Understanding the timing of genetic alterations in tumor development can often help define their roles in progression of disease, and this has important implications for detection and treatment. Unlike the well-defined sequence from adenoma to invasive carcinoma in the colorectum, the histological progression of pancreatic neuroendocrine tumors is not well defined because no clear-cut basement membrane separates noninvasive from invasive lesions. Tumor size can serve as a surrogate for progression of pancreatic neuroendocrine tumors, as the vast majority of small lesions (ie, microadenomas) behave in a benign fashion, while larger lesions (ie, pancreatic neuroendocrine tumors, by definition measuring ≥0.5 cm) demonstrate malignant potential.16,22,23
In this study, we found that loss of nuclear expression of ATRX and/or DAXX occurred in 6% of MEN-1 well-differentiated pancreatic neuroendocrine tumors and that loss of nuclear expression of ATRX and/or DAXX perfectly correlated with the presence of the alternative lengthening of telomeres phenotype. These findings establish the existence of ATRX and DAXX defects and the alternative lengthening of telomeres telomere maintenance mechanism in the setting of MEN-1 syndrome pancreatic neuroendocrine tumors and confirm the correlation between loss of nuclear expression of ATRX or DAXX and the occurrence of the alternative lengthening of telomeres phenotype.
We also found that loss of nuclear expression of ATRX and/or DAXX and the presence of the alternative lengthening of telomeres phenotype occurred only in larger (≥3 cm) pancreatic neuroendocrine tumors in patients with MEN-1 syndrome. This finding suggests that these changes are ‘late’ events in the pancreatic neuroendocrine tumorigenesis, occurring only after the neoplasms have grown well beyond the size of microadenomas.
As compared with the prior study of sporadic pancreatic neuroendocrine tumors by Jiao et al
the prevalence of ATRX and/or DAXX defects and the alternative lengthening of telomeres phenotype in the current study of MEN-1 tumors was considerably less (43% vs
6%). While inherent differences between these two groups of neuroendocrine tumors may certainly be responsible for this discrepancy, it may also reflect differences in the size (ie, progression) of the pancreatic neuroendocrine tumors studied: the mean size of pancreatic neuroendocrine tumors in the prior study was 4.9 cm, whereas the mean size in the current study was 1.9 cm. Even within the subset of MEN-1 pancreatic neuroendocrine tumors measuring ≥3 cm in the current study (albeit a sample size too small for definitive conclusions), the mean size was just 4.2cm, and notably, the proportion of these tumors showing ATRX and/or DAXX defects and the alternative lengthening of telomeres phenotype was 25% (a figure much closer to the 43% seen in the sporadic pancreatic neuroendocrine tumors). Additionally, it is worth noting that MEN1
mutation per se
does not preclude an ATRX or DAXX defect, as nearly one-quarter of the sporadic pancreatic neuroendocrine tumors in the previous study harbored dual mutations in MEN1
and either ATRX
Although the vast majority of neoplasms maintain their telomere lengths by increased activity of telomerase, ~ 4% of neoplasms maintain their telomere lengths through a homologous recombination-based mechanism known as alternative lengthening of telomeres.17
One of the hallmarks of this telomere maintenance mechanism is accumulation of large amounts of telomeric DNA in discrete nuclear foci, a feature that is the basis for the telomere-specific FISH assay used to detect alternative lengthening of telomeres. In all tumors tested to date, including those of the current study, loss of nuclear ATRX and/or DAXX shows a near 100% correlation with the alternative lengthening of telomeres phenotype,3,4
suggesting that inactivation of these proteins (and/or their associated genes) is a critical step in the development of this cancer-associated telomere maintenance mechanism. Although the clinical significance of these changes in pancreatic neuroendocrine tumors is still being unraveled, mutations in either ATRX
have been shown to be associated with improved survival (as compared with pancreatic neuroendocrine tumors without ATRX
Unfortunately, in the current study, the prevalence of ATRX and/or DAXX defects was too low to generate a statistically well-powered survival curve.
In summary, our findings establish the existence of ATRX and DAXX defects and the alternative lengthening of telomeres telomere maintenance mechanism in the setting of MEN-1 syndrome pancreatic neuroendocrine tumors, confirm the correlation between loss of nuclear ATRX or DAXX expression and the occurrence of the alternative lengthening of telomeres phenotype, and demonstrate that these changes occur as late events in pancreatic neuroendocrine tumor development in patients with MEN-1 syndrome. Furthermore, the presence of simultaneous defects in ATRX or DAXX and MEN1
in this study adds to a growing body of evidence that the signaling pathway of ATRX
is distinct from that of MEN1