Prognostic implications of immunohistochemically detected YKL-40 expression in breast cancer

YKL-40 was first discovered as a 40 kDa protein secreted by the MG63 human osteosarcoma cell line [1]. It was subsequently found to belong to a family of "Chitinase-Like Proteins" (CLP), named for their structural similarity to bacterial chitinases, although lacking their characteristic enzymatic activity. The family is notable for being highly phylogenetically conserved, thus not surprisingly, some of its members have been found to be fundamental mediators of collagen synthesis and extracellular matrix re-modeling as well as mitogenesis [2]. Although the unique functions of YKL-40 are unknown, its putative involvement in human disease seems to be protean. It has been implicated as playing a role in such diverse processes as bacterial sepsis, rheumatoid arthritis, inflammatory bowel disease, cirrhosis, and cancer [3‐7]. Serum levels of YKL-40 have been found to be elevated in a number of different human malignancies including those of breast, ovarian, colon, CNS, bone, and skin origin; this finding has invariably been associated with poor prognosis [8‐21]. The sheer diversity of these cancers suggests that this protein may play a fundamental role in the neoplastic process.

Breast cancer afflicts approximately 200,000 women per year in the United States and is the second leading cause of cancer death in this country [22]. Traditionally, primary tumor size and presence or absence of axillary lymph node metastases are considered the most important factors determining prognosis. Other factors such as degree of tumor differentiation and expression of estrogen and progesterone receptors also have prognostic import, although they are not usually used in clinical staging. In the past decade, numerous studies have reported that amplification/overexpression of the transmembrane protein kinase Her2/neu, may be associated with a more aggressive phenotype [23]. Although controversy persists as to this finding's true prognostic importance [24], this issue has largely been rendered moot by the recent demonstration of therapeutic efficacy of a monoclonal antibody directed against Her2 in patients with Her2-positive breast cancers [25‐27]. The relatively new technology of microarray analysis [28, 29] may offer further insight into new molecular prognostic markers/therapeutic targets, as it is becoming clear that this is the next evolutionary leap in cancer treatment. YKL-40 holds great promise in this regard; this assertion is supported by the finding that elevated levels of this protein are associated with poor outcome in women with invasive breast cancer [11, 12, 14] and ovarian cancer [15]. Heretofore, these studies have been mainly done with ELISA of patient sera. A previous immunohistochemical survey of breast cancer tissue demonstrated expression in over 70% of the samples, but the clinical significance of this finding was not reported [30]. In the present study, we also wished to determine the expression pattern of YKL-40 in breast cancer tissue, and further ascertain whether this had prognostic implications similar to what was found with data from patient sera.

Between May 1999 and June 2006, we prospectively collected and collated data on 265 women who underwent operative therapy for breast cancer at University Hospital in Newark. Of these, 215 patients had invasive disease (81%) and 50 patients (19%) had pure ductal-carcinoma-in-situ (DCIS). Of the patients with invasive disease, we had full clinical follow-up, tissue availability, and YKL-40 immunohistochemical data on 109 patients. These patients made up the cohort for the present study. Institutional Review Board approval was obtained via Protocol # 0120030318.

All immunohistochemical evaluation and scoring was performed by one of the authors (MH). Evaluation was always performed in an area of tumor where malignant cells were the most abundant. YKL-40 immunoreactivity was recognized as brown staining within cells, and was localized mainly to the cytoplasm of tumor cells (Figure 1). The staining intensity was more or less equal between the tumors and thus was not included as a scoring criterion. The number of reactive cells per specimen was variable, ranging from only a few per slide to more diffuse staining in focal areas of the tumor. No case, however, demonstrated immunoreactivity in all tumor cells, consistent with previous reports of expression in cancer tissues [10, 30]. We arbitrarily stratified these results on a scale of 0–2: a score of 0 denoted no detectable expression, 1 denoted "weak" expression (≤10 immunoreactive cells/tumor), and 2 denoted "strong" expression (>10 immunoreactive cells/tumor). A score of 1 or 2 was considered a "positive" result. Using these criteria, YKL-40-positive tumors were found in 37 patients (34%). Among the positive cases, 28/37 (76%) were considered "strong" expressors (score of 2).

We subsequently correlated the experimental findings with the clinical characteristics of our patient cohort. No significant difference was noted in the mean age of the patient subset with tumors in which YKL-40 was detected vs. the control group (52.3 years vs. 53 years, respectively). Furthermore, incidence of expression was not significantly different between invasive ductal vs lobular cancers. In contrast, several primary tumor factors associated with poor prognosis were found to correlate significantly with YKL-40 immunoreactivity (Table 1). Mean size of cancers in which the antigen was detected was 4.0 cm compared to 2.7 cm for those in which it was not (p < .05). Furthermore, the incidence of YKL-40 detection notably increased with worsening tumor differentiation (p < .001). Nearly 70% of YKL-40-positive tumors were poorly differentiated compared to only 18% of controls. The former group was also significantly more likely to be estrogen and progesterone receptor negative (p < .01). Factors demonstrating no significant correlation to YKL-40 status included presence or absence of axillary lymph node metastases, nodal stage (N0 – N3), or number of positive nodes in patients with node-positive disease. There was also no association between YKL-40 and Her2 immunoreactivity. These findings did not change when the weak YKL-40 expressing tumors (N = 9) were classified as "negative."

At a mean follow up of 3.2 years (median 2.7 years, range 0.4 – 7.3 years), actuarial 5-year disease-free survival (DFS) for the entire cohort was 66%. Factors that significantly predicted poor outcome on univariate analysis included primary tumor size, nodal stage, and immunohistochemically detectable YKL-40 expression. Degree of tumor differentiation was of borderline significance (p = .05). ER, PR, and Her2 status were not predictive of disease relapse on this cohort, perhaps due to the relatively limited follow up. For patients with YKL-40-positive tumors, median DFS was only 3.8 years (patients with non-expressing tumors had not yet reached median, Figure 2). Significant univariate factors associated with poor DFS were then examined by multivariate analysis using stepwise forward Cox regression, and demonstrated that YKL-40 status was independent of tumor size and nodal stage in predicting disease recurrence. Of note, although the numbers are small, no difference in survival was noted between patients having tumors with strong YKL-40 expression (N = 28) and those having tumors with weak expression (N = 9).

In our study, we found that expression of YKL-40 was detected in about one-third of archived breast cancer tissue specimens surveyed using immunohistochemical analysis. We noted cytoplasmic localization of the antigen within tumor cells, a finding consistent with that reported in a similar study of glioblastomas [10] and a previous survey of breast cancers [30]. This seems consistent with the fact that YKL-40 is a secreted protein and has been detected in sera of cancer patients. Although the number of immunoreactive cells per specimen was variable, this tumor-to-tumor variability in YKL-40 expression was not associated with any difference in its prognostic efficacy, that is, cancers with only sparse expression (score 1) associated with outcome that was similar to cancers with more diffuse expression (score 2). Our results differ somewhat from that of Roslind et al [30] in that we found a much smaller fraction of breast cancer specimens in which YKL-40 was detected (71% vs. 34%). This difference may simply be attributable to methodological discrepancy, however, the latter figure seems to be more consistent with the serum data in which 19–30% of breast cancer patients were found to have elevated levels of the antigen [11, 14].

Clinical correlation in our study showed that YKL-40 antigen detection was associated with larger tumor size, poorer tumor differentiation, and a higher likelihood of estrogen and progesterone receptor negativity. Furthermore, YKL-40 immunoreactivity was a predictor of poor disease-free survival in both univariate and multivariate analysis, being independent of both T- and N-stage. These results are in concordance with the serum data where high YKL-40 levels were predictive of worse outcome that also was independent of more traditional prognostic indices such as T and N stage [11, 12, 14]. There are, however, some notable differences between the serum studies and our results. The study by Johansen and colleagues showed a significant correlation between high serum YKL-40 level and node positivity, but no difference with intrinsic primary tumor factors such as size, grade (degree of differentiation), and estrogen receptor positivity [11]. Our results were essentially contrary: we found no statistical correlation with nodal disease however, there was a high degree of association with the above negative prognostic indices characteristic of the primary tumor (Table 1). These findings may indicate that the serum and immunohistochemical data, although both prognostically significant in determining outcome, may give different information. The immunohistochemical findings may be more strongly associated with intrinsic biology of the primary tumor, while the serum data may be more informative of nodal and distant metastasis [11, 12, 14]. Unquestionably, these two clinical phenomena are intimately related, so it will be useful in the future to clinically correlate the results of combined serum and immunohistochemical data from the same patients.

Our report does have some obvious limitations. The cohort size is relatively small and predominantly made up of women from ethnic minorities. The majority were Black females, a group that some have suggested may have an inherently worse breast cancer survivorship [33, 34]. Furthermore, because of the relative immaturity of our database, the length of follow-up was somewhat limited. Clearly, the results will require confirmation using greater patient numbers, longer periods of observation, as well as a patient population that is more reflective of the entire ethnic spectrum of women afflicted with breast cancer. However, despite these caveats, we feel that the findings are notable, and lay fertile ground for future studies of YKL-40 and its role in breast cancer progression.

Immunoreactivity for YKL-40 was a significant predictor of breast cancer relapse in this subset of patients. This was independent of T or N-stage and suggests that tumor immunohistochemistry for this protein may be a valuable prognostic marker in breast cancer