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.

MO036: Detection of copy number variations by single-nucleotide-polymorphism array in patients with end-stage renal disease of unknown etiology

BACKGROUND AND AIMS

Establishing the genetic basis of renal disease is pivotal to improve diagnostic and treatment strategies. Pathogenic copy number variations (CNVs) caused by chromosomal imbalances contribute significantly to the pediatric renal disease burden, especially in children with congenital anomalies of the kidney and urinary tract (CAKUT). The role of CNVs in chronic kidney disease of unknown etiology (uCKD) is scarcely investigated, although this group comprises about 20% of adults with end-stage renal disease (ESRD). We screened for causal CNVs in a preliminary sample of patients with ESRD from unknown etiology.

METHOD

We recruited 94 patients registered with ESRD ≤age 50 from uCKD or hypertension from the Danish Nephrology Registry. Medical records and previous kidney biopsies were initially reviewed, and patients were excluded if a specific cause of kidney disease other than hypertension was identified (n = 35). We performed genome-wide single-nucleotide-polymorphism (SNP) array in 59 patients to identify copy number gains and losses. All CNVs were systematically evaluated as possible causes of kidney disease and broadly classified into three main categories: ‘Normal’, ‘Pathogenic’ or ‘Variant of uncertain significance.’

RESULTS

The SNP array results were normal in 57 patients (96.6%). In the remaining two patients (3.4%), we detected pathogenic CNVs with incomplete penetrance in hot spot loci associated with CAKUT: One male patient with ESRD at age 18 was a heterozygous carrier of a 545 kb microdeletion on chromosome 16p11.2 (29.63 Mb to 30.18 Mb). He had left-side renal agenesis and a hypoplastic right kidney at presentation, but no history of developmental delay or dysmorphic features. The microdeletion was considered likely causal, because it included the CAKUT driver gene TBX6 similar to other heterozygous carriers of 16p11.2 microdeletions with isolated renal agenesis or dysplasia. A second patient with ESRD at age 49 had 3.66 Mb duplication on chromosome 1q21.1 (145.43 Mb to 149.09 Mb) in heterozygous form. The patient had no reported CAKUT or other features associated with CNVs in the 1q21.1 loci, and the duplication was therefore deemed unrelated to his renal disease.

CONCLUSION

Disease-causing CNVs are not frequent in this limited sample of patients with ESRD from unknown etiology. CNV analysis by SNP-array may be most effective in patients with uCKD combined with CAKUT features.

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.

MO175TROPONIN CUT-OFFS FOR ACUTE MYOCARDIAL INFARCTION IN PATIENTS WITH IMPAIRED RENAL FUNCTION - A SYSTEMATIC REVIEW AND META-ANALYSIS

Background and Aims

Background

Identifying acute myocardial infarction in patients with renal disease is notoriously difficult due to atypical presentation and chronically elevated troponin.

Aim

To generate an optimized troponin cut-off value for patients with impaired renal function and acute myocardial infarction via meta-analysis.

Method

Two investigators screened 2,580 publications from Medline, Embase, Pubmed, Web of Science and Cochrane library. Only studies that investigated alternative cut-offs according to renal impairment were included.

15 articles fulfilled the inclusion criteria and results were included in a meta-analysis. Study characteristics and cut-off values were extracted. Study quality and risk of bias were assessed by using QUADAS-2 score. Six studies were included in the meta-analysis. To calculate the optimal cut off value in accordance to AUC for troponin T and troponin I in patients with renal impairment a bivariat mixed effect model on the sensitivity and specificity transformed by way of the inverse probit function similar to the model implemented in the R-package diagmeta was employed.

Results

Review: There was a considerable diversity in study design, study population and endpoint definition. The cut-off value for patients on peritoneal dialyses was twice as high (144 ng/L) when compared to patients on hemodialysis (75 ng/l). Asian studies suggested a substantially higher troponin cut-off when compared to European and American studies. The risk of bias was low in the analyzed studies, yet several studies were considered to have a low applicability.

Meta-analysis

Cut-off value for troponin T in patients not in dialysis with eGFR <60 ml/min/1.73m2, a troponin T value of 47.89 ng (23.95; 71.83) was found. In patients on dialysis a troponin T value of 239.75 ng/l ( 69.27 ; 410.23) was demonstrated. The 99th percentile of the upper reference limit for troponin T was 14 ng/l. Cut-off value for troponin I: In patients not in dialysis with eGFR < 60 ml/min/1.73m² a troponin I value of 42.45 ng/l ( 33.83 ; 51.08 ) was demonstrated. The 99th percentile of the upper reference limit for troponin I ranged from 9-42 ng/l depending on the assays used. Troponin I cut-off for patients in dialysis could not be calculated due to limited data.

Conclusion

The new cut-off values could help to identify patients whose troponin suggests acute myocardial infarction rather than renal function related troponin elevation. The meta-analysis is based on only six studies in total. Further subdivision according to eGFR would be desirable in order to optimize troponin cut-off values especially for dialysis patients. A differentiation on troponin cut-offs for HD and PD patients may yield further benefits. Asian studies suggested a substantially higher troponin cut-off when compared to European and American studies. The factors behind these findings may be worth investigating.

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.