To our knowledge, this is the largest reported series characterizing pure PLCIS. We found that similar to CLCIS, PLCIS was characterized by 16q loss and 1q gain as well as abrogation of E-cadherin expression, suggesting a relationship between PLCIS and CLCIS. However, PLCIS showed different clinical findings, mammographic presentation and morphologic features than CLCIS. The biomarker expression of PLCIS was also distinct with an overall profile (i.e. decreased ER and PR expression and increased proliferation by Ki-67 labeling index) that is suggestive of a more aggressive phenotype. The distinct apocrine morphologic subtype of PLCIS had especially aggressive phenotypic features.
One of the most striking differences between PLCIS and CLCIS was the clinical presentation. Patients with PLCIS were significantly older than those with CLCIS. If PLCIS and CLCIS were indeed clonally related, it might be speculated that PLCIS requires a longer time for development. Unlike CLCIS which is usually an incidental microscopic finding without a detectable mammographic lesion, PLCIS typically presented as mammographic abnormalities including microcalcifications, architectural distortion and/or density. The clinical presentation and mammographic findings of PLCIS are similar to DCIS and different from CLCIS, as has been described previously (4
With regard to immunophenotype, we found both similarities and differences between PLCIS and CLCIS. All PLCIS were negative for E-cadherin expression, as is characteristic of CLCIS. However, unlike CLCIS lesions which are consistently positive for ER and PR, negative for HER2, and have low mitotic activity, PLCIS often showed low to negative ER and PR expression, a higher proliferation rate, and an increased incidence of HER2 overexpression. These findings are similar to those reported previously in PLCIS (1
). This “unfavorable” biomarker profile suggests that PLCIS represents a biologically more advanced lesion than CLCIS. All PLCIS, especially those of apocrine morphology, were strongly positive for AR. Activation of the AR pathway may be important in the progression of these lesions, and drugs targeting this pathway may constitute a novel therapeutic option for management of PLCIS.
To explore whether the differences in biomarker expression could be attributed to differences in genetic alterations, chromosomal changes were analyzed and compared between PLCIS and CLCIS by high resolution aCGH. The majority of PLCIS in our study demonstrated 1q gain and 16q loss, a molecular signature for lobular carcinomas including CLCIS (13
). Based on the more aggressive morphologic features and biomarker profile, a greater degree of genomic instability might be expected in PLCIS. Surprisingly, there was no significant difference in the extent of genomic alterations between the CLCIS and PLCIS groups. One possible explanation for this apparent lack of a difference in the extent of genomic alterations between these two groups may be that in our effort to obtain sufficient DNA for aCGH analysis, the analysis was biased toward cases with relatively extensive CLCIS, which may in turn result in a bias toward more advanced CLCIS lesions (5
). Another possibility is that the number of cases studied is insufficient to detect a significant difference. Alternatively, epigenetic alterations such as promoter methylation and/or histone deacetylation (14
) rather than genetic changes themselves may be the main mechanisms for alteration in the biomarker expression, and such changes would not be detected by aCGH analysis.
The apocrine subtype of PLCIS did demonstrate more genomic alterations than either CLCIS or non-apocrine PLCIS in our study. Although there was no specific genomic pattern that distinguished apocrine PLCIS from non-apocrine PLCIS or CLCIS, some molecular changes were either only present or more prevalent in apocrine PLCIS, including amplification of 17q11.2–17q12 (the region harboring HER2
gene), amplification of 11q13.3 (the region containing cyclin D1
gene), gain of 16p, and losses of 3q, 11q, 13q and 17p. Of note, many of these recurrent genetic alterations, including amplification of 11q13.3, gain of 16 p and losses of 11q, 13q and 17p, are shared by invasive classic lobular carcinoma and invasive pleomorphic lobular carcinoma as reported previously by our group (13
) and Simpson et al. (22
). Several important genes are mapped to these chromosomal regions and include p53
gene (17p13.1), MEN1
gene (multiple endocrine neoplasia 1, 11q13), ATM
gene (ataxia-telangiectasia mutated, 11q22.3) and CCNF
gene (cyclin F, 16p13.3). Furthermore, amplification of HER2
gene and loss of 13q (which harbors RB
gene at 13q14.1–14.2) have also been noted in invasive pleomorphic lobular carcinoma (22
) but not in invasive classic lobular carcinoma (13
). Deregulation of these genes could contribute to the aggressive morphologic features and biomarker profile in apocrine PLCIS.
Although four clusters emerged in the unsupervised clustering of all samples (), the various morphologic subtypes of LCIS did not segregate perfectly by aCGH clusters. This finding suggests that there is significant overlap in the pattern of genomic alteration among various LCIS types as defined by morphology and that there does not appear to be a particular genetic signature to define these LCIS subtypes, except the 1q gain and 16q loss for lobular neoplasia as a group. Of note, more apocrine PLCIS appeared in the cluster characterized by amplifications: 4 of 6 cases in this cluster being apocrine PLCIS, or 4 of the 8 apocrine PLCIS analyzed segregated to this cluster. Furthermore, none of the apocrine PLCIS fell into the genetically more stable cluster 4 (with lowest FGA). Data from our aCGH analysis thus suggest that although PLCIS are related to LCIS, they are a genetically heterogeneous group of lesions. The aCGH profiling also suggests that apocrine PLCIS may represent a genetically more distinct group, characterized by more extensive genome alteration, especially amplification.
We previously reported, using a similar aCGH platform and data analysis, the genomic profiles of 24 CLCIS with synchronous invasive lobular carcinoma (13
), 20 high-grade DCIS without invasion (12
), 35 invasive lobular carcinoma and 164 invasive ductal carcinoma (11
). Therefore the degree of genomic instability, as represented by FGA, in different tumor types could be compared. The mean FGA was 0.05 for non-apocrine PLCIS without invasion, 0.07 for CLCIS without invasion, 0.12 for apocrine PLCIS without invasion, 0.13 for CLCIS with invasion, 0.31 for high-grade DCIS without invasion, 0.20 for invasive lobular carcinoma and 0.32 for invasive ductal carcinoma. These comparisons indicate that pure CLCIS and non-apocrine PLCIS have the fewest genetic changes, significantly fewer than CLCIS with invasion. Pure apocrine PLCIS and CLCIS with invasion share a similar degree of genomic complexity. However, both apocrine PLCIS and CLCIS with invasion demonstrate less genomic alteration than invasive lobular carcinoma. Furthermore, apocrine PLCIS displays far less genetic instability than high-grade DCIS. These data support the current notion that CLCIS alone is more of a risk factor than immediate precursor for invasive carcinoma as these lesions harbor fewer genetic changes and thus will require longer time for progression to invasion. However, apocrine PLCIS appears to be a genetically more advanced lesion, similar to CLCIS that has evolved to invasive carcinoma. These data also clearly show that although PLCIS demonstrates some features similar to high-grade DCIS, these lesions have less genomic alteration than high-grade DCIS. Overall, the aCGH profiling changes in PLCIS qualitatively resemble lobular carcinoma more closely than high-grade DCIS.
Apocrine PLCIS appears to be a biologically distinct subtype. The tumor cells show prominent apocrine cytology and strong expression of GCDFP-15, a known marker for apocrine differentiaiton. The lesions are seen predominantly in postmenopausal women with a mean age that is about a decade older than that of patients with both CLCIS and non-apocrine PLCIS. Apocrine PLCIS lesions are mostly (~80%) negative for ER and PR, demonstrate HER2
gene amplification in about one third of cases, and exhibit more genomic alterations. In contrast, the non-apocrine PLCIS are positive for ER and PR and lack HER2
gene amplification. In previous studies on PLCIS including pure PLCIS and those with invasion, all the pure PLCIS were reported to be positive for ER and none showed HER2
gene amplification (4
). Those cases appear to be similar to the non-apocrine PLCIS in our study. On the other hand, a recent expression profiling study demonstrated the “molecular apocrine type” in 3 of 4 pleomorphic invasive lobular carcinomas (27
), a phenotype that is likely similar to apocrine PLCIS in our study.
The genetic signature of 1q gain and 16q loss combined with the lack of E-cadherin expression observed in PLCIS supports the lobular lineage for PLCIS along the multi-step cancer progression pathway. It is unclear whether PLCIS arises from CLCIS and represents a genetically more advanced lesion than CLCIS, or if PLCIS originates from a yet unidentified precursor lesion. Cases with synchronous CLCIS and PLCIS will provide a special opportunity to explore the origin for PLCIS. In one informative case with synchronous CLCIS and apocrine PLCIS in our series, both lesions shared 1q gain and 16 q loss with additional chromosomal changes present in the apocrine PLCIS. Our findings are in agreement with a recent report by Reis-Filho et al. (20
) and suggest that at least some PLCIS may evolve from the same precursor or through the same genetic pathway as CLCIS. Based on the results of the current study and previous work, we propose a hypothetical pathway for PLCIS along the multi-step model of breast cancer progression (). Future studies with more cases to compare the genetic alteration in synchronous CLCIS and PLCIS will help to define more clearly the genetic relationship between PLCIS and CLCIS.
Figure 6 Hypothesized pathway for lobular neoplasia along a multistep model of breast cancer progression. Solid arrows indicate the predominant pathway, while dashed arrows indicate a potential pathway. TDLUs, terminal duct lobular units; +, gain; −, loss; (more ...)
The natural history of PLCIS is unknown, and thus the optimal management of these lesions is presently unclear. Currently, the bias of most experts is that PLCIS should probably be treated more like DCIS than like CLCIS. However, there are admittedly no clinical follow-up data to support this view which is based on observational data such as those presented here.
In conclusion, PLCIS lacks E-cadherin expression and demonstrates the characteristic 1q gain and 16q loss aCGH pattern supporting a relationship to CLCIS at the molecular level. However, the histologic features, often unfavorable biomarker profile, and extent of genomic alterations are concerning and imply that PLCIS may be a more clinically significant lesion than CLCIS. Furthermore, our study demonstrates clear morphologic and molecular heterogeneity in PLCIS and suggests that the morphologically distinct apocrine PLCIS may be a particularly aggressive lesion. However, clinical follow-up studies will be required to define the natural history and most appropriate management of these lesions.