Recessive variants in MYO1C as a potential novel cause of proteinuric kidney disease

Background: Steroid-resistant nephrotic syndrome is the second leading cause of chronic kidney disease among patients <25 years of age. Through whole exome sequencing, identification of >65 monogenic causes has rendered insights into disease mechanisms of nephrotic syndrome. Methods: To elucidate novel monogenic causes of NS, we combined homozygosity mapping with ES in a worldwide cohort of 1649 pediatric patients with NS. Results: We identified homozygous missense variants in MYO1C in two unrelated children with nephrotic syndrome (c.292C>T, p.R98W; c.2273 A>T, p.K758M). We evaluated publicly available kidney single-cell RNA sequencing datasets and found MYO1Cto be predominantly expressed in podocytes. We then performed structural modeling in molecular viewer PyMol using the super function aligning shared regions within both partial structures of MYO1C (4byf and 4r8g). In both structures, calmodulin, a common regulator of myosin activity, is shown to bind to the IQ motif. At both residue sites (K758; R98), there are ion-ion interactions stabilizing intradomain and ligand interactions: R98 binds to nearby D220 within the Myosin Motor Domain and K758 binds to E14 on a calmodulin molecule. Variants of these charged residues to non-charged amino acids could ablate these ionic interactions, weakening protein structure and function establishing the impact of these variants. Conclusion: We here identified recessive variants in MYO1C as a potential novel cause of nephrotic syndrome in children.


Introduction
Nephrotic syndrome (NS) is characterized by substantial proteinuria (> 40 mg/m2/hour), leading to hypoalbuminemia, edema, and hyperlipidemia.Approximately 80% of pediatric and young adult patients go into remission with standard steroid therapy, termed steroid-sensitive nephrotic syndrome (SSNS) [1].Conversely, about 20% of individuals with steroid-resistant nephrotic syndrome (SRNS) are likely to progress to chronic kidney disease (CKD) and ultimately kidney failure [1].The predominant renal histological feature of SRNS is focal segmental glomerulosclerosis (FSGS), which stands as the second most prevalent cause of kidney failure in the rst two decades of life [2].
More than 65 genes have been identi ed as playing a role in monogenic SRNS, offering valuable insights into the complex mechanisms driving proteinuric kidney disease, SRNS, and podocyte biology [3].Despite the relative rarity of these genetic variants, their discovery substantially deepens our understanding of podocytopathies.Furthermore, the identi cation of novel genes associated with SRNS expands the repertoire of genes incorporated into diagnostic gene panels, which are becoming increasingly accessible and utilized in clinical practice.
The glomerular ltration apparatus comprises a three-layered structure consisting of fenestrated endothelium, the glomerular basement membrane, and interconnected podocytes linked through the slit diaphragm.The actin network within podocytes is critical in maintaining foot process integrity, facilitating slit diaphragm turnover, and providing structural support amidst varying glomerular pressures.Genetic variants in genes encoding actin-interacting proteins are prevalent among the monogenic causes of SRNS, underscoring the central role of actin in podocyte physiology [4] [5].
Myosins serve as pivotal actin-based molecular motors, playing important roles in a wide array of cellular functions such as intracellular tra cking, cell adhesion, motility, and maintenance of membrane tension.Among them, MYO1C, a non-muscle myosin motor protein, exhibits high expression levels in glomerular podocytes [6].It plays a crucial role in the function of the glomerular lter by interacting directly with nephrin and neph1, facilitating their transport to podocyte intercellular junctions [7].Depletion of myo1c disrupts the localization of these slit diaphragm proteins at the podocyte cell membrane and leads to an edematous phenotype and abnormal podocyte morphology, as evidenced in zebra sh studies [7].
Based on our prior investigations, we have established that whole exome sequencing (WES) can uncover a monogenic basis for steroid-resistant nephrotic syndrome (SRNS) in 11-45% of familial cases, often by detecting variants within one of the > 65 published genes [8][9][10].
Therefore, we hypothesized that previously unidenti ed single-gene causes of SRNS may be discovered in children and young adults affected by nephrotic syndrome.To investigate this hypothesis, we conducted ES on patients with SRNS/FSGS, leading to the identi cation of variants in MYO1C in two unrelated families.

Results
To identify potential candidate genes for SRNS, we employed WES on an international cohort of 1649 individuals with NS.We discovered two distinct homozygous missense variants in the MYO1C gene, which encodes the motor protein Myosin 1C, in two unrelated families with proteinuric kidney disease.
In family B3913, a homozygous missense variant c.292C > T (p.R98W) was identi ed, while in family B3934, a homozygous variant c.2273A > T (p.K758M) was detected.We con rmed the presence of both variants by Sanger sequencing (Supp Fig. 2).Importantly, neither variant was present homozygously in GnomAD, and only 5 and 14 heterozygous variants, respectively, were listed in the database of this control population (n = ~ 200,000 individuals) [17].Evolutionary conservation analysis indicates that at the modi ed amino acid position, a positively charged amino acid-either arginine or lysine-is conserved across species down to Danio rerio.Two out of three bioinformatic prediction programs (SIFT, Mutation Taster and Polyphen-2) classi ed both variants as pathogenic (Table 1).The identi ed variants were located within well-de ned functional domains, speci cally the N-terminal myosin motor domain and the calmodulin-binding IQ domain, respectively (Fig. 1D).The affected individual in family B3913 is an Arabic male who developed steroid-dependent nephrotic syndrome at the age of 3.He has no extrarenal manifestation and has not had a kidney biopsy.In family B3943, an Arabic male developed steroidresistant nephrotic syndrome at the age of 13, with no extrarenal manifestation, and a kidney biopsy revealed membranoproliferative glomerulonephritis.Consanguinity was reported in both families, and it was con rmed with homozygosity mapping showing runs of homozygosity spanning 154.6 Mb and 170.3 Mb respectively.Notably, the candidate variants in MYO1C coincided with a peak indicating a region of homozygosity by descent (Fig. 1).
We examined kidney single-cell RNA sequencing datasets available publicly and identi ed strong podocyte expression of MYO1C in comparison to positive (nephrin and podocin) and negative controls (ACTA2) ([26]) (Supp Fig. 1).Next, using the molecular viewer PyMol software, we conducted structural modeling to align shared regions within both partial structures of MYO1c.(4byf and 4r8g), aligned with the common regulator of myosin activity, calmodulin (Fig. 2).Calmodulin was observed to bind to the IQ motif in both structures.At residue sites (K758, R98), ion-ion interactions were discerned, stabilizing intradomain and ligand interactions: speci cally, R98 forms a bond with nearby D220 within the Myosin Motor Domain, while K758 interacts with E14 on a calmodulin molecule.We analyzed the effects of the variants on protein tertiary structure utilizing the AlphaMissense software [27].The R98W exchange in the myosin domain (Fig. 2A) was predicted to be benign, whereas K758M, which interacts with glutamate E14 on calmodulin was predicted to ablate this ionic bond and was deemed likely pathogenic (Fig. 2C).

Discussion
In this study, we identi ed two homozygous missense variants in the gene MYO1C in two unrelated families with proteinuric kidney disease.Reported consanguinity and measured homozygosity in both families suggest a recessive cause of disease.Indeed, homozygous MYO1C variants were detected in peaks on homozygosity mapping.Both variants are missense alterations resulting in a positively charged amino acid, arginine or lysine, substituted by a neutral amino acid.Evolutionary conservation, pathogenicity prediction by in-silico programs and 3D structure prediction programs, and position within protein domain structures, support the pathogenic role of these MYO1C variants.The majority of causative SRNS genes exhibit high level expression in glomerular podocytes, strongly suggesting podocytes to be the main site of injury in NS and underscoring its crucial role in upholding the ltration barrier.MYO1C is predominantly expressed in podocytes as evidenced by single-cell RNA sequencing datasets [26] (Supp Fig. 1).This supports a potential pathogenic role of MYO1C in SRNS.Moreover, 3D structural modeling revealed ion-ion interactions at both residue sites (K758, R98), which likely play an important role in stabilizing intradomain and ligand interactions.Modifying these charged residues to non-charged amino acids could potentially disrupt these ionic interactions, leading to a weakening of protein structure and its function.Published functional data further supports the pivotal role of MYO1C in podocyte physiology.MYO1C was shown to be a direct interactor of the podocyte slit diaphragm structural proteins, nephrin and neph1, and a mediator of their transport to the podocyte intercellular junction [7].Arif et al. demonstrated abnormal developmental phenotype of myo1c knockout zebra sh, characterized by pericardial edema and dilated renal tubules [6].Subsequent analysis of the glomerular ultrastructure in myo1c depleted zebra sh revealed absence of the slit diaphragm and abnormal podocyte morphology.Furthermore, their research highlighted the central role of Myo1cmediated regulation of TGF-b in the pathogenesis of podocyte injury, as evidenced by ndings from a Myo1c knockout mouse model [7].
Although the number of known genes associated with SRNS has increased from 27 to 69 over the last decade, the impact on the solve rate has been limited [Supp Table 1].This can be explained by the rarity of the newly discovered genes as shown in Fig. 3. Analyzing this chart reveals a discernible trend: there appears to be a correlation between the year a gene was discovered as being causative of SRNS, and its prevalence.Notably, the genes implicated latest as monogenic causes of SRNS are exceptionally rare, being present in only a few families.
Based on our prior experience, an initial discovery of a rare variant becomes the basis for identifying additional variants in the candidate gene.With su cient genetic evidence, usually when four different families with distinct alleles in the same gene are discovered, functional studies are, then, warranted to support the biological impact of disease-causing variants.Nevertheless, when lacking this threshold, we still nd it crucial to report these discoveries, once genetic evidence, computational methods, and existing literature strongly support a potential role of the gene-product in the pathogenesis of SRNS.For instance, KANK1 and CRB2, initially identi ed by our lab in one and four families respectively [28,29], have now been reported in seven and thirty-one families in Human Gene Mutation Database, indicating the evolving landscape of genetic discoveries in nephrotic syndrome.
In conclusion, we here present variants in MYO1C in two families, which may point to MYO1C as a potential new candidate gene for SRNS.Discovery of further families with NS who carry variants in the MYO1C gene, together with functional evidence to support their pathogenicity, are necessary to assess the role of MYO1C as a candidate gene for SRNS.

Figures Figure 1 WES 2
Figures

Table 1 is
available in the Supplementary Files section.