Primary Hyperoxaluria Type 2

Excerpt

Clinical characteristics: Primary hyperoxaluria type 2 (PH2), caused by deficiency of the enzyme glyoxylate reductase/hydroxypyruvate reductase (GR/HPR), is characterized by recurrent nephrolithiasis (deposition of calcium oxalate in the renal pelvis / urinary tract), nephrocalcinosis (deposition of calcium oxalate in the renal parenchyma), and end-stage kidney disease (ESKD). After ESKD, oxalosis (widespread tissue deposition of calcium oxalate) usually develops. Symptom onset is typically in childhood.

Diagnosis/testing: The diagnosis of PH2 is established in a proband by identification of biallelic pathogenic variants in GRHPR by molecular genetic testing. If no pathogenic variants or only one pathogenic variant is identified by molecular genetic testing, identification of reduced glyoxylate reductase enzyme activity on liver biopsy can establish the diagnosis of PH2.

Management: Treatment of manifestations: Reduction of urinary calcium oxalate supersaturation through adequate daily fluid intake and treatment with inhibitors of calcium oxalate crystallization (orthophosphate, potassium citrate, and magnesium); temporary intensive dialysis for ESKD, followed by transplantation.

Surveillance: Biannual assessment of kidney function, urinalysis with measurements of urine oxalate excretion (using 24-hour collection if easy to facilitate or spot urine oxalate to creatinine ratio), and calcium oxalate saturation if available, blood pressure, and full blood count including hematocrit; assessment of kidney stone burden every six to 12 months by urinary tract imaging (renal ultrasound or CT); assessment of cardiac, skin, bone, joint, eye, thyroid, and hematologic involvement annually after progression to ESKD.

Agents/circumstances to avoid: Dehydration. Ascorbate (vitamin C) ingestion and foods rich in oxalate (chocolate, rhubarb, and star fruit) may cause additional minimal increase in urinary oxalate levels in select individuals; excess should be discouraged; high salt (sodium) diet should be discouraged; excessive stone interventions with extracorporal shock wave lithotripsy.

Evaluation of relatives at risk: For asymptomatic at-risk relatives offer urine analysis and, if indicated by the results of urine analysis, molecular genetic testing (if the pathogenic variants in the family are known) so that early diagnosis can inform treatment.

Genetic counseling: PH2 is inherited in an autosomal recessive manner. Each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk family members and prenatal testing for a pregnancy at increased risk are possible if the pathogenic variants in the family are known.