The role of genomic disorders in chronic kidney failure of undetermined aetiology ≤50 years.

Background: Genomic disorders caused by copy number variations (CNVs) are prevalent in patients with kidney disease; however, their contribution to chronic kidney failure (KF) of undetermined aetiology (uKF) is unclear. We screened patients with uKF aged 50 years or younger to establish the prevalence of causative CNVs. Methods: We enrolled patients with an onset of KF ≤50 years from suspected undetermined aetiology for initial review of medical records to exclude patients with clear-cut clinical or histopathological kidney diagnoses or patients with already established genetic kidney diseases. Next, we performed single nucleotide polymorphism (SNP) array-based CNV screening. All the detected CNVs were systematically classified and evaluated as possible causes of the patient's kidney disease. Patients with CNVs not explaining the kidney phenotype were additionally screened for causal variants in 540 genes using whole-genome sequencing. Results: We enrolled 172 patients, of whom 123 underwent SNP-array. Pathogenic CNVs corresponding to known genomic disorders were identified in 12 patients (9.8%). The identified genomic disorders provided a causative kidney diagnosis in three patients, all of whom had reached KF by age 18 years. The remaining nine patients had CNVs with unclear kidney disease causality. Subsequently, whole-genome sequencing provided a causative genetic diagnosis in an additional four patients, including two diagnostic sequence variants unrelated to the detected CNVs. Conclusions: Genomic disorders were prevalent in this cohort with uKF, and causative CNVs were identified in 5 of 123 patients. Further studies combining the analysis of CNVs and sequence variants are needed to clarify the causal role of genomic disorders in kidney disease.

Reverse Phenotypes of Patients with Genetically Confirmed Liddle Syndrome.

BACKGROUND: Liddle syndrome was initially characterized by hypertension, hypokalemia, metabolic alkalosis, and suppressed plasma renin and aldosterone, resulting from gain-of-function variants in the epithelial Na + channel (ENaC). Efficient treatment with ENaC inhibitors is available, but the phenotypic spectrum of genetically confirmed Liddle syndrome is unknown, and some patients may remain undiagnosed and at risk of inefficient treatment. In this study, we used a reverse phenotyping approach to investigate the Liddle syndrome phenotypic spectrum and genotype-phenotype correlations. METHODS: Pubmed, Embase, Scopus, and the Human Gene Mutation Database were searched for articles reporting Liddle syndrome variants. The genetic variants were systematically classified to identify patients with genetically confirmed Liddle syndrome. We identified 62 articles describing 45 unique variants within 86 Liddle syndrome families, and phenotypic data were pooled for 268 patients with confirmed Liddle syndrome. RESULTS: The Liddle syndrome variants localized to exon 13 of SCNN1B and SCNN1G , disrupting the PPPxY motif critical for downregulating ENaC activity. Hypertension sensitive to ENaC inhibition was present in 97% of adults carrying Liddle syndrome variants while hypokalemia, metabolic alkalosis, and plasma renin and aldosterone suppression showed incomplete penetrance. In addition, 95% and 55% of patients had a family history of hypertension or cerebrovascular events, respectively. The genotype had minor phenotypic effects; however, probands compared with relatives showed significant phenotypic discrepancies consistent with selection bias for initial genetic screening. CONCLUSIONS: Patients with genetically confirmed Liddle syndrome displayed a phenotypic spectrum, with ENaC-sensitive hypertension and family history of hypertension being the most common features. The phenotype seemed independent of the specific gene or variant type involved.

Troponin Cut-Offs for Acute Myocardial Infarction in Patients with Impaired Renal Function-A Systematic Review and Meta-Analysis.

Identifying acute myocardial infarction in patients with renal disease is notoriously difficult, due to atypical presentation and chronically elevated troponin. The aim of this study was to identify a specific troponin T/troponin I cut-off value for diagnosis of acute myocardial infarction in patients with renal impairment via meta-analysis. Two investigators screened 2590 publications from MEDLINE, Embase, PubMed, Web of Science, and the Cochrane library. Only studies that investigated alternative cut-offs according to renal impairment were included. Fifteen articles fulfilled the inclusion criteria. Six studies were combined for meta-analysis. The manufacturer's upper reference level for troponin T is 14 ng/L. Based on the meta-analyses, cut-off values for troponin in patients with renal impairment with myocardial infarction was 42 ng/L for troponin I and 48 ng/L for troponin T. For patients on dialysis the troponin T cut-off is even higher at 239 ng/L. A troponin I cut-off value for dialysis patients could not be established due to lack of data. The 15 studies analyzed showed considerable diversity in study design, study population, and the definition of myocardial infarction. Further studies are needed to define a reliable troponin cut-off value for patients with kidney disease, especially in dialysis patients, and to allow necessary subanalysis.

Family History is Important to Identify Patients with Monogenic Causes of Adult-Onset Chronic Kidney Disease.

Monogenic causes of chronic kidney disease (CKD) are more prevalent in adults than previously thought, as causative gene variants are found in almost 10% of unselected patients with CKD. Even so, genetic testing in patients with adult-onset CKD is uncommon in clinical practice and the optimal criteria for patient selection remain unclear. A family history of kidney disease emerges as one marker associated with a high diagnostic yield of genetic testing. We present 3 cases of adult-onset CKD with underlying monogenic causes exemplifying different modes of inheritance. Case 1 is a 60-year-old male with slowly progressive CKD initially ascribed to hypertension and diabetes despite a family history with several affected first-degree relatives. A pathogenic MUC1 variant was found, and thus we identified the first Danish family of MUC1-associated autosomal dominant tubulointerstitial kidney disease. Case 2 is a 40-year-old female with nephrocalcinosis, nephrolithiasis, and unexplainable hypercalcemia consistent with vitamin D intoxication. The family history indicated autosomal recessive inheritance, and genetic testing revealed 2 pathogenic CYP24A1 variants in compound heterozygous form associated with idiopathic infantile hypercalcemia. Case 3 is a 50-year-old male with microscopic hematuria, proteinuria, and hearing loss. Electron microscopy of renal biopsy showed thin basal membrane syndrome, and the family history indicated X-linked inheritance. A novel missense variant in COL4A5 was identified, suggesting an atypical late-onset form of X-linked Alport syndrome. This case series illustrates the heterogeneous presentations of monogenic kidney disease in adults and emphasizes the importance of family history for initiating genetic testing to identify underlying monogenic causation. Moreover, we discuss the potential impact of genetic diagnostics on patient management and genetic family counseling.

Bacterial Peptide Display for the Selection of Novel Biotinylating Enzymes.

Biotin is an attractive post-translational modification of proteins that provides a powerful tag for the isolation and detection of protein. Enzymatic biotinylation by the E. coli biotin-protein ligase BirA is highly specific and allows for the biotinylation of target proteins in their native environment; however, the current usage of BirA mediated biotinylation requires the presence of a synthetic acceptor peptide (AP) in the target protein. Therefore, its application is limited to proteins that have been engineered to contain the AP. The purpose of the present protocol is to use the bacterial display of a peptide derived from an unmodified target protein to select for BirA variants that biotinylates the peptide. The system is based on a single plasmid that allows for the co-expression of BirA variants along with a scaffold for the peptide display on the bacterial surface. The protocol describes a detailed procedure for the incorporation of the target peptide into the display scaffold, creation of the BirA library, selection of active BirA variants and initial characterization of the isolated BirA variants. The method provides a highly effective selection system for the isolation of novel BirA variants that can be used for the further directed evolution of biotin-protein ligases that biotinylate a native protein in complex solutions.

A bacterial display system for effective selection of protein-biotin ligase BirA variants with novel peptide specificity.

Biotinylation creates a sensitive and specific tag for purification and detection of target proteins. The E. coli protein-biotin ligase BirA biotinylates a lysine within a synthetic biotin acceptor peptide (AP) and allow for specific tagging of proteins fused to the AP. The approach is not applicable to unmodified proteins, and we sought to develop an effective selection system that could form the basis for directed evolution of novel BirA variants with specificity towards unmodified proteins. The system was based on bacterial display of a target peptide sequence, which could be biotinylated by cytosolic BirA variants before being displayed on the surface. In a model selection, the bacterial display system accomplished >1.000.000 enrichment in a single selection step. A randomly mutated BirA library was used to identify novel variants. Bacteria displaying peptide sequences from 13 out of 14 tested proteins were strongly enriched after 3-5 selection rounds. Moreover, a clone selected for biotinylation of a C-terminal peptide from red-fluorescent protein TagRFP showed biotinylation of the native protein. Thus, active BirA variants with novel activity are effectively isolated with our bacterial display system and provides a basis for the development of BirA variants for site-selective biotinylation.