Hyperparathyroidism (HPT) is a common endocrine disorder with incompletely understood etiology, characterized by enlarged hyperactive parathyroid glands and increased serum concentrations of parathyroid hormone and ionized calcium. We have recently reported activation of the Wnt signaling pathway by accumulation of β-catenin in all analyzed parathyroid tumors from patients with primary HPT (pHPT) and in hyperplastic parathyroid glands from patients with uremia secondary to HPT (sHPT). Mechanisms that may account for this activation have not been identified, except for a few cases of β-catenin (CTNNB1) stabilizing mutation in pHPT tumors.
Methods and Findings
Reverse transcription PCR and Western blot analysis showed expression of an aberrantly spliced internally truncated WNT coreceptor low-density lipoprotein receptor–related protein 5 (LRP5) in 32 out of 37 pHPT tumors (86%) and 20 out of 20 sHPT tumors (100%). Stabilizing mutation of CTNNB1 and expression of the internally truncated LRP5 receptor was mutually exclusive. Expression of the truncated LRP5 receptor was required to maintain the nonphosphorylated active β-catenin level, transcription activity of β-catenin, MYC expression, parathyroid cell growth in vitro, and parathyroid tumor growth in a xenograft severe combined immunodeficiency (SCID) mouse model. WNT3 ligand and the internally truncated LRP5 receptor strongly activated transcription, and the internally truncated LRP5 receptor was insensitive to inhibition by DKK1.
The internally truncated LRP5 receptor is strongly implicated in deregulated activation of the WNT/β-catenin signaling pathway in hyperparathyroid tumors, and presents a potential target for therapeutic intervention.
Gunnar Westin and colleagues report the expression of an aberrantly spliced LRP5 receptor in primary and spontaneous parathyroid tumors and implicate it in the deregulated activation of the Wnt/β-catenin signaling pathway.
The parathyroid glands—four rice-sized glands in the neck—maintain a normal calcium balance in the body, to maintain strong bones and essential cellular functions. The glands release parathyroid hormone as a response to a decrease in blood calcium level. By stimulating calcium release from bone and its absorption in the gut, parathyroid hormone restores the blood calcium level. However, 100,000 new individuals in the US develop hyperparathyroidism (HPT) annually, characterized by enlarged, overactive parathyroid glands and high blood levels of calcium. Primary HPT (pHPT) is usually caused by a benign tumor (a non-life-threatening growth) in one of the parathyroid glands. Secondary HPT (sHPT) occurs in response to calcium regulatory disturbances, linked to vitamin D deficiency, and more or less invariably develops in patients with uremic kidney disease.
Why Was This Study Done?
HPT is usually treated by surgical removal of the enlarged parathyroid glands, which is done with great efficiency. However, ideally, doctors would like to know what drives the overgrowth of the parathyroid glands to be able to develop drugs for treatment or disease prophylaxis. Researchers recently reported that the cells in enlarged parathyroid glands from patients with HPT contain high amounts of β-catenin. This protein is part of the Wnt signaling pathway, which has been found to be disrupted in many tumor entities in other organs. In the absence of Wnt proteins, a group of proteins called the β-catenin destruction complex marks β-catenin so that it is rapidly destroyed. When Wnt proteins bind to a cell-surface receptor called Frizzled and a coreceptor called LRP5, the destruction complex is inhibited and β-catenin accumulates. This accumulation induces the production of other proteins (in particular, c-Myc) that stimulate cell growth and division. The accumulation of β-catenin in the enlarged parathyroid glands of patients with HPT could, therefore, significantly contribute to the overgrowth of their glands—but what causes β-catenin accumulation? In this study, the researchers have investigated this question to try to identify a target for drugs to treat HPT.
What Did the Researchers Do and Find?
The researchers looked for genetic changes (mutations) in β-catenin that stabilize the protein and measured the expression of LRP5 in abnormal parathyroid gland tissue from 37 patients with pHPT and 20 with uremia and sHPT. All the samples contained high levels of β-catenin, but only four contained a β-catenin–stabilizing mutation. All the sHPT samples and 32 pHPT samples (but none of the samples containing the β-catenin stabilizing mutation) expressed a mutated LRP5, with the central region deleted. To investigate the functional consequences of this internally deleted LRP5 protein, the researchers used a technique called RNA interference to block its expression in a human parathyroid tumor cell line. They found that expression of the mutated, short LRP5 is required for accumulation of β-catenin, expression of c-Myc, and continued growth of the cell line in test tubes and in animals.
What Do These Findings Mean?
The accumulation of β-catenin in all the enlarged parathyroid glands examined so far strongly implicates abnormal Wnt/β-catenin signaling in the development of pHPT and sHPT. These new findings identify which part of the signaling pathway is altered. The expression data and functional data together suggest that an internally deleted LRP5 coreceptor is often responsible for the accumulation of β-catenin. The functional data also show that expression of shortened LRP5 is necessary for the abnormal growth of parathyroid tumor cells. Exactly how the internally deleted coreceptor activates β-catenin signaling in parathyroid gland cells, or why a shorter-than-normal LRP5 is made, are not yet known. However, because these findings indicate that internally deleted LRP5 has a fundamental role in activating Wnt signaling in HPT, drugs that inactivate this aberrant protein but leave the normal protein unscathed might provide a nonsurgical treatment for this common hormone disorder.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040328.
edlinePlus has encyclopedia pages on hyperparathyroidism, primary hyperparathyroidism, and secondary hyperparathyroidim (in English and Spanish)
Information is available for patients from the US National Institute of Diabetes and Digestive and Kidney Diseases on hyperparathyroidism, which includes links to organizations that help people with hyperparathyroidism
Wikipedia maintains pages on Wnt signaling pathway and on β-catenin (note that Wikipedia is a free online encyclopedia that anyone can edit; available in several languages)