to cytoplasmic leaflet of cell membranes, a property crucial for many biological processes. Mutations in P4-ATPases are associated with severe inherited and complex human disorders. We determined the expression, localization and ATPase activity of four variants of ATP8A2, the P4-ATPase associated with the neurodevelopmental disorder known as cerebellar ataxia, impaired intellectual development and disequilibrium syndrome 4 (CAMRQ4). Two variants, G447R and A772P, harboring mutations in catalytic domains, expressed at low levels and mislocalized in cells. In contrast, the E459Q variant in a flexible loop displayed wild-type expression levels, Golgi-endosome localization and ATPase activity. The R1147W variant expressed at 50% of wild-type levels but showed normal localization and activity. These results indicate that the G447R and A772P mutations cause CAMRQ4 through protein misfolding. The E459Q mutation is unlikely to be causative, whereas the R1147W may display a milder disease phenotype. Using various programs that predict protein stability, we show that there is a good correlation between the experimental expression of the variants and in silico stability assessments, suggesting that such analysis is useful in identifying protein misfolding disease-associated variants.

Kidney length standardized to body length predicts outcome in infants with a solitary functioning kidney.

BACKGROUND: Infants with a solitary functioning kidney (SFK) are at risk for chronic kidney injury (CKI). Lack of compensatory kidney growth (CKG) is associated with CKI, but measuring CKG is challenging since it is typically reported relative to normal kidneys. This study aims to (1) standardize SFK growth in infants, (2) investigate the relationship between standardized kidney length and clinical outcomes, and (3) use these results to develop a risk-based prediction model and local clinical pathway for SFK care. METHODS: This was a quality improvement study of 166 infants with an SFK. Linear regression was used to assess kidney growth from 0 to 180 days of life. Univariate binary regression analysis was used to identify kidney length to body length thresholds associated with the development of CKI, defined as the composite outcome of chronic kidney disease (eGFR < 60 mL/min/1.73 m2), hypertension, or proteinuria. RESULTS: Kidneys grew in length from 0 to 180 days, and growth was constant when standardized to body length. Over follow-up, infants with a baseline kidney length to body length ≤ 0.088 were more likely to experience CKI than the rest of the cohort (27 vs. 8%, p = 0.04). Kidney length to body length ≤ 0.088 was also significantly associated with CKI development (OR 4.17, 95% CI 1.14-15.28, p = 0.04). CONCLUSIONS: In this study, kidney length to body length ratio was a stable CKG metric over 0-180 days, and a baseline ratio ≤ 0.088 was a risk factor for CKI. Results will aid in developing a practical, point-of-care risk assessment tool, and overarching risk-stratified clinical pathway for infants with an SFK. A higher resolution version of the Graphical abstract is available as Supplementary information.

Outcomes of solitary functioning kidneys-renal agenesis is different than multicystic dysplastic kidney disease.

BACKGROUND: Multicystic dysplastic kidney (MCDK) disease and unilateral renal agenesis (URA) are well-known causes of a solitary functioning kidney (SFK) and are associated with long-term kidney injury. The aims of this study were to characterize the natural history of SFK at our center, define the risk factors associated with chronic kidney injury, and identify distinguishing features between URA and MCDK that predict outcome. METHODS: This was a retrospective cohort study of 230 SFK patients. We compared MCDK (n=160) and URA (n=70) according to clinical features at diagnosis and kidney outcomes over follow-up. Univariate and multivariate binary regression analysis was used to determine independent risk factors for chronic kidney injury, defined as the composite outcome of hypertension, proteinuria, or chronic kidney disease (eGFR <60 mL/min/1.73m2). RESULTS: URA had a higher prevalence of comorbid genetic syndromes (15 vs. 6%, p=0.04), non-renal anomalies (39 vs. 11%, p<0.001), and congenital anomalies of the kidney and urinary tract (CAKUT) (51 vs. 26%, p<0.001) than MCDK. Over follow-up, URA experienced more hypertension (19 vs. 3%, p=0.002), proteinuria (12 vs. 3%, p=0.03), and the composite outcome (19 vs. 6%, p=0.003) than MCDK. Independent risk factors for chronic kidney injury included CAKUT (OR 5.01, p=0.002) and URA (OR 2.71, p=0.04). CONCLUSIONS: In our population, URA was more likely to have associated syndromes or anomalies, and to have worse outcomes over time than MCDK. URA diagnosis was an independent risk factor for chronic kidney injury. Our results will be used to develop a standardized clinical pathway for SFK management. A higher resolution version of the Graphical abstract is available as Supplementary information.

The impact of small kidneys.

BACKGROUND: Small kidneys due to renal hypodysplasia (RHD) result from a decrease in nephron number. The objectives of this study were to identify clinical variables that determine long-term renal outcome in children with RHD and to define the role of kidney size as a predictor of developing end-stage renal disease (ESRD). METHODS: This was a single-center retrospective cohort analysis. The primary outcome was development of ESRD. We identified 202 RHD cases, with 25 (12%) reaching ESRD at mean age of 8.9 (±6.6) years. RESULTS: Children with RHD with a known genetic syndrome had the smallest kidneys while those with posterior urethral valves (PUV) had the largest kidneys at diagnosis. Cases with bilateral RHD were most likely to develop ESRD. Younger gestational age (OR 0.8, CI 0.69-0.99, p = 0.05), smaller kidney size at diagnosis (OR 0.13, CI 0.03-0.47, p = 0.002), lower best-estimated glomerular filtration rate (eGFR) (OR 0.74, CI 0.58-0.93, p = 0.01), proteinuria (OR 1.03, CI 1.01-1.05, p < 0.001) and high blood pressure (OR 1.02, CI 1.01-1.04, p = 0.01) were associated with development of ESRD, while kidney size at diagnosis was independently associated with ESRD (HR 0.03, CI 0.01-0.72, p = 0.043). CONCLUSIONS: In children with RHD, kidney size at diagnosis predicts the likelihood of developing ESRD.