Genetic assessment in primary hyperoxaluria: why it matters.

Accurate diagnosis of primary hyperoxaluria (PH) has important therapeutic consequences. Since biochemical assessment can be unreliable, genetic testing is a crucial diagnostic tool for patients with PH to define the disease type. Patients with PH type 1 (PH1) have a worse prognosis than those with other PH types, despite the same extent of oxalate excretion. The relation between genotype and clinical phenotype in PH1 is extremely heterogeneous with respect to age of first symptoms and development of kidney failure. Some mutations are significantly linked to pyridoxine-sensitivity in PH1, such as homozygosity for p.G170R and p.F152I combined with a common polymorphism. Although patients with these mutations display on average better outcomes, they may also present with CKD stage 5 in infancy. In vitro studies suggest pyridoxine-sensitivity for some other mutations, but confirmatory clinical data are lacking (p.G47R, p.G161R, p.I56N/major allele) or scarce (p.I244T). These studies also suggest that other vitamin B6 derivatives than pyridoxine may be more effective and should be a focus for clinical testing. PH patients displaying the same mutation, even within one family, may have completely different clinical outcomes. This discordance may be caused by environmental or genetic factors that are unrelated to the effect of the causative mutation(s). No relation between genotype and clinical or biochemical phenotypes have been found so far in PH types 2 and 3. This manuscript reviews the current knowledge on the genetic background of the three types of primary hyperoxaluria and its impact on clinical management, including prenatal diagnosis.

Biallelic TMEM260 variants cause truncus arteriosus, with or without renal defects.

Only two families have been reported with biallelic TMEM260 variants segregating with structural heart defects and renal anomalies syndrome (SHDRA). With a combination of genome, exome sequencing and RNA studies, we identified eight individuals from five families with biallelic TMEM260 variants. Variants included one multi-exon deletion, four nonsense/frameshifts, two splicing changes and one missense change. Together with the published cases, analysis of clinical data revealed ventricular septal defects (12/12), mostly secondary to truncus arteriosus (10/12), elevated creatinine levels (6/12), horse-shoe kidneys (1/12) and renal cysts (1/12) in patients. Three pregnancies were terminated on detection of severe congenital anomalies. Six patients died between the ages of 6 weeks and 5 years. Using a range of stringencies, carrier frequency for SHDRA was estimated at 0.0007-0.007 across ancestries. In conclusion, this study confirms the genetic basis of SHDRA, expands its known mutational spectrum and clarifies its clinical features. We demonstrate that SHDRA is a severe condition associated with substantial mortality in early childhood and characterised by congenital cardiac malformations with a variable renal phenotype.

Positive trends in paediatric renal biopsy service provision in the UK: a national survey and re-audit of paediatric renal biopsy practice.

BACKGROUND: Paediatric renal biopsy standards introduced in the UK in 2010 were intended to reduce variation and improve practice. A concurrent national drive was aimed at building robust paediatric nephrology networks to ensure services cater for the needs of the family and minimise time away from home. We aimed to identify current national practice since these changes on behalf of the British Association for Paediatric Nephrology. METHODS: All UK paediatric nephrology centres were invited to complete a survey of their biopsy practice, including advance preparation. From 1 January to 30 June 2012, a national prospective audit of renal biopsies was undertaken at participating centres comparing practice with the British Association for Paediatric Nephrology (BAPN) standards and audit results from 2005. RESULTS: Survey results from 11 centres demonstrated increased use of pre-procedure information leaflets (63.6 % vs 45.5 %, P = 0.39) and play preparation (90.9 % vs 9.1 %, P = 0.0001). Audit of 331 biopsies showed a move towards day-case procedures (49.5 % vs 32.9 %, P = 0.17) and reduced major complications (4.5 % vs 10.4 %, P = 0.002). Biopsies with 18-gauge needles had significantly higher mean pass rates (3.2 vs 2.3, P = 0.0008) and major complications (15.3 % vs 3.3 %, P = 0.0015) compared with 16-gauge needles. CONCLUSIONS: Percutaneous renal biopsy remains a safe procedure in children, thus improving family-centered service provision in the UK.

Clinical practice recommendations for primary hyperoxaluria: an expert consensus statement from ERKNet and OxalEurope.

Primary hyperoxaluria (PH) is an inherited disorder that results from the overproduction of endogenous oxalate, leading to recurrent kidney stones, nephrocalcinosis and eventually kidney failure; the subsequent storage of oxalate can cause life-threatening systemic disease. Diagnosis of PH is often delayed or missed owing to its rarity, variable clinical expression and other diagnostic challenges. Management of patients with PH and kidney failure is also extremely challenging. However, in the past few years, several new developments, including new outcome data from patients with infantile oxalosis, from transplanted patients with type 1 PH (PH1) and from patients with the rarer PH types 2 and 3, have emerged. In addition, two promising therapies based on RNA interference have been introduced. These developments warrant an update of existing guidelines on PH, based on new evidence and on a broad consensus. In response to this need, a consensus development core group, comprising (paediatric) nephrologists, (paediatric) urologists, biochemists and geneticists from OxalEurope and the European Rare Kidney Disease Reference Network (ERKNet), formulated and graded statements relating to the management of PH on the basis of existing evidence. Consensus was reached following review of the recommendations by representatives of OxalEurope, ESPN, ERKNet and ERA, resulting in 48 practical statements relating to the diagnosis and management of PH, including consideration of conventional therapy (conservative therapy, dialysis and transplantation), new therapies and recommendations for patient follow-up.

Treatment of primary hyperoxaluria type 1.

Supportive treatment for primary hyperoxaluria type 1 (PH1) focuses on high fluid intake and crystallization inhibitors. A subset of patients with specific PH1 genotypes (c.508G>A and c.454T>A) will respond to pyridoxine, defined as a >30% reduction in urinary oxalate excretion. Response to pyridoxine is variable and in some patients, urinary oxalate may normalize. The first focused treatment for PH1 using an RNA interference agent to reduce urinary oxalate was approved in 2020, and such therapies may significantly alter treatment approaches and long-term outcomes in PH1. Currently PH1 often presents with kidney function impairment and frequently results in end-stage kidney disease (ESKD). With kidney dysfunction, urinary oxalate clearance decreases and multisystem deposition of oxalate (oxalosis) occurs, commonly in bones, eyes, heart and skin. Once plasma oxalate levels exceed 30 µmol/L, aggressive haemodialysis is indicated to prevent oxalosis, even if the glomerular filtration rate (GFR) remains better than for typical dialysis initiation. Peritoneal dialysis alone does not achieve the needed oxalate clearance. Dialysis is a bridge to future transplantation. Liver transplantation restores hepatic alanine-glyoxylate transaminase enzyme activity, allowing glyoxylate detoxification and preventing further oxalosis. The native liver must be removed as part of this process to avoid ongoing pathologic oxalate production. The timing and type of liver transplantation are dependent on pyridoxine sensitivity, age, weight, residual GFR and evidence of systemic oxalate deposition in extrarenal organs. Liver transplant can be isolated or combined with kidney transplantation in a sequential or simultaneous fashion. Isolated kidney transplantation is generally reserved for pyridoxine-sensitive patients only. Although liver transplantation is curative for PH1 and kidney transplantation treats ESKD, ensuing necessary immunosuppression and potential allograft dysfunction impart significant long-term risks.

Improved Outcome of Infantile Oxalosis Over Time in Europe: Data From the OxalEurope Registry.

Introduction: Infantile oxalosis is the most severe form of primary hyperoxaluria type 1 (PH1), with onset of end-stage kidney disease (ESKD) during infancy. We aimed to analyze the outcome of these patients as our current understanding is limited owing to a paucity of reports. Methods: A retrospective registry study was conducted using data from the OxalEurope registry. All PH1 patients with ESKD onset at age <1 year were analyzed. Results: We identified 95 patients born between 1980 and 2018 with infantile oxalosis. Median (interquartile range [IQR]) age at ESKD was 0.4 (0.3-0.5) year. There were 4 patients diagnosed by family screening who developed ESKD despite early diagnosis. There were 11 patients who had biallelic missense mutations associated with vitamin B6 responsiveness. Of 89 patients, 27 (30%) died at a median age of 1.4 (0.6-2.0) years (5-year patient survival of 69%). Systemic oxalosis was described in 54 of 56 screened patients (96%). First transplantation was performed at a median age of 1.7 (1.3-2.9) years. In 42 cases, this procedure was a combined liver-kidney transplantation (LKTx), and in 23 cases, liver transplantations (LTx) was part of a sequential procedure. Survival rates of both strategies were similar. Patient survival was significantly higher in patients born after 2000. Intrafamilial phenotypic variability was present in 14 families of patients with infantile oxalosis. Conclusion: Nearly all screened patients with infantile oxalosis developed systemic disease. Mortality is still high but has significantly improved over time and might further improve under new therapies. The intrafamilial phenotypic variability warrants further investigation.

Guidelines for Genetic Testing and Management of Alport Syndrome.

Genetic testing for pathogenic COL4A3-5 variants is usually undertaken to investigate the cause of persistent hematuria, especially with a family history of hematuria or kidney function impairment. Alport syndrome experts now advocate genetic testing for persistent hematuria, even when a heterozygous pathogenic COL4A3 or COL4A4 is suspected, and cascade testing of their first-degree family members because of their risk of impaired kidney function. The experts recommend too that COL4A3 or COL4A4 heterozygotes do not act as kidney donors. Testing for variants in the COL4A3-COL4A5 genes should also be performed for persistent proteinuria and steroid-resistant nephrotic syndrome due to suspected inherited FSGS and for familial IgA glomerulonephritis and kidney failure of unknown cause.

Consensus statement on standards and guidelines for the molecular diagnostics of Alport syndrome: refining the ACMG criteria.

The recent Chandos House meeting of the Alport Variant Collaborative extended the indications for screening for pathogenic variants in the COL4A5, COL4A3 and COL4A4 genes beyond the classical Alport phenotype (haematuria, renal failure; family history of haematuria or renal failure) to include persistent proteinuria, steroid-resistant nephrotic syndrome, focal and segmental glomerulosclerosis (FSGS), familial IgA glomerulonephritis and end-stage kidney failure without an obvious cause. The meeting refined the ACMG criteria for variant assessment for the Alport genes (COL4A3-5). It identified 'mutational hotspots' (PM1) in the collagen IV α5, α3 and α4 chains including position 1 Glycine residues in the Gly-X-Y repeats in the intermediate collagenous domains; and Cysteine residues in the carboxy non-collagenous domain (PP3). It considered that 'well-established' functional assays (PS3, BS3) were still mainly research tools but sequencing and minigene assays were commonly used to confirm splicing variants. It was not possible to define the Minor Allele Frequency (MAF) threshold above which variants were considered Benign (BA1, BS1), because of the different modes of inheritances of Alport syndrome, and the occurrence of hypomorphic variants (often Glycine adjacent to a non-collagenous interruption) and local founder effects. Heterozygous COL4A3 and COL4A4 variants were common 'incidental' findings also present in normal reference databases. The recognition and interpretation of hypomorphic variants in the COL4A3-COL4A5 genes remains a challenge.

Compound heterozygous RMND1 gene variants associated with chronic kidney disease, dilated cardiomyopathy and neurological involvement: a case report.

BACKGROUND: Nuclear gene mutations are being increasingly recognised as causes of mitochondrial disease. The nuclear gene RMND1 has recently been implicated in mitochondrial disease, but the spectrum of pathogenic variants and associated phenotype for this gene, has not been fully elucidated. CASE PRESENTATION: An 11-month-old boy presented with renal impairment associated with a truncal ataxia, bilateral sensorineural hearing loss, hypotonia, delayed visual maturation and global developmental delay. Over a 9-year period, he progressed to chronic kidney disease stage V and developed a dilated cardiomyopathy. Abnormalities in renal and muscle biopsy as well as cytochrome c oxidase activity prompted genetic testing. After exclusion of mitochondrial DNA defects, nuclear genetic studies identified compound heterozygous RMND1 (c.713A>G, p. Asn238Ser and c.565C>T, p.Gln189*) variants. CONCLUSION: We report RMND1 gene variants associated with end stage renal failure, dilated cardiomyopathy, deafness and neurological involvement due to mitochondrial disease. This case expands current knowledge of mitochondrial disease secondary to mutation of the RMND1 gene by further delineating renal manifestations including histopathology. To our knowledge dilated cardiomyopathy has not been reported with renal failure in mitochondrial disease due to mutations of RMND1. The presence of this complication was important in this case as it precluded renal transplantation.