A biomarker that can accurately and reliably distinguish cancer or high-grade dysplasia among mucinous pancreatic cystic neoplasms remains an important clinical need. The most accepted cyst fluid biomarker currently is CEA, which is good at differentiating mucinous from non-mucinous cysts. CEA, however, is not reliable for differentiating cancer or high-grade dysplasia among pre-malignant mucinous cysts. As a result, current practice relies on clinical and radiographic data to help clinicians decide which cystic lesions warrant immediate surgery over observation . While helpful, cases of unnecessary surgery or missed opportunities to resect cancer occur [19–21].
AREG's discovery as a potential cyst fluid biomarker arose from observations of increased Anterior Gradient 2 (AGR2) gene expression among pancreatic adenocarcinomas . AGR2 is a highly conserved gene that is associated with mucus secreting cells. AGR2 stimulates adenocarcinoma cell growth and supports the development of many features associated with malignant transformation [14, 15]. Closer examination of the gene expression studies showed that AGR2 expression was significantly higher in MCN cysts compared to SCA lesions. Recent studies revealed that AREG, a secreted epidermal growth factor receptor ligand, is specifically induced by AGR2 .
In this study, we examined the diagnostic utility of AREG in pancreatic cyst fluid and observed no difference in cyst AREG concentrations between non-mucinous and benign mucinous cysts. Malignant mucinous cysts that included high-grade dysplastic lesions, however, expressed a significantly higher AREG level (median 986 pg/ml) compared to benign mucinous cysts (median 63 pg/ml) and non-mucinous cysts (median 85 pg/ml). By receiver operator curve analysis, an AREG level of 300 pg/ml provided a diagnostic accuracy for cancer of 78% (sensitivity 83%, specificity 73%). The higher cyst AREG levels observed in malignant cysts is likely a function of the total cellular mass of AREG producing cells. As a benign cyst transitions to a malignant cyst, a hallmark of dysplasia includes a change from simple to stratified epithelium. We hypothesized that this results in a significant increase in the cellular mass of a cyst leading to increased cyst AREG expression. The similarities in cyst AREG levels between non-mucinous and benign mucinous cysts may be related to the physiologic expression of AREG as part of a reparative process in combination with a smaller cellular mass of mucin producing cells. Recent studies have determined that AREG serves an important role in tissue repair after damage in the gastrointestinal tract [22, 23].
Because cyst CEA is fairly accurate in differentiating non-mucinous from mucinous cysts, the diagnostic utility of combining both CEA and AREG was considered. There were 21 of the 33 samples where cyst fluid CEA and AREG levels were available for analysis. The median (IQR) CEA levels for the 4 non-mucinous cysts, 11 benign mucinous, and 6 malignant mucinous cysts were 127 ng/ml (36-844), 1294 ng/ml (171-8600), and 2400 ng/ml (1245-11962), respectively. Mucinous cysts (n = 17) had an elevated CEA (median (IQR) 1311 ng/ml (277-8600)) compared to non-mucinous cysts (n = 4) (126 ng/ml (36-844) (p = 0.09). Although this difference was not statistically significant, this is likely due to the small sample size. Using a cutoff of 192 ng/ml, the sensitivity and specificity of CEA to differentiate non-mucinous from mucinous cysts was 76% and 75%, respectively- an observation similar to previous reports [7, 24]. The small size of this sample may also explain why no difference in sensitivity and specificity for cancer was observed when combining AREG and CEA compared to AREG alone. When an AREG threshold of 300 pg/ml was used for the diagnosis of malignant mucinous cysts, the sensitivity was 67% and the specificity 80%. When AREG was sequentially tested only on pancreatic cysts with a CEA level greater than 192 ng/ml, neither the sensitivity nor specificity changed for cancer.
There are several features of this study that limit the generalizability of these observed results. First, this is a retrospective single tertiary center with a relatively small sample of cyst fluid samples. The small sample size is due in part to restricting the study to surgical patients. Although recruitment was difficult because patients with pancreatic cysts often do not undergo surgery, it was felt that as an initial proof-of-concept study, the use of pathology and surgically resected samples was a necessary gold standard to establish the correct diagnosis. As a result, the impact of a small sample size (in particular the limited cases of non-mucinous cysts) may include inadequate power to demonstrate a difference between non-mucinous and mucinous AREG levels should one truly exist. Second, the 12 cancer cases (including high grade dysplasia) were relatively advanced cases and could likely be identified by current practices without cyst AREG. It is unclear how AREG will perform in cases when imaging and clinical characteristics are non-specific. Many of these limitations, however, can be addressed in the future with prospective, longitudinal validation incorporating a larger sample size and multi-center collaboration.