#5158 AUTOSOMAL DOMINANT TUBULAR KIDNEY DISEASE: FROM PHENOTYPE TO GENOTYPE
Authors: Jorge Sofia , Pais Telma , Gameiro Joana , Godinho Iolanda , Nogueira Estela , Outerelo Cristina , Rodrigues Márcia , Lopes José António 
Published: June 2023

Abstract

Background and Aims
Autosomal dominant tubulointerstitial kidney disease (ADTKD) represent a recently described group of kidney diseases, characterized by chronic interstitial nephritis (CIN), in most cases hereditary, with an autosomal dominant inheritance pattern, subclassified according to its genetic cause, when identified. Mutations in UMOD, MUC1, REN and HNF1B genes have been implicated in their pathogenesis. Recent molecular diagnostic technologies, such as Next-Generation Sequencing (NGS), and particularly the use of restriction and Snapshot method for the detection of a cytosine insertion on the coding variable-number tandem repeat (VNTR) sequence in the MUC1 gene encoding mucin 1, have allowed the nephrologists to diagnose renal diseases of previously unknown cause, with a real prevalence probably underestimated. We present the cohort of patients identified with ADTKD in the Nephrogenetics’ clinic from the Nephrology department of our hospital from 2015 to 2022.

Method
Patients with chronic kidney disease (CKD) of unknown etiology and renal phenotype suggesting CIN (bland urine analysis, hyperuricemia and/or anemia unrelated to the degree of CKD, hypo/hypercalcemia, acidemia, urinary concentration defects, absent hypertension, renal cysts), especially with a family history of similar CKD, were referred for the Nephrogenetics’ clinic and ADTKD was investigated. In these cases, we performed a genetic study of ADTKD by the following steps: 1) UMOD, REN, HNF1B mutations studied through NGS; 2) if negative, search for the insertion of a single cytosine of the repeat unit comprising the extremely long (∼1.5-5 kb), GC-rich (&gt;80%) coding variable-number tandem repeat (VNTR) sequence in the MUC1 gene encoding mucin 1 (see Kirby et al Nat Genet. 2013). It is used a restriction and Snapshot method. 3) if negative, search for HNF1B Large deletion, detected by MLPA (kit P241-D2, MRC-Holland). Pre-genetic counselling was performed by the nephrologist, and post-genetic counselling was performed by the nephrologist and geneticist, namely to proceed with family screening of potentially affected family members when a pathogenic mutation was identified or when segregation studies were necessary.

Results
31 families were studied and a confirmed genetic diagnosis was obtained in 11 (35%), allowing the identification of 30 patients with ADTKD; the mutations identified were in MUC1 in 4 families (13 patients), UMOD in 4 families (11 patients), HNF1B (2 families, 5 patients) and REN: 1 patient with a phenotype compatible with ADTKD but with a genotype compatible with renal tubular dysgenesis (two mutations in REN gene from each of the parents). One patient with HNF1B deletion also presented with a heterozygous variant of unknown significance in PKD1 gene, and a pathogenic variant of SLC12A3, heterozygous. Three families are still under study: 2 families are being studied with segregation studies to evaluate the pathogenicity of mutations in UMOD (1 family) and HNF1B (1 family), and 1 family with a phenotype of ADTKD (3 young patients with positive family history) with negative results for mutations in ADTKD genes is in study, with a negative result in WES. The study will be pursued in a specialized laboratory to better study MUC1, but at this moment we classify this family as ADTKD-NOS. If positive results are obtained for these 3 families, the percentage of families identified using this protocol will increase to 43,3% of the families studied through this model.

Conclusion
In our cohort, a genetic diagnosis was established in 35% of the families studied. MUC1 and UMOD are the most prevalent genes implicated in ADTKD at our center, similarly to what has been reported in other cohorts of ADTKD. Given the rarity of these diseases and the fact that they require specific molecular techniques for accurate diagnosis, we believe that Nephrogenetics’ clinics with dedicated nephrologists and geneticists in close cooperation are critical for an accurate diagnosis.