Tubulopathies

Proximal renal tubular acidosis (pRTA) is an ultra-rare genetic disorder caused by defective Na+/HCO3−cotransporters (NBC) in the renal proximal tubules (Igarashi T, Nat Genet, 1999). NBC facilitates the majority of basolateral bicarbonate reabsorption and is encoded by SLC4A4. To date, 25 pathogenic mutations in SLC4A4 have been reported (HGMD professional 2023.1), impacting the electrogenic activities of NBC (Horita S, J Am Soc Nephrol, 2005). This dysfunction leads to Fanconi syndrome, featuring renal bicarbonate loss, subsequent metabolic acidosis, and hypokalemia. Concurrent hypercalciuria often results in kidney damage, including nephrolithiasis and nephrocalcinosis, progressing to chronic kidney disease (CKD) and kidney failure. Additionally, patients with pRTA commonly exhibit short stature and ocular abnormalities, such as glaucoma, cataracts, and band keratopathy due to disrupted bicarbonate homeostasis and corneal calcium deposition. Further associated manifestations include dental abnormalities, as well as psychomotor and intellectual retardation due to by basal ganglia calcifications (Igarashi T, J Am Soc Nephrol, 2002).
Currently, no specific therapy exists for pRTA, necessitating the supplementation of high amounts of bicarbonate and potassium preparations. However, these interventions frequently yield inadequate and short-term corrections, resulting in pH fluctuations. A novel therapeutic avenue for distal RTA involves the prolonged-release formulation of potassium citrate and potassium bicarbonate (Sibnayal), facilitating continuous bicarbonate and potassium release, thereby improving metabolic control and mitigating pH fluctuations (Bertholet-Thomas A, Pediatr Nephrol, 2020; Bertholet-Thomas A, Pediatr Nephrol, 2021). Moreover, improved patient comfort and compliance are anticipated due to the reduced dosing frequency (twice daily) and diminished gastrointestinal side effects. Clinical trials assessing its efficacy have demonstrated favorable impacts on bone metabolism, suggesting a potential delay in nephrocalcinosis development and CKD progression through improved metabolic control. In April 2021, extended-release potassium citrate received approval from the EMA for dRTA.

We aim to further explore the ultra-rare disease of proximal renal tubular acidosis (pRTA), including functional characterization of the disease's underlying mechanisms, characterization of kidney and extrarenal phenotypes, and investigation of a new therapeutic approach using prolonged-release formulation of potassium citrate and potassium bicarbonate (Sibnayal).

The following are the key objectives:

(1) Genotypic and phenotypic characterization of patients carrying SLC4A4 variants causing isolated or syndromic proximal renal tubular acidosis (RTA).

(a) Deep phenotyping of a multiple affected family to characterize of kidney involvement, ocular manifestations, skeletal defects, and
     other organ manifestations.

(b) Identification and characterization of patients/families with pRTA carrying (likely) pathogenic SLC4A4 variants by screening the ERKNet registry.

(2) Functional characterization of the activity of Na+/HCO3−cotransporters (NBC) in patients' derived cells.

(a) Investigation of SLC4A4 expression in tubular epithelial cells

(b) pH measurement in tubular epithelial cells to assess electrogenic transport activity

(3) Investigating a new therapeutic approach of Sibnayal, a treatment that is currently approved for distal RTA.

Adult and pediatric patients with proximal renal tubular acidosis due to (likely) pathogenic SLC4A4 variants should be included.
All stages of CKD (stage 1-5T KDIGO) are eligible.

Data Sources: ERKReg, Data Sources (click all that apply) Surveys and HCP centre data

Data Elements:
Genetic data:

• genetic analysis (panel, exome sequencing, genome sequencing).

• SLC4A4 variant, homozygous/compound-heterozygous state, segregation, c. and p. position, transcript ID.

Clinical data:

• family history

• age at diagnosis

• kidney phenotype (e.g., kidney stones, nephrocalcinosis), CKD stage

• laboratory values: pH and calcium level in urine; pH, bicarbonate, potassium, sodium, chloride level in serum

• patient height

• skeletal affection

• ocular manifestations

• psychomotor and intellectual development

Treatment:

• bicarbonate and potassium substitution

• other medication (e.g., diuretics)

• other therapeutic approaches (e.g., fluid intake)

Objective (1)
Genotypic and phenotypic characterization of patients carrying SLC4A4 variants causing isolated or syndromic proximal renal tubular acidosis (RTA).
Deep phenotyping of a family with three affected daughters out of six children in total. The affected 31, 25, and 23-year-old women were diagnosed via exome sequencing, revealing homozygous likely pathogenic SLC4A4 variant c.1170G>C, p.(Arg390Ser) (NM_001098484.3). Segregation analysis identified heterozygous variants in both healthy parents. The three patients exhibit Fanconi syndrome with severe metabolic acidosis (e.g., pH 7.12, K+ 2.8mmol/l, and HCO3- 12.2 mmol/l) under treatment with Blemaren 3-3-3, Bicanorm 1g 3-3-3-3, and Kalinor 2-1-0. Additionally, they present with varying degrees of growth retardation, ocular involvement, skeletal anomalies, and intellectual disability.

(a) Deep phenotyping encompasses comprehensive assessments, including kidney examination utilizing ultrasound and laboratory analyses, ocular investigations involving Optical Coherence Tomography (OCT) to reveal calcium deposition in the corneal stroma, slit lamp examination, Pentacam documentation of corneal structure, and measurement of pH in tear fluid. Additionally, skeletal status will be evaluated through X-ray imaging, along with the examination of other organ manifestations.

(b) Identification and characterization of patients with pRTA carrying SLC4A4 variants by screening the ERKNet registry based on the specified inclusion criteria.

Objective (2)
Functional characterization of the activity of Na+/HCO3−cotransporters (NBC) in patients' derived cells. Urine-Derived Epithelial Cells (URECs) generated from patients' urine from our index family will be utilized for tubular epithelial cell function assessment using a detailed protocol (Zhou T, Nat Protoc, 2012).

(a) Investigation of quantitative and qualitative SLC4A4 expression in tubular epithelial cells by immunofluorescence and qPCR.

(b) pH measurement in tubular epithelial cells to asses electrogenic transport activity

Objective (3)
Investigating a new therapeutic approach of Sibnayal, a treatment for distal RTA. The cost commitment for Sibnayal therapy has already been approved by the patient's health insurance. Assessment will include quality-of-life questionnaires and monitoring for side effects (e.g., gastrointestinal). Additionally, laboratory values (pH and calcium level in urine; pH, bicarbonate, potassium, sodium, chloride in serum) and clinical manifestations will be evaluated.

12 weeks