Pheochromocytoma and abdominal paraganglioma : clinical and genetic aspects

Abstract

Pheochromocytomas and abdominal paragangliomas are rare catecholamine producing tumours arising from sympathoadrenal paraganglia located within and outside of the adrenal medulla, respectively. The great majority of tumours are benign and cured by surgery. However, a minority of patients will develop metastases and/or invasive tumour growth, which is associated with a poor prognosis. Differentiation between benign and malignant tumours based on histopathology has generally been found unreliable and prognostic markers are lacking.

The aims of this project were firstly to determine whether there are clinical features which might distinguish benign from malignant tumours, to assess operative results and optimal management based on experience derived from long term follow up of a large number of patients. Secondly, to develop markers which may aid in the prognostic assessment of these tumours. Such markers might establish that a tumour is more likely to metastasise in which case close follow up would be mandatory, or that a tumour is likely to be benign and that clinical surveillance might be reduced. Since the genetic background of these tumours is largely unknown, a third aim was to explore genetic events of significance to the development of these tumour types.

Clinical and therapeutic features were reviewed retrospectively in 85 patients who underwent surgery for pheochromocytoma or abdominal paraganglioma at the Karolinska Hospital between 1976 and 1999. There was no significant difference in either clinical or biochemical parameters between patients with benign and malignant disease, except that paragangliomas were significantly more often malignant than pheochromocytomas (7/15 vs. 7/70). Nine of the 14 patients with tumours classified as malignant according to the AFIP criteria, i.e. presence of metastasis and/or extensive local invasion, developed distant metastasis and/or invasive local recurrence during follow-up. On the other hand, none of 71 patients with tumours initially classified as benign had relapsed after a median follow-up time of 144 months (7-287).

Five potential prognostic markers were evaluated in 32 primary pheochromocytoma and abdominal paraganglioma. Assessment was made of proliferative activity measured as Ki-67/MIB-1 immunoreactivity, telomerase activity using the telomeric repeat amplification protocol (TRAP), hTERT (human telomerase reverse transcriptase) gene expression by reverse transcriptase (RT)-PCR as well as mRNA expression of metalloproteinase MMP-2 and the MMP inducer EMMPRIN using in situ hybridization. Ki-67/MIB-1 and hTERT expression were both significantly associated with malignancy. When combined, the sensitivity of these markers was 88% with a 90% specificity for malignancy and a negative predictive value of 95%.

Approximately 10% of pheochromocytomas are associated with familial cancer syndromes. One of these, Multiple Endocrine Neoplasia type 2A (MEN 2A) is caused by activating mutations in the RET proto-oncogene encoding a receptor tyrosine kinase. Since a number of neurotrophic factors had been found to activate the RET receptor, we investigated the expression of RET and two of its ligand complexes (GDNF/GFRa-1 and NTN/GFRa-2) in 25 tumours using mRNA in situ hybridisation. No expression was detected for these ligand complexes, indicating they do not have a significant role in tumourigenesis.

In an attempt to find chromosomal regions which may harbour oncogenes or tumour suppressor genes involved in tumour development, 34 tumours were screened for gross genetic alterations using comparative genomic hybridization (CGH). Deletions on chromosome arm 1p was found in >80 % of tumours. Losses were also frequently detected on chromosome arms 3q, 3p, 11p and 4q and gains on chromosome arms 19p, 19q and 17q. Pheochromocytomas and abdominal paragangliomas had very similar CGH pattern indicating a common genetic origin. Using 53 tumours, regions on chromosome 1p were analysed for loss of heterozygosity (LOH) which was detected in 74% of tumours. Three regions of overlapping deletions were identified, which are potential sites for tumour suppressor genes.

In summary, at the time of primary surgery, patients without metastases or extensive local invasion have a very low risk of developing metastatic disease. Ki-67/MIB-1 inummoreactivity and hTERT expression are valuable prognostic markers of malignancy, which can be used in addition to histopathological evaluation. At least three regions on 1p have been defined as potential sites for tumour suppresser genes and deletion of chromosome 1p is likely to be an early and important event in the development of pheochromocytomas and abdominal paragangliomas.