Electrolyte Disorders in Mitochondrial Cytopathies: A Systematic Review.

SIGNIFICANCE STATEMENT: Several recent studies identified mitochondrial mutations in patients with Gitelman or Fanconi syndrome. Mitochondrial cytopathies are generally not considered in the diagnostic workup of patients with electrolyte disorders. In this systematic review, we investigated the presence of electrolyte disorders in patients with mitochondrial cytopathies to determine the relevance of mitochondrial mutation screening in this population. Our analysis demonstrates that electrolyte disorders are commonly reported in mitochondrial cytopathies, often as presenting symptoms. Consequently, more clinical attention should be raised for mitochondrial disease as cause for disturbances in electrolyte homeostasis. Further prospective cohort studies are required to determine the exact prevalence of electrolyte disorders in mitochondrial cytopathies. BACKGROUND: Electrolyte reabsorption in the kidney has a high energy demand. Proximal and distal tubular epithelial cells have a high mitochondrial density for energy release. Recently, electrolyte disorders have been reported as the primary presentation of some mitochondrial cytopathies. However, the prevalence and the pathophysiology of electrolyte disturbances in mitochondrial disease are unknown. Therefore, we systematically investigated electrolyte disorders in patients with mitochondrial cytopathies. METHODS: We searched PubMed, Embase, and Google Scholar for articles on genetically confirmed mitochondrial disease in patients for whom at least one electrolyte is reported. Patients with a known second genetic anomaly were excluded. We evaluated 214 case series and reports (362 patients) as well as nine observational studies. Joanna Briggs Institute criteria were used to evaluate the quality of included studies. RESULTS: Of 362 reported patients, 289 had an electrolyte disorder, with it being the presenting or main symptom in 38 patients. The average number of different electrolyte abnormalities per patient ranged from 2.4 to 1.0, depending on genotype. Patients with mitochondrial DNA structural variants seemed most affected. Reported pathophysiologic mechanisms included renal tubulopathies and hormonal, gastrointestinal, and iatrogenic causes. CONCLUSIONS: Mitochondrial diseases should be considered in the evaluation of unexplained electrolyte disorders. Furthermore, clinicians should be aware of electrolyte abnormalities in patients with mitochondrial disease.

Long-Read Sequencing Identifies Novel Pathogenic Intronic Variants in Gitelman Syndrome.

BACKGROUND: Gitelman syndrome is a salt-losing tubulopathy characterized by hypokalemic alkalosis and hypomagnesemia. It is caused by homozygous recessive or compound heterozygous pathogenic variants in SLC12A3 , which encodes the Na + -Cl - cotransporter (NCC). In up to 10% of patients with Gitelman syndrome, current genetic techniques detect only one specific pathogenic variant. This study aimed to identify a second pathogenic variant in introns, splice sites, or promoters to increase the diagnostic yield. METHODS: Long-read sequencing of SLC12A3 was performed in 67 DNA samples from individuals with suspected Gitelman syndrome in whom a single likely pathogenic or pathogenic variant was previously detected. In addition, we sequenced DNA samples from 28 individuals with one variant of uncertain significance or no candidate variant. Midigene splice assays assessed the pathogenicity of novel intronic variants. RESULTS: A second likely pathogenic/pathogenic variant was identified in 45 (67%) patients. Those with two likely pathogenic/pathogenic variants had a more severe electrolyte phenotype than other patients. Of the 45 patients, 16 had intronic variants outside of canonic splice sites (nine variants, mostly deep intronic, six novel), whereas 29 patients had an exonic variant or canonic splice site variant. Midigene splice assays of the previously known c.1670-191C>T variant and intronic candidate variants demonstrated aberrant splicing patterns. CONCLUSION: Intronic pathogenic variants explain an important part of the missing heritability in Gitelman syndrome. Long-read sequencing should be considered in diagnostic workflows for Gitelman syndrome.

Higher SBP in female patients with mitochondrial disease.

BACKGROUND: Previous research suggests that hypertension is more prevalent among patients with mitochondrial diseases. Blood pressure (BP) is linearly related to increased cardiovascular risk, and this relationship is strongest for SBP; nevertheless, studies on SBP and DBP in mitochondrial diseases have not yet been performed. METHOD: In a retrospective case-control study design, BP in mitochondrial disease patients was compared with BP in a population cohort. Secondly, using multiple linear regression, we examined blood pressure differences in various genetic mitochondrial diseases. Lastly, we explored additional predictors of BP in a subgroup with the m.3243A > G variant. RESULTS: Two hundred and eighty-six genetically confirmed mitochondrial disease patients were included. One hundred and eighty of these patients carried the m.3243A>G mitochondrial DNA variant. SBP was 9 mmHg higher in female mitochondrial disease patients than in the general female population (95% CI: 4.4-13.3 mmHg, P  < 0.001), whereas male patients had similar BP compared with controls. BP was not significantly different in patients with m.8344A>G and m.8363G>A, a mtDNA deletion or a nuclear mutation compared with m.3243A>G patients. Higher SBP was a predictor for left ventricular hypertrophy in the m.3243A>G subgroup (P  = 0.04). CONCLUSION: Novel aspects of the role of mitochondrial dysfunction in blood pressure regulation are exposed in this study. Compared with the general population, female mitochondrial disease patients have a higher SBP. Left ventricular hypertrophy is more prevalent in patients with higher SBP. Clinicians should be aware of this to prevent hypertensive complications in mitochondrial disease patients.

Functional tests to guide management in an adult with loss of function of type-1 angiotensin II receptor.

BACKGROUND: Genetic loss of function of AGT (angiotensinogen), REN (renin), ACE (angiotensin-converting enzyme), or AGTR1 (type-1 angiotensin II receptor) leads to renal tubular dysgenesis (RTD). This syndrome is almost invariably lethal. Most surviving patients reach stage 5 chronic kidney disease at a young age. METHODS: Here, we report a 28-year-old male with a homozygous truncating mutation in AGTR1 (p.Arg216*), who survived the perinatal period with a mildly impaired kidney function. In contrast to classic RTD, kidney biopsy showed proximal tubules that were mostly normal. During the subsequent three decades, we observed evidence of both tubular dysfunction (hyperkalemia, metabolic acidosis, salt-wasting and a urinary concentrating defect) and glomerular dysfunction (reduced glomerular filtration rate, currently ~30 mL/min/1.73 m2, accompanied by proteinuria). To investigate the recurrent and severe hyperkalemia, we performed a patient-tailored functional test and showed that high doses of fludrocortisone induced renal potassium excretion by 155%. Furthermore, fludrocortisone lowered renal sodium excretion by 39%, which would have a mitigating effect on salt-wasting. In addition, urinary pH decreased in response to fludrocortisone. Opposite effects on urinary potassium and pH occurred with administration of amiloride, further supporting the notion that a collecting duct is present and able to react to fludrocortisone. CONCLUSIONS: This report provides living proof that even truncating loss-of-function mutations in AGTR1 are compatible with life and relatively good GFR and provides evidence for the prescription of fludrocortisone to treat hyperkalemia and salt-wasting in such patients.

Genetics of renovascular hypertension in children.

OBJECTIVE: In most cases of renovascular hypertension in children, the cause is unclear. The aim of this study was to investigate genetic variation as a factor in the development of renovascular hypertension in children. METHODS: In a cohort of 37 unrelated children from a single tertiary referral center, exome sequencing was performed. We assessed variants in recognized and suspected disease genes and searched for novel ones with a gene-based variant-burden analysis. RESULTS: In the majority of patients, exome sequencing could not identify causative variants. We found a pathogenic variant in a recognized associated disease gene in five patients (three pathogenic variants in NF1, one in ELN and a deletion of chromosome 7q11.23, consistent with Williams syndrome). In two other patients, (likely) pathogenic variants were found in putative renovascular hypertension genes (SMAD6 and GLA), with clinical implications for both. Ten additional patients carried variants of uncertain significance (VUS) in known (n = 4) or putative (n = 6) renovascular hypertension disease genes. Rare variant burden analysis yielded no further candidate genes. CONCLUSION: Genetic contributors, such as germline mutations in NF1, ELN, 7q11.23del were present in only 5 out of 37 (14%) children with renovascular hypertension. Twelve other children (32%) had potentially causal variants identified, including a pathogenic variant in SMAD6; a vasculopathy gene hitherto unknown to link with renovascular hypertension. Most importantly, our data show that exome sequencing can rarely identify the cause of renovascular hypertension in nonsyndromic children. We suggest that nongenetic factors or somatic genetic variation will play a more important role.

Polycystic Kidney Disease with Hyperinsulinemic Hypoglycemia Caused by a Promoter Mutation in Phosphomannomutase 2.

Hyperinsulinemic hypoglycemia (HI) and congenital polycystic kidney disease (PKD) are rare, genetically heterogeneous disorders. The co-occurrence of these disorders (HIPKD) in 17 children from 11 unrelated families suggested an unrecognized genetic disorder. Whole-genome linkage analysis in five informative families identified a single significant locus on chromosome 16p13.2 (logarithm of odds score 6.5). Sequencing of the coding regions of all linked genes failed to identify biallelic mutations. Instead, we found in all patients a promoter mutation (c.-167G>T) in the phosphomannomutase 2 gene (PMM2), either homozygous or in trans with PMM2 coding mutations. PMM2 encodes a key enzyme in N-glycosylation. Abnormal glycosylation has been associated with PKD, and we found that deglycosylation in cultured pancreatic ß cells altered insulin secretion. Recessive coding mutations in PMM2 cause congenital disorder of glycosylation type 1a (CDG1A), a devastating multisystem disorder with prominent neurologic involvement. Yet our patients did not exhibit the typical clinical or diagnostic features of CDG1A. In vitro, the PMM2 promoter mutation associated with decreased transcriptional activity in patient kidney cells and impaired binding of the transcription factor ZNF143. In silico analysis suggested an important role of ZNF143 for the formation of a chromatin loop including PMM2 We propose that the PMM2 promoter mutation alters tissue-specific chromatin loop formation, with consequent organ-specific deficiency of PMM2 leading to the restricted phenotype of HIPKD. Our findings extend the spectrum of genetic causes for both HI and PKD and provide insights into gene regulation and PMM2 pleiotropy.

Genetic causes of hypomagnesemia, a clinical overview.

Magnesium is essential to the proper functioning of numerous cellular processes. Magnesium ion (Mg2+) deficits, as reflected in hypomagnesemia, can cause neuromuscular irritability, seizures and cardiac arrhythmias. With normal Mg2+ intake, homeostasis is maintained primarily through the regulated reabsorption of Mg2+ by the thick ascending limb of Henle's loop and distal convoluted tubule of the kidney. Inadequate reabsorption results in renal Mg2+ wasting, as evidenced by an inappropriately high fractional Mg2+ excretion. Familial renal Mg2+ wasting is suggestive of a genetic cause, and subsequent studies in these hypomagnesemic families have revealed over a dozen genes directly or indirectly involved in Mg2+ transport. Those can be classified into four groups: hypercalciuric hypomagnesemias (encompassing mutations in CLDN16, CLDN19, CASR, CLCNKB), Gitelman-like hypomagnesemias (CLCNKB, SLC12A3, BSND, KCNJ10, FYXD2, HNF1B, PCBD1), mitochondrial hypomagnesemias (SARS2, MT-TI, Kearns-Sayre syndrome) and other hypomagnesemias (TRPM6, CNMM2, EGF, EGFR, KCNA1, FAM111A). Although identification of these genes has not yet changed treatment, which remains Mg2+ supplementation, it has contributed enormously to our understanding of Mg2+ transport and renal function. In this review, we discuss general mechanisms and symptoms of genetic causes of hypomagnesemia as well as the specific molecular mechanisms and clinical phenotypes associated with each syndrome.

P2X6 Knockout Mice Exhibit Normal Electrolyte Homeostasis.

ATP-mediated signaling is an important regulator of electrolyte transport in the kidney. The purinergic cation channel P2X6 has been previously localized to the distal convoluted tubule (DCT), a nephron segment important for Mg2+ and Na+ reabsorption, but its role in ion transport remains unknown. In this study, P2x6 knockout (P2x6-/-) mice were generated to investigate the role of P2X6 in renal electrolyte transport. The P2x6-/- animals displayed a normal phenotype and did not differ physiologically from wild type mice. Differences in serum concentration and 24-hrs urine excretion of Na+, K+, Mg2+ and Ca2+ were not detected between P2x6+/+, P2x6+/- and P2x6-/- mice. Quantitative PCR was applied to examine potential compensatory changes in renal expression levels of other P2x subunits and electrolyte transporters, including P2x1-5, P2x7, Trpm6, Ncc, Egf, Cldn16, Scnn1, Slc12a3, Slc41a1, Slc41a3, Cnnm2, Kcnj10 and Fxyd2. Additionally, protein levels of P2X2 and P2X4 were assessed in P2x6+/+ and P2x6-/- mouse kidneys. However, significant changes in expression were not detected. Furthermore, no compensatory changes in gene expression could be demonstrated in heart material isolated from P2x6-/- mice. Except for a significant (P<0.05) upregulation of P2x2 in the heart of P2x6-/- mice compared to the P2x6+/+ mice. Thus, our data suggests that purinergic signaling via P2X6 is not significantly involved in the regulation of renal electrolyte handling under normal physiological conditions.