Kidney Transplantation and Genetic Counseling

For many families living with FHHNC, kidney transplantation is a long-term option when advanced kidney failure occurs. While the journey to transplant can be complex and is not a simple “cure,” it offers a definitive way to resolve the severe mineral wasting that defines the condition ^[1][2].

The Role of Transplantation

In FHHNC, the native kidneys have a genetic defect in the “tight junctions” of their filtering tubes, which causes them to lose magnesium and calcium ^[1]. A transplant corrects these specific tubular defects because the “new” donor kidney comes with its own healthy set of CLDN16 or CLDN19 proteins. The defect generally does not recur in the new graft ^[1][2].

Once the new kidney is in place, biochemical improvements are often seen:

• Magnesium: Serum magnesium levels often stabilize ^[3][4].
• Calcium Balance: The excessive loss of calcium in the urine (hypercalciuria) usually improves ^[3].

However, a transplant involves significant trade-offs. It requires major surgery, carries risks of rejection, infection, and malignancy, and requires lifelong immunosuppressive medications. Notably, calcineurin inhibitors (common anti-rejection drugs) can themselves cause hypomagnesemia and other electrolyte disturbances. Therefore, stopping magnesium supplements is not guaranteed, and regular post-transplant lab monitoring remains vital. Furthermore, the transplant does not change the genetic makeup of the rest of the body; for those with CLDN19 mutations, any existing eye abnormalities will still require ongoing care from an ophthalmologist ^[5].

Genetic Counseling for the Family

FHHNC is an autosomal recessive disorder, meaning an affected child has inherited one mutated gene from each parent ^[2]. Genetic counseling is a vital part of the transplant and family planning process:

• Risk to Future Children: Each pregnancy for the same two parents has a 25% (1 in 4) chance of resulting in a child with FHHNC, a 50% chance the child is a carrier, and a 25% chance they are unaffected ^[2].
• Carrier Testing: Siblings and other close relatives can be tested to see if they are carriers of the mutation ^[6].
• Reproductive Options: Counselors can discuss options such as prenatal diagnosis or preimplantation genetic testing for families who wish to have more children ^[2].

Carrier Parents as Living Donors

Because parents are typically “carriers” (heterozygous), they may consider living kidney donation. Current research shows that heterozygous parents have safely donated a kidney, provided they undergo a very strict medical evaluation ^[3][4]. However, this is a complex decision with strict safeguards.

• No Expectation to Donate: A carrier parent is never obligated to donate. They will be assigned an independent donor advocate and a separate medical team to ensure their decision is free from pressure.
• Thorough Evaluation: Eligibility is determined on a case-by-case basis by an independent transplant program. A parent must be screened for subclinical (hidden) symptoms. This includes normal blood magnesium, calcium, and PTH levels, kidney function (eGFR), blood pressure, albuminuria, and no excessive calcium in their urine ^[3][7]. Detailed imaging is required to confirm the parent has no nephrocalcinosis or kidney stones, though the specific imaging (ultrasound vs. CT, which involves radiation) is individualized ^[3][8].

While many carriers are healthy, some studies note that a small number may show mild signs of mineral imbalance, such as slightly higher urine calcium or a history of stones ^[7][4]. The long-term risks for a carrier living with only one kidney are not fully established, which is why the donor evaluation is so thorough. When transplantation is indicated, the medical team will discuss whether living donation, a deceased donor, or dialysis as a bridge is the safest path forward ^[3][4].