Comprehensive clinical approach to renal tubular acidosis

Genetic Diagnosis and Treatment of Inherited Renal Tubular Acidosis

Background

Renal tubular acidosis (RTA) is caused by various disruptions to the secretion of H+ by distal renal tubules and/or dysfunctional reabsorption of HCO3- by proximal renal tubules, which causes renal acidification dysfunction, ultimately leading to a clinical syndrome characterized by hyperchloremic metabolic acidosis with a normal anion gap. With the development of molecular genetics and gene sequencing technology, inherited RTA has also attracted attention, and an increasing number of RTA-related pathogenic genes have been discovered and reported.

Summary

This paper focuses on the latest progress in the research of inherited RTA and systematically reviews the pathogenic genes, protein functions, clinical manifestations, internal relationship between genotypes and clinical phenotypes, diagnostic clues, differential diagnosis, and treatment strategies associated with inherited RTA. This paper aims to deepen the understanding of inherited RTA and reduce the missed diagnosis and misdiagnosis of RTA.

Key Messages

This review systematically summarizes the pathogenic genes, pathophysiological mechanisms, differential diagnosis, and treatment of different types of inherited RTA, which has good clinical value for guiding the diagnosis and treatment of inherited RTA.

Renal Tubular Acidosis in Patients with Systemic Lupus Erythematosus

OBJECTIVE

Renal tubular acidosis (RTA) is a clinical manifestation that occurs with insufficiency in restoring bicarbonate or disruption in hydrogen ion elimination as a result of a disruption in tubulus functions, causing normal anion gap-opening metabolic acidosis. In the present study, we aimed to investigate the prevalence of RTA in the largest systemic lupus erythematosus (SLE) patient population to date.

MATERIALS AND METHODS

SLE patients, who were followed up in 2 different healthcare centers, were included. Patients with metabolic acidosis (pH <7.35 and HCO3 <22 mEq/L) in venous blood gas analysis were determined. The serum and urine anion GAP of these patients were estimated, and the urine pH was assessed. RTA presence was evaluated as metabolic acidosis with a normal serum anion gap and a positive urine anion GAP.

RESULTS

A total of 108 patients were included in the present study. The mean age of the patients was 41.5 ± 1.2 and 87% were female. The SLE diagnosis duration was 75 ± 5 months. The mean creatinine value ​​was 0.6 ± 0.1 mg/dL and the mean eGFR was 111 ± 2 mL/min. According to the blood gas analysis, 18 patients (16.7% of the total) had RTA. Sixteen of these patients had type 1 RTA and 2 had type 2 RTA; type 4 RTA was not determined in any of the patients.

CONCLUSION

RTA should be considered in SLE patients even if they have normal eGFR values. This is the largest study to examine the prevalence of RTA in SLE patients in the literature.

Factors influencing peak bone mass gain

Bone mass is a key determinant of osteoporosis and fragility fractures. Epidemiologic studies have shown that a 10% increase in peak bone mass (PBM) at the population level reduces the risk of fracture later in life by 50%. Low PBM is possibly due to the bone loss caused by various conditions or processes that occur during adolescence and young adulthood. Race, gender, and family history (genetics) are responsible for the majority of PBM, but other factors, such as physical activity, calcium and vitamin D intake, weight, smoking and alcohol consumption, socioeconomic status, age at menarche, and other secondary causes (diseases and medications), play important roles in PBM gain during childhood and adolescence. Hence, the optimization of lifestyle factors that affect PBM and bone strength is an important strategy to maximize PBM among adolescents and young people, and thus to reduce the low bone mass or osteoporosis risk in later life. This review aims to summarize the available evidence for the common but important factors that influence bone mass gain during growth and development and discuss the advances of developing high PBM.

[Renal involvement in Sjögren's syndrome]

Primary Sjögren syndrome is an autoimmune disorder characterized by lymphoplasmacytic infiltration of the exocrine (salivary and lachrymal) glands resulting in sicca symptoms (dryness). Systemic complications can occur in primary Sjögren syndrome, but renal involvement is rare, affecting<10% patients. The most frequent form of nephropathy in primary Sjögren syndrome is tubulointerstitial nephritis, where infiltration of the kidney by plasma cells is a key feature and shows similarity to the lymphoplasmacytic infiltration of the salivary glands. Electrolyte disturbances may occur in primary Sjögren syndrome, such as renal distal tubular acidosis, diabetes insipidus, Gitelman syndrome, or Fanconi syndrome. Glomerular involvement is less frequently detected in patients with primary Sjögren syndrome, but can take the form of membranoproliferative glomerulonephritis secondary to cryoglobulinaemia. The renal prognosis in patients with primary Sjögren syndrome and TIN or glomerular disease is usually good, but the risk of chronic kidney disease remains significant for some patients. Appropriate screening must be performed at least once a year in patients with systemic primary Sjögren syndrome in order to facilitate the early detection of renal complications. In this Review, we discuss the epidemiology, pathophysiology, differential diagnosis, and treatment of renal disease in primary Sjögren syndrome.

Biomarkers and Diagnostic Testing for Renal Disease in Sjogren's Syndrome

Primary Sjogren's syndrome (pSS) is an autoimmune disorder in which lymphocytic infiltration leads to lacrimal and salivary glands dysfunction, which results in symptoms of dryness (xerophthalmia and xerostomia). Extraglandular features are common and may affect several organs. Renal involvement has long been known as one of the systemic complications of pSS. The most classical lesion observed in pSS is tubulointerstitial nephritis (TIN) and less frequently membranoproliferative glomerulonephritis (MPGN), which is related to cryoglobulinemia. In some cases, renal biopsy is necessary for the definitive diagnosis of kidney involvement. Patients may present with proximal renal tubular acidosis, distal renal tubular acidosis and chronic kidney disease. Response to treatment is usually favorable. However, occasionally severe and rarely lethal outcomes have been described. Recently, several case series and cross-sectional studies have been published which investigated the factors associated with renal involvement in pSS and the most accurate screening tests for early detection. The presence of xerophthalmia, anti-SSA and rheumatoid factor positivity, low C3 levels and other features have all shown either positive or inverse associations with the development of renal complications. Serum creatinine, alpha-1-microglobulin, cystatin-C have been evaluated as early detection biomarkers with variable accuracy. More advanced techniques may be necessary to confirm proximal and distal renal tubular acidosis, along with nephrogenic diabetes insipidus. The aim of the current paper is to summarize and critically examine these findings in order to provide updated guidance on serum biomarkers and further testing for kidney involvement in pSS.

Teenager with a Primary Renal Lymphoma: A Case Report

We report a case of an 18-year-old male patient who was diagnosed with primary renal lymphoma (PRL) through biopsy findings and imaging studies. The patient presented with clinical manifestations of distal renal tubular acidosis including polyuria, polydipsia, lower limb weakness, involuntary weight loss, asthenia and dyspnea. No personal background or relevant medical history was reported. A kidney biopsy showed high grade immature B-cell lymphoproliferative process (Non-Hodgkin's Lymphoma) with a Ki67 value greater than 90%. Complementary studies excluded primary lymphoid migration sites, which confirmed the diagnosis of PRL. The oncology unit initiated treatment with a combination of medications due to lack of protocols for the specific treatment. Besides the fact that this condition is rare, it also shows a unique symptoms presentation and non-typical findings in imaging methods. Also, it is important to underline the fact that the treatment is not yet specified for such type of cancer and different combinations are needed to control the disease.

[Etiology and genetic diagnosis of short stature in children]

OBJECTIVE

To study the etiology and genetic diagnosis of children with short stature.

METHODS

A retrospective analysis was performed to study the etiological distribution and clinical features of 86 children with short stature.

RESULTS

A total of 6 causes were observed in these children, among which idiopathic short stature (ISS, 41%) and growth hormone deficiency (GHD, 29%) were the most common causes, followed by genetic diseases (14%). There were no significant differences in age at the time of diagnosis, body height, body length and weight at birth, body height of parents and insulin-like growth factor-1 levels between the genetic disease group and the ISS/GHD groups (P>0.05). Compared with the ISS group, the genetic disease group had significantly lower deviation from the 3rd percentile for the height of children of the same age and sex (ΔP3) and height standard deviation score (P<0.05), while there were no significant differences between the genetic disease and GHD groups (P>0.05). The analysis of the clinical manifestations for the genetic disease group showed heterogeneity and phenotypic overlap in children with different genetic diseases.

CONCLUSIONS

ISS, GHD and genetic diseases are major causes of short stature in children. For children with severe short stature, genetic testing should be performed to make a definitive diagnosis after GHD has been excluded.

Presence of serum autoantibodies to vacuolar H+ -ATPase in patients with renal tubular acidosis

OBJECTIVE

Concomitant presence of renal tubular acidosis (RTA) and autoimmune diseases is indicative of the potential role of immune factors in the pathogenesis of RTA. Our study aimed to detect the serum antibodies to renal tubular epithelial cells in RTA patients.

METHODS

We enrolled 11 RTA patients, eight primary Sjögren's syndrome (pSS) patients and eight healthy controls (HC). Serum biochemical test, urinary regular test, and 24 hours urinary protein quantification were measured using a fully automated analyzer. Immunofluorescence assay was performed to detect the antibodies to subunit B1 and subunit B2 of v-H+-ATPases (adenosine triphosphatases) in the serum of the participants.

RESULTS

Clinically, RTA patients showed hyperchloremia, acidosis and paradoxical alkaline urine. We detected the serum antibodies to renal tubular epithelial cells and there were 6/11 positive in RTA patients, much higher than that in the pSS group (0/8) and the HC group (0/8). Subsequently, we demonstrated that in normal renal tissue, the B1 subunit of v-H+-ATPase specifically expressed in intercalated cells, while the B2 subunit continuously expressed along the lumen of renal tubular epithelial cells. Moreover, the antibody to subunit B1/B2 of v-H+-ATPase was positive in the sera of 6 RTA patients (54%), while it was negative in both the pSS and HC group.

CONCLUSIONS

We detected the presence of serum autoantibodies to subunit B1 and subunit B2 of v-H⁺ -ATPase in RTA patients. Our findings may provide novel mechanism insights into the pathogenesis of RTA and the potential diagnostic utility of antibodies to v-H⁺ -ATPase in the classification of RTA.

Classification of Uniparental Isodisomy Patterns That Cause Autosomal Recessive Disorders: Proposed Mechanisms of Different Proportions and Parental Origin in Each Pattern

Patients with autosomal recessive (AR) disorders are usually born to parents both of whom are heterozygous carriers of the disease. However, in some instances only one of the parents is a carrier and a mutation is segregated to the patient through uniparental isodisomy (UPiD). Recently, an increasing number of such case reports has been published, and it has become clear that there are several different UPiD patterns that cause AR disorders. In this article, we report 3 remarkable patients with different patterns of UPiD. We then review 85 cases collected in the literature. We realized that they can be classified into 3 patterns: UPiD of the whole chromosome, segmental UPiD with uniparental heterodisomy (UPhD), and segmental UPiD caused by post-zygotic mitotic recombination (MiRe). Whole chromosomal UPiD accounted for the majority of cases, with paternal origin accounting for approximately twice as many cases as maternal origin. Most cases of segmental UPiD with UPhD were of maternal origin, with a dominancy of nondisjunction in meiosis I, while segmental UPiD through MiRe is the smallest pattern with equal parental origin. These differences in proportion and parental origin in each pattern can be explained by considering nondisjunction during oogenesis as the starting point and UPiD as subsequent events.

Response of Npt2a knockout mice to dietary calcium and phosphorus

Mutations in the renal sodium-dependent phosphate co-transporters NPT2a and NPT2c have been reported in patients with renal stone disease and nephrocalcinosis, but the relative contribution of genotype, dietary calcium and phosphate to the formation of renal mineral deposits is unclear. We previously reported that renal calcium phosphate deposits persist and/or reappear in older Npt2a-/- mice supplemented with phosphate despite resolution of hypercalciuria while no deposits are seen in wild-type (WT) mice on the same diet. Addition of calcium to their diets further increased calcium phosphate deposits in Npt2a-/-, but not WT mice. The response of PTH to dietary phosphate of Npt2a-/- was blunted when compared to WT mice and the response of the urinary calcium x phosphorus product to the addition of calcium and phosphate to the diet of Npt2a-/- was increased. These finding suggests that Npt2a-/- mice respond differently to dietary phosphate when compared to WT mice. Further evaluation in the Npt2a-/- cohort on different diets suggests that urinary calcium excretion, plasma phosphate and FGF23 levels appear to be positively correlated to renal mineral deposit formation while urine phosphate levels and the urine anion gap, an indirect measure of ammonia excretion, appear to be inversely correlated. Our observations in Npt2a-/- mice, if confirmed in humans, may be relevant for the optimization of existing and the development of novel therapies to prevent nephrolithiasis and nephrocalcinosis in human carriers of NPT2a and NPT2c mutations.

Renal involvement in primary Sjögren syndrome

Primary Sjögren syndrome (pSS) is an autoimmune disorder characterized by lymphoplasmacytic infiltration of the exocrine (salivary and lachrymal) glands that results in sicca symptoms (dryness of the eyes and mouth). Systemic complications can occur in pSS, but renal involvement is rare, affecting <10% patients. The most frequent form of nephropathy in pSS is tubulointerstitial nephritis (TIN), in which infiltration of the kidney by plasma cells is a key feature and shows similarity to the lymphoplasmacytic infiltration of the salivary glands. Electrolyte disturbances may occur in pSS, such as renal distal tubular acidosis, diabetes insipidus, Gitelman syndrome or Fanconi syndrome. Glomerular involvement is less frequently detected in patients with pSS, but usually takes the form of membranoproliferative glomerulonephritis secondary to cryoglobulinaemia. The renal prognosis in patients with pSS and TIN or glomerular disease is usually favourable, but the risk of chronic kidney disease remains high in patients with TIN. Appropriate screening must be performed at least once a year in patients with systemic pSS in order to facilitate the early detection of renal complications. In this Review we discuss the epidemiology, pathophysiology, differential diagnosis and treatment of renal disease in pSS.