1,25-dihydroxyvitamin D-mediated hypercalcemia in a renal transplant recipient

Vitamin D-Mediated Hypercalcemia: Mechanisms, Diagnosis, and Treatment

Hypercalcemia occurs in up to 4% of the population in association with malignancy, primary hyperparathyroidism, ingestion of excessive calcium and/or vitamin D, ectopic production of 1,25-dihydroxyvitamin D [1,25(OH)₂D], and impaired degradation of 1,25(OH)₂D. The ingestion of excessive amounts of vitamin D₃ (or vitamin D₂) results in hypercalcemia and hypercalciuria due to the formation of supraphysiological amounts of 25-hydroxyvitamin D [25(OH)D] that bind to the vitamin D receptor, albeit with lower affinity than the active form of the vitamin, 1,25(OH)₂D, and the formation of 5,6-trans 25(OH)D, which binds to the vitamin D receptor more tightly than 25(OH)D. In patients with granulomatous disease such as sarcoidosis or tuberculosis and tumors such as lymphomas, hypercalcemia occurs as a result of the activity of ectopic 25(OH)D-1-hydroxylase (CYP27B1) expressed in macrophages or tumor cells and the formation of excessive amounts of 1,25(OH)₂D. Recent work has identified a novel cause of non-PTH-mediated hypercalcemia that occurs when the degradation of 1,25(OH)₂D is impaired as a result of mutations of the 1,25(OH)₂D-24-hydroxylase cytochrome P450 (CYP24A1). Patients with biallelic and, in some instances, monoallelic mutations of the CYP24A1 gene have elevated serum calcium concentrations associated with elevated serum 1,25(OH)₂D, suppressed PTH concentrations, hypercalciuria, nephrocalcinosis, nephrolithiasis, and on occasion, reduced bone density. Of interest, first-time calcium renal stone formers have elevated 1,25(OH)₂D and evidence of impaired 24-hydroxylase-mediated 1,25(OH)₂D degradation. We will describe the biochemical processes associated with the synthesis and degradation of various vitamin D metabolites, the clinical features of the vitamin D-mediated hypercalcemia, their biochemical diagnosis, and treatment.

A lifetime of hypercalcemia and hypercalciuria, finally explained

CONTEXT

Hypercalcemia, hypercalciuria, and recurrent nephrolithiasis are all common clinical problems. This case report illustrates a newly described but possibly not uncommon cause of this presenting complex.

OBJECTIVE

We report on a patient studied for over 30 years, with the diagnosis finally made with modern biochemical and genetic tools.

DESIGN AND SETTING

This study consists of a case report and review of literature conducted in a University Referral Center.

PATIENT AND INTERVENTION

A single patient with hypercalcemia, hypercalciuria, and recurrent nephrolithiasis was treated with low-calcium diet, low vitamin D intake, prednisone, and ketoconazole.

MAIN OUTCOME MEASURE

We measured the patient's clinical and biochemical response to interventions above.

RESULTS

Calcium absorption measured by dual isotope absorptiometry was elevated at 37.4%. Serum levels of 24,25-dihydroxyvitamin D were very low, as measured in two laboratories (0.62 ng/mL [normal, 3.49 ± 1.57], and 0.18 mg/mL). Genetic analysis of CYP24A1 revealed homozygous mutation E143del previously described. The patient's serum calcium and renal function improved markedly on treatment with ketoconazole but not with prednisone.

CONCLUSIONS

Chronic hypercalcemia, hypercalciuria, and/or nephrolithiasis may be caused by mutations in CYP24A1 causing failure to metabolize 1,25-dihydroxyvitamin D.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

Hypercalcaemia as a prodromal feature of indolent Pneumocystis jivorecii after renal transplantation

Following renal transplantation, hypercalcaemia is frequently caused by persisting hyperparathyroidism. Unregulated extrarenal 1,25-dihydroxyvitamin D (1,25(OH)(2)D) synthesis, which is well recognized as a cause of hypercalcaemia in granulomatous diseases, may also occur after kidney transplantation. This mechanism is also likely to be responsible for hypercalcaemia reported during treatment of cytomegalovirus and associated with acute symptomatic pneumocystis jivorecii pneumonia (PCP). Hypercalcaemia as a prodromal feature of indolent PCP has not been described. We report a renal transplant recipient who developed hypercalcaemia 30 months post-transplant due to extrarenal production of 1,25(OH)(2)D. Two months later, PCP was diagnosed and hypercalcaemia resolved after initiation of treatment.

Hypercalcemia in a renal transplant recipient suffering with Pneumocystis carinii pneumonia

Hypercalcemia occurs frequently after renal transplantation. Preexisting hyperparathyroidism is the most common cause of post-transplantation hypercalcemia. We describe a renal transplant recipient infected with Pneumocystis carinii pneumonia (PCP) who developed hypercalcemia, elevated 1,25-dihydroxyvitamin D, and suppressed parathyroid hormone levels. This phenomenon mimics the extrarenal production of 1,25-dihydroxyvitamin D by activated alveolar macrophages in granulomatous diseases with hypercalcemia. To the best of our knowledge, this is the first report of 1,25-dihydroxyvitamin D-mediated hypercalcemia caused by PCP in a renal transplant recipient. This entity should be included in the differential diagnosis for renal transplant recipients with hypercalcemia, especially in patients who develop lung infections.

Hypercalcemia following renal transplantation in a cat

An 11-year-old 3.0-kg (6.6-lb) neutered male Persian was referred for renal transplantation. Serum total calcium concentration was slightly high prior to surgery, but the week after surgery, total and ionized calcium concentrations were extremely high, and a small mass was palpable on the right side of the trachea at the level of the thyroid and parathyroid glands. Exploratory surgery of the ventral aspect of the neck was performed, and a right external parathyroid mass was removed. One hour after surgery, the serum ionized calcium concentration was within reference limits, and the serum calcium concentration remained normal for the next 14 months without any specific treatment. The gross and histologic appearance of the mass, combined with the rapid decrease in serum calcium concentration following its removal, confirmed that the mass was a functional parathyroid adenoma. Although a common postoperative complication in people, hypercalcemia following renal transplantation appears to be a rare complication in cats. Surgery should be considered if the condition is a result of a parathyroid adenoma.