Nontruncated amino-terminal parathyroid hormone overproduction in two patients with parathyroid carcinoma: a possible link to HRPT2 gene inactivation

Abstract

Objective: Some patients with parathyroid carcinoma present with an over-production of nontruncated amino-terminal (NT-N) parathyroid hormone (PTH), a post-transcriptionally modified form of PTH(1-84). This is usually picked up on an elevated whole (W) PTH (third-generation)/total (T) (second-generation) PTH assay ratio (N > 0·8).

Patients and design: Two parathyroid cancer patients with several episodes of hypercalcaemia and multiple surgeries are described. In both patients, W-PTH, T-PTH and circulating PTH molecular forms separated by high-pressure liquid chromatography (HPLC) were measured with the same assays. qPCR was used to study HRPT2 gene mutation.

Results: The first patient had total calcium of 3·8 and 3·22 mmol/l before the fourth and fifth surgeries, and third/second-generation PTH ratios of 2·95 and 3·6, respectively. After the fourth surgery, the ratio remained normal for 1 year and increased progressively to 3·6 over 15 months. This preceded hypercalcaemia by 6 months. The ratio became normal after the fifth surgery. HPLC analysis disclosed an over-expression of NT-N PTH to 82·2% (N < 10%) relative to hPTH(1-84) before the fifth surgery. A deletion of all the tested exons of the HRPT2 gene was identified. In the second patient, W-PTH/T-PTH ratio was 0·89 when serum calcium was 3·3 mmol/l. NT-N PTH was also over-expressed at 51·9%. An inactivating mutation of the HRPT2 gene was also identified.

Conclusions: This may suggest that a progressive rise in third/second-generation ratio may have possible clinical utility to monitor parathyroid cancer recurrence. A possible association between NT-N PTH overproduction and HRPT2 gene inactivation is also suggested.

Figure 1

HPLC profiles of circulating PTH molecular forms, obtained with Whole PTH (—) and total PTH (----) assays, are illustrated after the 4^th surgery (A) and before (B) and after 5^th surgery (C) for the first patient and during Cinacalcet HCl therapy for the second patient (D). The regions of elution of hPTH(1-84), NT-N PTH and non(1-84) PTH fragments are indicated at the top (A, B). NT-N PTH overexpression is seen in B and D where it represents 82.2% and 51.9% of W-PTH immunoreactivity respectively. The different elution positions observed in A, B, C vs D are related to the use of different HPLC columns.