Understanding SLC34A3 Gene Transmission: Why Kidney Stones Occur Alongside Bone Loss

Understanding SLC34A3 Gene Transmission: Why Kidney Stones Occur Alongside Bone Loss

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare disorder caused by mutations in the SLC34A3 gene. This gene encodes the sodium-phosphate co-transporter NaPi-IIc in the kidney’s proximal tubule. When NaPi-IIc function is impaired, phosphate is lost in urine, leading to low blood phosphate levels, weakened bones, and unexpectedly, high urinary calcium that can form kidney stones. Understanding how SLC34A3 mutations drive both bone loss and stone formation helps guide diagnosis and treatment.

Inheritance Pattern of SLC34A3 Mutations

• Autosomal recessive transmission

• Affected individuals inherit two mutated copies of SLC34A3 (one from each parent).
• Parents are typically healthy carriers, each with one mutated and one normal gene.
  • Carrier frequency and family risk
• If both parents are carriers, each child has a 25% chance of being affected, 50% chance of being a carrier, and 25% chance of having two normal copies.
• Siblings of an affected person may also carry one or two mutated copies; genetic counseling is recommended.

How SLC34A3 Mutations Cause HHRH

• NaPi-IIc dysfunction

• Normal role: reabsorbs 20–30% of filtered phosphate in the proximal tubule.
• Mutation: reduces transporter activity or expression, causing phosphate to spill into urine.
  • Consequences of phosphate wasting
• Hypophosphatemia (low serum phosphate) impairs bone mineralization.
• The body responds by increasing production of 1,25-dihydroxyvitamin D (calcitriol), enhancing intestinal calcium absorption.
  • Lab findings in HHRH
• Low serum phosphate, high urinary phosphate.
• Elevated serum calcitriol, normal or low parathyroid hormone (PTH).
• Hypercalciuria (high urinary calcium) despite normal serum calcium.

Link Between Phosphate Loss, Bone Health, and Kidney Stones

1. Bone Loss and Rickets/Osteomalacia
   • Phosphate is essential for hydroxyapatite formation in bone.
   • Chronic hypophosphatemia leads to soft, poorly mineralized bone—rickets in children, osteomalacia in adults.
   • Symptoms include bone pain, delayed growth, bowed legs, muscle weakness.

2. Hypercalciuria and Stone Formation
   • Elevated calcitriol boosts calcium absorption from the gut.
   • Extra calcium filtered by the kidneys exceeds reabsorptive capacity, leading to hypercalciuria.
   • High urinary calcium can precipitate with oxalate or phosphate, forming kidney stones (nephrolithiasis) or calcium deposits in the kidney tissue (nephrocalcinosis).

Clinical Presentation of HHRH

Children and adults with HHRH may show:

• Skeletal features

• Bowing of legs, delayed walking, short stature (in kids)
• Bone pain in hips, knees, or lower back
• Fractures with minimal trauma
  • Renal and urinary signs
• Recurrent kidney stones or flank pain
• Blood in urine (hematuria)
• Possible urinary tract infections
  • Laboratory abnormalities
• Persistent hypophosphatemia
• Elevated calcitriol levels
• Hypercalciuria on 24-hour urine collection
• Normal to low PTH, normal serum calcium

Diagnosing HHRH

Accurate diagnosis combines biochemical tests, imaging, and genetics:

• Blood tests

• Serum phosphate (low)
• Calcium (normal)
• Calcitriol (high)
• PTH (normal or low)
  • Urine studies
• 24-hour phosphate excretion (high)
• 24-hour calcium excretion (high)
  • Imaging
• X-rays: signs of rickets or osteomalacia (e.g., widened growth plates, Looser zones).
• Ultrasound or CT: detect kidney stones or nephrocalcinosis.
  • Genetic testing
• Sequencing of SLC34A3 confirms pathogenic variants.
• Helpful for family counseling and carrier detection.

Managing HHRH: Balancing Bone and Kidney Health

Treatment aims to correct phosphate levels, improve bone mineralization, and reduce stone risk:

• Oral phosphate supplements

• Primary therapy to raise serum phosphate.
• Given multiple times per day with meals.
  • Vitamin D considerations
• Active vitamin D (calcitriol) is typically not needed, since endogenous production is already high.
• Avoid excessive vitamin D to prevent worsening hypercalciuria.
  • Thiazide diuretics
• Reduce urinary calcium excretion.
• May help prevent stone formation.
  • Dietary guidance
• Moderate calcium intake—meet but do not exceed daily needs.
• Adequate hydration to dilute urine and prevent crystals.
  • Monitoring
• Regular blood tests (phosphate, calcium)
• Periodic urine studies (calcium excretion)
• Imaging as needed for stones or nephrocalcinosis
• Bone density assessments in adults

Prognosis and Long-Term Care

With early diagnosis and proper management, many individuals with HHRH can achieve improved bone health and fewer kidney stones. Long-term follow-up by a metabolic bone or kidney specialist is important to:

• Adjust therapy based on growth (in children) and lab results
• Monitor for complications such as nephrocalcinosis or secondary hyperparathyroidism
• Address quality-of-life issues, including pain management and physical function

Next Steps if You Have Symptoms

If you experience unexplained bone pain, muscle weakness, recurrent kidney stones, or lab findings of low phosphate with high urinary calcium, consider a free, online symptom check, using the doctor approved Ubie Symptom Checker. This tool can guide you through common and rare causes of symptoms and help you decide what to discuss with your healthcare provider.

When to Speak to a Doctor

Although HHRH is rare, symptoms like severe bone pain, difficulty walking, or passing kidney stones warrant prompt medical evaluation. Always speak to a doctor about anything that could be life-threatening or serious. Early recognition and targeted treatment can greatly improve outcomes for hereditary hypophosphatemic rickets with hypercalciuria (HHRH).