#4317 CLINICAL DIVERSITY OF STEROID-RESISTANT NEPHROTIC SYNDROME CAUSED BY TRPC6 MUTATIONS
Authors: Tran Quynh , Tran Linh , Ueda Hiroko , Nakazato Hitoshi , Sugimoto Keisuke , Kagami Shoji , Hattori Motoshi , Nozu Kandai , Tsukaguchi Hiroyasu 
Published: June 2023

Abstract

Background and Aims
Steroid-resistant nephrotic syndrome (SRNS) is a clinically and genetically heterogeneous disorder caused by either genetic or immunological factors or their combination. Approximately 30–40% of patients with SRNS make a fast progression to ESRD. SRNS represents the second leading cause of end-stage renal disease in individuals under the age of 25 years worldwide. More than 60 podocyte-related gene mutations have thus far been reported in monogenic SRNS. Recent studies indicate that almost 30% of patients with childhood-onset SRNS have monogenic causative variants in one of the 27 SRNS genes. Among these, a mutation in Transient Receptor Potential Canonical 6 (TRPC6) gene, encoding non-selective cation channel, accounts for 6% of familial SRNS and approximately 2% of sporadic cases. We recently detected eight patients with TRPC6 mutations in our SRNS cohort (n = 39, 21%) and found inter- and intra-familial heterogeneity of clinical phenotypes in terms of disease onset and progression.

Method
A total of 39 Japanese SRNS patients were studied according to the protocol approved by the institutional review board of each affiliation. Subjects who manifested SRNS at age one to 14-year-old with biopsy-proven FSGS were enrolled. A total of 39 SRNS patients, whose average onset of proteinuria at median age 4 (range 0.9 to 17 yr) and ESRD at median age 7 (49% of total n = 19, range 3.0 to 16.0 yr) were studied. Family history was found in 44% (n = 17), including 13 autosomal recessive, 3 dominant, and one X-linked transmission. Sixteen percent (n = 10) of the patients who underwent renal transplantation win no recurrence. Sequence analysis revealed the pathogenic variants in 62% (n = 24) of all cases. Library for whole-exome sequencing or targeted panel sequencing were prepared from genomic DNA by use of SureSelect Human All Exon V5 (Agilent) or a HaloPlex target enrichment system kit (Agilent Technologies), or Ampli-Seq (Thermo), Prepared samples were run on a HiSeq 2000, or MiSeq (Illumina), or Ion PGM (Thermo). The sequence reads with 101-bp paired-end reads and 7-bp index reads were mapped to the human reference sequence (hg19, GRCh38.p7). After filtering the superfluous begin variants, only variants that fulfill the “pathogenic” criteria of ACMG and/or ClinVar, Varsome were selected.

Results
Sequence analysis revealed the pathogenic variants in 62% (n = 24) of all cases. TRPC6 mutations were the most frequent cause of SRNS (n = 8), which was followed by NUP107 (n = 5), PLCE1 (n = 3), COL4A3 (n = 2), COL4A5 (n = 1), and others were identified in only single case. The eight TRPC6 mutations clustered into the two distinctive cytoplasmic domains; N-terminal ANK (Y173D, R175W, R175G) and C-terminal coiled-coil domain (p.E875V, p.867_868Del, p.S893N, p.R895C). The p.R895C is deposited as the pathogenic in ClinVar and may be a gain-of-function. The remaining 7 variants are of unknown functions: six have not yet been deposited in ClinVar, whereas one is VUS. Four patients were de novo occurrence, three followed the dominant transmission, and one arose from gonadal mosaicism. Two index patients exhibited SRNS in early childhood (&lt; age 3), four manifested it in childhood (age 3–12), while the remaining two developed it in adolescence (age &gt;12). In familial cases, for example, affected ones harboring R175W or p.E875V showed a remarkable discordant age onset and disease severity.

Conclusion
TRPC6 channelopathy was found to be the most frequent cause in our SRNS cohort, of which age onset ranges from age 0.9 to 17 year. Heterogeneity of disease severity may reflect that channel activity is regulated by the mode of action (gain- or loss-of-function), the combination of channel subunit assembly, and interaction with other modifying factors. Mutations in these TRPCs are associated with relatively common kidney diseases as well as other pathologies. Further expression studies will improve our mechanistic understanding of TRP channel pathology and may help the development of novel therapeutic targets.