Multidisciplinary approaches for elucidating genetics and molecular pathogenesis of urinary tract malformations

Advances in clinical diagnostics and molecular tools have improved our understanding of the genetically heterogeneous causes underlying congenital anomalies of kidney and urinary tract (CAKUT). However, despite a sharp incline of CAKUT reports in the literature within the past 2 decades, there remains a plateau in the genetic diagnostic yield that is disproportionate to the accelerated ability to generate robust genome-wide data. Explanations for this observation include (i) diverse inheritance patterns with incomplete penetrance and variable expressivity, (ii) rarity of single-gene drivers such that large sample sizes are required to meet the burden of proof, and (iii) multigene interactions that might produce either intra- (e.g., copy number variants) or inter- (e.g., effects in trans) locus effects. These challenges present an opportunity for the community to implement innovative genetic and molecular avenues to explain the missing heritability and to better elucidate the mechanisms that underscore CAKUT. Here, we review recent multidisciplinary approaches at the intersection of genetics, genomics, in vivo modeling, and in vitro systems toward refining a blueprint for overcoming the diagnostic hurdles that are pervasive in urinary tract malformation cohorts. These approaches will not only benefit clinical management by reducing age at molecular diagnosis and prompting early evaluation for comorbid features but will also serve as a springboard for therapeutic development

Whole genome homozygosity mapping results.

<p>A red peak reaching a 1.0 max score of statistical significance depicts a homozygous region on chromosome 19. The region coincides with the 0.49 Mbp locus identified using two-point linkage analysis and is bracketed by recombination spots (indicated in red). The identified haploblock fulfills the zygosity criterion by displaying homozygosity in affected animals (indicated in red) and heterozygosity in unaffected carriers (indicated in green).</p

Sanger sequencing confirmation of the disease segregating variant identified in the linked region.

<p>Candidate region identified using linkage analysis in PACG pedigree animals (left). Sequence chromatograms from a carrier (C), confirms heterozygous genotype whereas affected animals (A) display a homozygous state of variant identified in <i>NEB</i> (g.5588214 A->G) (Right).</p

A Fisher exact contingency table of genotypes observed in a confirmatory animal cohort for the <i>RIF1</i> variant (g.55723957 C->T).

<p>Forty-four additional unaffected and affected Basset Hounds were selected for confirmatory sequencing of the <i>RIF1</i> variant (g.55723957 C->T). The observed total of individuals and percentage of individuals displaying a specific genotype is shown for each cell respectively. The two-tailed P value is 0.57 (Fisher Exact Probability Test)</p><p>A Fisher exact contingency table of genotypes observed in a confirmatory animal cohort for the <i>RIF1</i> variant (g.55723957 C->T).</p

A Fisher exact contingency table of genotypes observed in a confirmatory animal cohort for the <i>NEB</i> variant (g.55885214 A->G).

<p>Forty-four additional unaffected and affected Basset Hounds were selected for confirmatory sequencing of the <i>NEB</i> variant (g.55885214 A->G). The observed total of individuals and percentage of individuals displaying a specific genotype is shown for each cell respectively.</p><p>The two-tailed P value is 0.00034 (Fisher Exact Probability Test)</p><p>A Fisher exact contingency table of genotypes observed in a confirmatory animal cohort for the <i>NEB</i> variant (g.55885214 A->G).</p

Alignment of the amino acid sequence of exon 48 in Nebulin in several vertebrate species.

<p>The Lysine (K) residue (highlighted in blue) at position p.2051 is conserved among 23 vertebrate species.</p

Basset Hound Pedigree used in this study.

<p>The affected Basset 5a in the second generation was duplicated twice (5b and 5c) in order to break two otherwise computationally confounding breeding loops. Genotypes of typed markers within region uncovered following two-point linkage analysis are shown. Shading indicates the transmission pattern of heterozygous parental haplotypes to affected, homozygous offspring. Additional patterns of shading including diagonal lines indicate variation to the same haplotype identified in the other pedigree members. Complete concordance of homozygous haplotype inheritance with the disease phenotype is observed in all affected animals.</p

Genome-wide linkage analysis of SNP genotype data.

<p>Using two-point linkage analysis, a maximum LOD_two-point score of 3.07 was achieved for a 0.49 Mb locus on chromosome 19. The red line of statistical significance indicates LOD scores values > 3 (A). Using multipoint linkage analysis, an increased maximum LOD_multipoint score of 3.24 was achieved for a locus mapped to the same location (B). A schematic view of the maximum LOD scores achieved across chromosome 19. The statistically significant locus is located at the distal end of chromosome 19 (Chr19: 54,949,124–56,765,346) and spans 1.82 Mbp (C).</p

De novo TRIM8 variants impair its protein localization to nuclear bodies and cause developmental delay, epilepsy, and focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is the main pathology underlying steroid-resistant nephrotic syndrome (SRNS) and a leading cause of chronic kidney disease. Monogenic forms of pediatric SRNS are predominantly caused by recessive mutations, while the contribution of de novo variants (DNVs) to this trait is poorly understood. Using exome sequencing (ES) in a proband with FSGS/SRNS, developmental delay, and epilepsy, we discovered a nonsense DNV in TRIM8, which encodes the E3 ubiquitin ligase tripartite motif containing 8. To establish whether TRIM8 variants represent a cause of FSGS, we aggregated exome/genome-sequencing data for 2,501 pediatric FSGS/SRNS-affected individuals and 48,556 control subjects, detecting eight heterozygous TRIM8 truncating variants in affected subjects but none in control subjects (p = 3.28 × 10-11). In all six cases with available parental DNA, we demonstrated de novo inheritance (p = 2.21 × 10-15). Reverse phenotyping revealed neurodevelopmental disease in all eight families. We next analyzed ES from 9,067 individuals with epilepsy, yielding three additional families with truncating TRIM8 variants. Clinical review revealed FSGS in all. All TRIM8 variants cause protein truncation clustering within the last exon between residues 390 and 487 of the 551 amino acid protein, indicating a correlation between this syndrome and loss of the TRIM8 C-terminal region. Wild-type TRIM8 overexpressed in immortalized human podocytes and neuronal cells localized to nuclear bodies, while constructs harboring patient-specific variants mislocalized diffusely to the nucleoplasm. Co-localization studies demonstrated that Gemini and Cajal bodies frequently abut a TRIM8 nuclear body. Truncating TRIM8 DNVs cause a neuro-renal syndrome via aberrant TRIM8 localization, implicating nuclear bodies in FSGS and developmental brain disease.</p