Clinical Description
Individuals with hereditary distal renal tubular acidosis (dRTA) typically present in infancy with failure to thrive, although later presentations can occur, especially in individuals with autosomal dominant SLC4A1-dRTA. Initial clinical manifestations may also include emesis, polyuria, polydipsia, constipation, diarrhea, decreased appetite, and episodes of dehydration [Besouw et al 2017, Palazzo et al 2017].
Electrolyte manifestations include hypokalemia and hyperchloremic non-anion gap metabolic acidosis with inappropriately elevated urine pH (which may lead to secondary tachypnea if severe [Besouw et al 2017, Palazzo et al 2017]). Some individuals may present with evidence of proximal tubular dysfunction (e.g., amino aciduria, decreased reabsorption of phosphate, and low molecular weight proteinuria); however, this resolves with correction of the acidosis [Besouw et al 2017].
Renal complications in dRTA include nephrocalcinosis, nephrolithiasis, medullary cysts, and impaired renal function, which may occur in childhood [Igarashi et al 1991, Besouw et al 2017].
Nephrocalcinosis, typically bilateral, results from calcium deposition in the renal parenchyma. Reports on the frequency vary in the literature, ranging from approximately 25% [Chang & Lin 2002] to 100% of children with dRTA [Pirojsakul et al 2011, Besouw et al 2017, Enerbäck et al 2018]. In a large mostly European cohort of 340 individuals with dRTA, more than 90% of individuals with molecularly confirmed hereditary dRTA had nephrocalcinosis [Lopez-Garcia et al 2019]. The occurrence appears to increase with age and with later onset of alkalinizing therapy [Caldas et al 1992].
Medullary cysts develop in many individuals during childhood or in adulthood, likely secondary to hypokalemia [Igarashi et al 1991, Besouw et al 2017].
A mild-to-moderate decrease in glomerular filtration rate can occur and increase in prevalence with age but may be present in childhood [Besouw et al 2017, Palazzo et al 2017].
Hypokalemia (blood potassium level <3.5 mEq/L) is found in the majority of individuals with dRTA [Caldas et al 1992, Domrongkitchaiporn et al 2001, Pirojsakul et al 2011]. Individuals with ATP6V1B1- or ATP6V0A4-dRTA tend to have more severe hypokalemia than individuals with autosomal dominant SLC4A1-dRTA [Karet 2002, Alonso-Varela et al 2018]. Symptoms of hypokalemia include muscle weakness and muscle cramps [Nilwarangkur et al 1990, Domrongkitchaiporn et al 2001, Pirojsakul et al 2011]. Paralysis and respiratory depression as a result of muscle weakness may occur with severe hypokalemia [Caldas et al 1992, Domrongkitchaiporn et al 2002a]. Renal cysts are likely related to hypokalemia.
Skeletal manifestations. The metabolic acidosis in dRTA results in the release of bicarbonate and phosphate – which are complexed with calcium – from bone. These salts act as alkalizing buffers to promote restoration of physiologic blood pH [Bushinsky & Frick 2000, Chan et al 2001, Domrongkitchaiporn et al 2001, Bushinsky et al 2003, Fry & Karet 2007].
Bone demineralization can cause rickets in children and osteomalacia in adults [Escobar et al 2013]. These conditions increase the risk of fractures and may cause bone pain. The frequency and severity of bone findings reported in the literature vary significantly. Rickets can cause bone deformities; ambulation may be impaired as a result of leg deformities [Bushinsky & Frick 2000, Domrongkitchaiporn et al 2001, Bushinsky et al 2003]. In a European cohort, rickets was present in 25% of children with dRTA [Caldas et al 1992]; however, others failed to identify any radiographic evidence of rickets in their cohort [Brenner et al 1982]. The reported prevalence of osteomalacia ranges from 10% to 23% [Nilwarangkur et al 1990, Jha et al 2011]. Low bone mass has been commonly reported in individuals of Thai descent with dRTA [Domrongkitchaiporn et al 2001]. Alkali treatment has been shown to improve bone mineral density in these individuals [Domrongkitchaiporn et al 2002b].
Growth. dRTA is often diagnosed during the evaluation of infants or young children with failure to thrive (principally manifesting as poor linear growth with normal weight for height) [Besouw et al 2017]. The majority of children with dRTA have short stature prior to adequate alkali therapy [Besouw et al 2017].The height deficit at diagnosis can be severe [Domrongkitchaiporn et al 2001, Bajpai et al 2005]. Children treated with adequate alkali therapy have improved growth velocity and catch-up growth is common, frequently allowing achievement of a normal height [Besouw et al 2017, Lopez-Garcia et al 2019].
Sensorineural hearing loss occurs in individuals with pathogenic variants in ATP6V1B1 or ATP6V0A4. Both early-childhood onset and later-adult onset can occur; the hearing loss can be profound [Karet et al 1999, Vargas-Poussou et al 2006, Enerbäck et al 2018]. Increased severity and earlier-onset hearing loss is more common in individuals with ATP6V1B1-dRTA. Progression or appearance of hypoacusia is not prevented by medical treatment. Hearing impairment is treated with hearing aids or cochlear implants when necessary.
Hematologic manifestations. A small number of individuals with SLC4A1-dRTA will also have hereditary hemolytic anemia. Pathogenic variants causing both dRTA and hemolytic anemia most commonly occur in Southeast Asia and have also been reported in families in the Middle East and India. The combination of dRTA and hemolytic anemia usually presents in infants and children [Fawaz et al 2012, Khositseth et al 2012]. In one series including 78 affected individuals, hemoglobin values ranged from 4.4 to 15.7 g/dL [Khositseth et al 2012]. Biallelic SLC4A1 pathogenic variants can result in morphologic changes in erythrocytes. These altered erythrocytes are vulnerable to hemolysis under conditions of metabolic acidosis. Alkali therapy is associated with correction of anemia and reticulocytosis [Khositseth et al 2008]. Affected individuals also respond to transfusion and iron therapy [Khositseth et al 2012].
Heterozygotes for pathogenic variants in genes typically associated with autosomal recessive complete dRTA. Heterozygous ATP6V1B1 pathogenic variants have been identified in a few individuals with mild renal acidification defects that do not result in altered blood pH; these individuals are said to have incomplete dRTA [Zhang et al 2014, Dhayat et al 2016]. The diagnosis can be made with an ammonium chloride or fludrocortisone/furosemide challenge, as these individuals also fail to adequately acidify their urine. These individuals also commonly have hypercalciuria and kidney stones.
Phenotype Correlations by Gene
ATP6V0A4 may be associated with a more severe metabolic acidosis and later onset of deafness [Karet et al 1999, Stover et al 2002, Vargas-Poussou et al 2006, Besouw et al 2017].
ATP6V1B1 is associated with symptom onset in infancy or childhood, and deafness typically with onset in infancy.
FOXI1 is associated with autosomal recessive dRTA and early-onset deafness [Enerbäck et al 2018].
SLC4A1 is typically associated with a milder form of dRTA; affected individuals may have a compensated hyperchloremic metabolic acidosis (low serum bicarbonate, but normal pH) [Besouw et al 2017]. Symptom onset occurs in childhood or later, with less impact on growth than in the autosomal recessive forms.