Research Article: An LRP5 Receptor with Internal Deletion in Hyperparathyroid Tumors with Implications for Deregulated WNT/β-Catenin Signaling

Date Published: November 27, 2007

Publisher: Public Library of Science

Author(s): Peyman Björklund, Göran Åkerström, Gunnar Westin, Hans Clevers

Abstract: BackgroundHyperparathyroidism (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 FindingsReverse 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.ConclusionsThe 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.

Partial Text: Primary hyperparathyroidism (pHPT) is characterized by hypersecretion of parathyroid hormone and generally also hypercalcemia, due to one or several parathyroid tumors (adenoma). Secondary hyperparathyroidism (sHPT) develops in patients with uremia because of phosphate retention, hypocalcemia, and reduced 1,25-dihydroxyvitamin D3 levels, causing parathyroid hyperplasia and eventually development of parathyroid tumors and hypercalcemia [1–4]. Parathyroidectomy is the only considered therapy for most patients. We recently reported aberrant β-catenin (CTNNB1) accumulation in all analyzed parathyroid tumors from patients with pHPT and in hyperplastic parathyroid glands from patients with uremia secondary to HPT [5]. MYC, a direct target of the Wnt/β-catenin signaling pathway in colorectal cancer cells and established as the critical mediator of the early stages of intestinal neoplasia [6,7], was found to be overexpressed at the protein level in 79% of parathyroid tumors [5]. Maintained activity of endogenous β-catenin was found to be necessary for the expression of MYC and cyclin D1 (CCND1), as well as growth and survival of a unique human parathyroid tumor cell line [8]. Overexpression of cyclin D1 has been reported in 20%–40% of pHPT tumors [2], and overexpression of cyclin D1 in the parathyroid glands of transgenic mice caused development of pHPT [9]. In a small fraction of parathyroid adenomas, overexpression is due to activation of the CCND1 gene by pericentromeric inversions of Chromosome 11, involving the parathyroid hormone (PTH) promoter [10]. Augmented cyclin D1 expression in some parathyroid adenomas could also be a consequence of aberrant β-catenin accumulation [5], although it remains to be determined whether CCND1 constitutes a β-catenin target [11] in parathyroid cells. We also reported CTNNB1 stabilizing mutations in a few cases (3 out of 20) of pHPT tumors, while no mutation was found in uremic secondary HPT tumors, and inactivating truncations of adenomatosis polyposis coli (APC) were not seen [5]. Mutation or deregulated expression of other Wnt-signaling components leading to β-catenin accumulation was therefore anticipated.

β-catenin accumulation has been observed by us in all so far analyzed parathyroid tumors of primary origin and hyperplastic parathyroid glands of HPT secondary to uremia ([5] and this work), strongly suggesting dysregulated WNT/β-catenin signaling as a common pathogenic pathway for these different conditions. We reported CTNNB1-stabilizing mutations in a few cases (3 out of 20) of pHPT tumors, while no mutation was found in uremic secondary HPT tumors, and inactivating truncations of APC were not seen [5]. Recently, parathyroid adenomas from 97 patients with pHPT who had undergone parathyroidectomy in the United States were analyzed regarding CTNNB1-stabilizing mutations in exon 3 [33]. In agreement with one previous smaller study of Japanese patients [34], no mutations were identified. Taken together, these results indicated a low CTNNB1 mutation frequency in pHPT tumors [35]. We are currently analyzing a large number of additional pHPT tumors from Swedish patients. A few additional tumors with CTNNB1-stabilizing mutations has been found so far, suggesting an overall mutation frequency of approximately 6% (Björklund et al., unpublished data). This frequency is slightly lower than that observed for colorectal cancers, in which approximately 10% of the tumors harbored missense mutations or interstitial deletions of exon 3 [13,14].