Given its high histological grade, IBC is more frequently estrogen-receptor (ER)-negative than non-IBC 11020. Up to 60% of IBCs lack hormone receptor expression, whereas other forms of LABC are more likely to be ER-positive 810.

Higher proliferation rates than in non-IBC, including other forms of LABC, have been observed with several methods (labeling index, Ki67 and S-phase fraction) 101221.

As alterations to ERBB2, EGFR and TP53 can be associated with greater breast tumor aggressiveness, their roles in IBC have also been investigated.

In a series of 80 IBC samples, ERBB2 assessed using Southern blotting was amplified in 41% of cases, compared with 19% of 141 non-IBC samples 22. Using the same method, Prost and colleagues found that 39% of 96 IBC tumors were amplified for ERBB2, compared with only 18% of 224 non-IBC tumors 23. ERBB2 mRNA expression, studied with northern blotting, is also high in IBC (overexpressed in 61%, compared with 39% in non-IBC) 22. In the same study, EGFR transcripts were observed more frequently in IBC than in non-IBC 22. Based on real-time quantitative RT-PCR, ERBB2 mRNA overexpression is also more frequent in IBC (36%) than in non-IBC (26%) 17.

TP53 is mutated in approximately 30% of breast tumors. TP53 mutations can be detected at the nucleic acid level or at the protein level using immunohistochemistry, although this technique is not always reliable 24. The TP53 gene is more frequently mutated in advanced-stage breast cancer and/or aggressive breast cancer. Likewise, TP53 is mutated in 30–60% of IBCs, with most mutations located in exons 5–8, the mutational hot-spot of TP53 17252627. Based on a functional TP53 expression assay in yeast 57% of 63 IBC samples were mutated, compared with 37% of 27 non-inflammatory LABC samples 17.

Despite a few discrepancies 2128, the protein expression of the ERBB2, EGFR and TP53 genes studied by means of immunohistochemistry is higher in IBCs than in non-IBCs 162129. Two recently published tissue microarray-based studies of 80 and 34 IBCs, compared with 552 and 41 non-IBCs, respectively, confirmed these results 1230.

IBC is thus more likely than non-IBC to be hormone receptor-negative, rapidly proliferative, and ERBB2-positive, EGFR-positive, and TP53-positive.

The rapid onset of inflammatory signs and the very high rate of distant metastasis suggest that cytokines, growth factors, and angiogenic factors are involved in IBC. It appears, however, that IBC tumors produce negligible levels of most inflammatory cytokines, including interferon gamma, IL-1, and IL-12 9.

We used real-time quantitative RT-PCR to quantify the mRNA expression levels of 538 selected cancer genes in human IBC relative to non-inflammatory LABC. None of the best-known inflammatory cytokines (IFNγ, TNF, IL-1A, IL-1B, IL-8, and IL-10) was overexpressed in IBC compared with the other samples, tending to confirm that the inflammatory phenotype of IBC is due to dermal lymphatic blockage by tumor cells rather than due to infiltration by inflammatory cells 31.

In addition to being lymphotactic, IBC tends to be highly angiogenic. Using the Weidner method of quantification to study 45 IBC and 22 non-stage-matched non-IBC samples, McCarthy and colleagues observed a significant increase in intratumoral microvessel density in IBC 28. Colpaert and colleagues obtained histological evidence of intense angiogenesis in samples of 35 IBC samples compared with 104 non-IBC samples, especially in terms of the endothelial cell percentage 32. This intense angiogenesis could be due to hypoxia, as a correlation has been observed between the hypoxia marker carbonic anhydrase IX and endothelial cell proliferation 32. Among nine angiogenic factors quantified by means of RT-PCR (vascular endothelial growth factor [VEGF], VEGFR1, VEGFR2, Ang-1, Ang-2, TIE-1, TIE-2, COX-2, and basic fibroblast growth factor [bFGF]), the same authors found that Ang-1, TIE-1, TIE-2, and bFGF were strongly expressed in IBC when compared with non-IBC 33. High levels of VEGF, bFGF, IL-6 and IL-8 have been found in IBC samples by another group 9.

We identified several angiogenesis-related genes (namely VEGF, TBXA2R, PTGS2/COX2, THBD/thrombomodulin, and ANGPT2/angiopoietin 2) that were upregulated in IBC. However, other major angiogenic genes (VEGF2, VEGF3, VEGF4, VEGFR1, VEGFR2, and VEGFR3) had similar expression levels in IBC and non-IBC 31.

SUM-149 and SUM-190 are two cell lines established from primary IBC tumors 34. Both are able to form tumors in nude mice after mammary fat pad injection. They have been characterized with respect to their ER, TP53, and other classical gene expression status. These cell lines have been studied by comparative genomic hybridization and by differential display technology, and the results are outlined in the next section 3435.

Some insights into the pathogenesis of IBC have also been obtained by studying two human IBC xenografts designated MARY-X and WIBC-9 3637. Nude mice transplanted with MARY-X inflammatory breast carcinoma xenografts develop tumors within lymphatics and blood vessels, and erythema of the overlying skin, as in human IBC 36. Like most human IBC samples, MARY-X is ER-negative, progesterone receptor-negative, p53-positive, and epidermal growth factor receptor-positive, but is Her-2/neu-negative. MARY-X has been screened for many molecules: integrin and immunoglobulin families, angiogenic factors of the VEGF, fibroblast growth factor, and transforming growth factor families, and candidate proteases and their receptors.

Only two molecules were overexpressed relative to non-inflammatory xenografts, namely E-cadherin and MUC1 36. Expression of E-cadherin is thought to be 'lost' during malignant progression. Surprisingly, E-cadherin overexpression and overfunction is present in MARY-X relative to normal non-IBC cell lines and xenografts 3638. This overexpression was observed using a primary anti-E-cadherin antibody to examine the pulmonary lymphovascular emboli spontaneously produced in MARY-X-grafted animals 38. Interestingly, the anti-E-cadherin antibody caused the emboli to disappear. According to that finding, dominant-negative E-cadherin mutants transfected with MARY-X are weakly tumorigenic and do not form lymphovascular emboli. Two studies have reported E-cadherin immunoreactivity within lymphovascular emboli in 90–100% of human IBC samples 1638. At the functional level, E-cadherin may cooperate with sialyl-Lewis X/A-deficient MUC1 to favor passive dissemination of tumor emboli, resulting in more frequent and larger pulmonary metastases in MARY-X 39.

The other inflammatory breast cancer xenograft WIBC-9, originating from a patient with IBC, frequently metastasized and induced erythema of the overlying skin, like MARY-X and human IBC 37. WIBC-9 exhibits aneuploidy and ERBB2 gene amplification, and lacks hormone receptors. Among the four epidermal growth factors, 18 angiogenic genes and six cytokines, only seven genes (namely IL-8, VEGF, bFGF, angiopoietin 1, flt-1, Tie-2 and Tie-1) are overexpressed in WIBC-9 when compared with non-inflammatory models 37. The therapeutic potential of VEGF and angiopoietin pathway blockade was explored by injecting vectors encoding soluble Flt-1 and Tie-2 into WIBC-9. Both treatments inhibited the growth of WIBC-9 and suppressed WIBC-9 lung metastasis 40.

Despite some discordant results, these in vitro and in vivo models have facilitated the characterization of IBC.

None of the genetic alterations cited is specific to the inflammatory phenotype of IBC. Other genes are likely to be specifically involved in IBC.

The two inflammatory breast cancer cell lines SUM-149 and SUM-190 have been characterized by means of comparative genomic hybridization: frequent deletions were observed at 3p, 8p, 11p, 11q and 13q, with gains at 1q, 7q, 8q and 17q, suggesting the presence of candidate genes in these regions 34.

In a series of 66 human IBC samples, we found a 52% overall rate of loss of heterozygosity using 71 micro-satellite markers located at 21 chromosomal regions on 12 chromosomal arms associated with primary breast cancer (1p, 3p, 6p, 6q, 7q, 8p, 9p, 11p, 11q, 16q, 17p, and 17q), pointing to high genomic instability in inflammatory breast tumor cells 41. Furthermore, 3p21-p14, 6p, 8p22, 11q, 13q14, and 17q21 were more frequently altered in IBC than in non-IBC. These findings should facilitate the identification of candidate suppressor genes in IBC.

Candidate genes may also be identified by global analytical approaches to gene expression. By comparing the expression of transcripts from SUM-149, actively growing normal mammary epithelial cells, and the patients' matched lymphocytes, van Golen and colleagues identified 17 differentially expressed genes. Only two genes, RhoC GTPase and the lost in inflammatory breast cancer gene LIBC (also known as WISP3), were respectively overexpressed and underexpressed in inflammatory versus stage III non-inflammatory breast tumor samples by in situ hybridization 35. LIBC was underexpressed in 80% of IBC samples versus 21% in non-inflammatory LABC samples, and RhoC was overexpressed in 90% and 36%, respectively 35.

LIBC/WISP3 appears to be a good candidate gene in IBC: it is located at 6q22-q23, a frequently deleted region in breast tumors, and LIBC/WISP3 seems to be a member of the IGFBP family, already known for a role in cancer progression 35. Finally, functional data indicate that LIBC/WISP3 acts as a tumor suppressor gene in the breast, and that loss of its expression contributes to the proliferative and invasive phenotype of IBC 42.

Using stable human mammary epithelial-RhoC transfectants, van Golen and colleagues demonstrated that RhoC GTPase is a transforming oncogene in human mammary cells, leading to a highly invasive phenotype akin to that seen in IBC 43. RhoC GTPase expression seems to be modulated by WISP3, and these two genes might thus act in concert to give rise to IBC 44. Using quantitative RT-PCR, however, we found RhoC overexpression only in three of 32 inflammatory breast tumor samples 17.

Among the 538 selected genes that we studied by real-time quantitative RT-PCR in IBC samples, 27 (5.0%) were significantly upregulated compared with non-inflammatory LABC 31. None of the genes was downregulated. The 27 upregulated genes encoded seven transcription factors (JUN, EGR1, JUNB, FOS, FOSB, MYCN, and SNAIL1), four of which are components of the AP-1 transcription factor family (JUN, JUNB, FOS, and FOSB). AP-1 has been implicated in a variety of tumoral processes 45. The other 20 upregulated genes encoded growth factors (VEGF, DTR/HB-EGF, IGFBP7, IL-6, ANGPT2, EREG, CCL3/MIP1A, and CCL5/RANTES) or growth factor receptors (TBXA2R, TNFRSF10A/ TRAILR1, and ROBO2), mainly involved in angiogenesis. Finally, IBC and non-IBC showed similar expression levels of the genes WISP3/LIBC, RhoC and E-cadherin, a finding that conflicts with several other reports 31.

One should be cautious in interpreting biological studies on IBC since many of them have been performed at the RNA level, which may not reflect the functional protein level.

By definition, patients diagnosed with IBC have at least stage IIIB disease. Up to 30% of these patients become long-term disease-free survivors, however, and it would be of interest to be able to identify those patients requiring intensified, prolonged, or novel therapies. Unfortunately, specific prognostic factors are lacking in IBC.

Although a large initial tumor size has been linked to a poorer prognosis 4647, this criterion would not be very useful as a measurable tumor mass is rare in patients with IBC. Lymph node involvement at diagnosis has also been linked to poorer outcome 4849. However, physical examination can overestimate lymph node status in up to 50% of cases and cytologic evaluation is rarely done.

In an attempt to refine prognostication in IBC, Gustave-Roussy investigators developed a staging system named 'Poussee Evolutive' (PEV) based on signs of inflammation and tumor aggressiveness 50. At diagnosis, about one-third of IBC patients have PEV3 tumors, with inflammation involving the entire breast, whereas the remaining patients (PEV2) have only localized breast inflammation (less than 50% of the breast). We sought correlations between several molecular features, clinicopathological features, and clinical outcome in a series of 64 IBC patients 51. Respectively one-third and two-thirds of the 64 patients had PEV3 tumors and PEV2 tumors. With the exception of stage IV disease, extensive breast inflammation (PEV3) at the first clinical examination was the main factor associated with poor outcome, in keeping with previous studies 48495152.

The presence of dermal lymphatic invasion, while not necessary for diagnosis, could also be associated with poor prognosis 1853, but this is controversial 1124654.

The response to chemotherapy may be the best prognostic indicator in IBC, as in other forms of LABC and operable breast tumors first treated with first-line chemotherapy. The prognostic value of the response to chemotherapy has mainly been studied in terms of clinical characteristics. There is increasing evidence, however, that the pathologic response, and particularly a complete response, could predict outcome more reliably 5556. In IBC, the clinical response has been correlated with survival 48525758. The few papers reporting pathological responses tend to confirm the prognostic value of this criterion 4596061.

It is now recognized that IBC patients who respond to chemotherapy are candidates for a mastectomy and axillary lymph node dissection 46263. As in 'not otherwise specified' breast tumors, lymph node involvement at this stage has prognostic value 71157.

Classical markers of poor outcome in breast cancer have been thought to contribute to poor outcome in IBC too. For example, ER negativity has been linked to poor prognosis in inflammatory breast tumors by some authors 1205262, but not by others 1051.

As in non-IBC, the prognostic value of ERBB2 and TP53 is controversial 12222326. ERBB2 amplification was not associated with poorer outcome in a series of 67 IBC patients, contrary to a series of 178 non-IBC patients 23. Applying multivariate analysis to data on 24 patients with IBC, Riou and colleagues found a positive correlation between p53 nuclear overexpression and poor clinical outcome 26. In another series of 32 IBC patients, TP53 mutations were associated with large tumors and metastases at diagnosis 25.

In a recent immunophenotypic study of 80 IBC patients compared with 552 non-IBC controls, tissue arrays were used to determine the protein expression of ER, progesterone receptor, EGFR and ERBB2, MIB1, P53, MUC1 and E-cadherin 12. Five variables were significantly associated with IBC in multivariate analysis: MIB1, ERBB2 and E-cadherin overexpression, ER negativity, and MUC1 cytoplasmic staining. This five-gene molecular signature of IBC was further explored for its possible prognostic significance. The 5-year survival rates of patients with IBC were not significantly different from those of non-IBC controls fulfilling four or five of these criteria. Furthermore, this non-IBC group had a significantly worse outcome than their counterparts with between none and three of these criteria 12.

Among 538 selected genes, we identified a three-gene expression profile – based on MYCN, EREG, and SHH – which discriminated subgroups of IBC patients with good outcome, intermediate outcome, and poor outcome 31. EREG (epiregulin) belongs to the EGF growth factor family that binds both ERBB1 and ERBB4. SHH codes for the most important molecule of the Hedgehog-Gli signaling pathway. Inappropriate activation of the SHH pathway occurs in several malignancies 64. Although unexpected in the context of breast tumorigenesis, MYCN was recently identified as a major direct target of SHH pathways 65. This three-gene expression signature of poor-prognosis IBC warrants validation in larger series.