Background
Inflammatory breast cancer (IBC) is a breast adenocarcinoma defined by a rapid onset of inflammatory signs involving at least one-third of the breast, such as erythema and edema (also known as ‘peau d’orange’) [
1]. Although IBC is rare, constituting 1–5% of breast cancer cases, it harbors aggressive behavior with poor a prognosis and accounts for roughly 10% of breast cancer mortality annually [
2]. Compared to non-IBC, IBC frequently presents resistance to conventional therapies and early recurrence. Although therapeutic progress in the past two decades in the context of non-IBC has also had a positive impact in women with IBC, with a more than 22-month improvement in median breast cancer-specific survival (BCSS) and a 14% improvement in 2-year BCSS [
3], IBC is still a challenge for breast cancer physicians because of poor survival and lack of specific treatment. The clinical presentation and outcome of IBC are obviously different from those of non-IBC but there is no significant difference in treatment between IBC and stage III non-IBC. The poor understanding of the specific biological and molecular characteristics of IBC precludes specific therapeutic interventions. We urgently need to identify how and why IBC is distinct from non-IBC.
The ability to exploit the genetic information of a tumor for any clinical potential has only recently become evident. In this evidence-based precision medicine, genetic data have been exploited to identify therapies appropriate for an individual and has led to changes in drug oversight policy and the way certain drugs have been designated. As a special case of breast cancer mostly defined by clinical symptoms, IBC genome-specific maps are barely understood. Thus far, in previous studies, the IBC gene expression profiles demonstrated high transcriptional heterogeneity and heavy overall mutation burden compared with non-IBC [
4,
5]. The largest molecular biology research on IBC mainly focused on the transcriptome and demonstrated the presence of molecular subtypes similar to those of non-IBC tumors, although with over-representation of human epidermal growth factor receptor 2 (HER2)-enriched tumors and a low prevalence of Luminal A tumors, and suggested the deregulation of the expression of few genes in IBC compared with non-IBC, in particular those involved in cell motility, invasion, inflammatory pathways, and transforming growth factor (TGF)Beta signaling [
6‐
8]. Recently, some studies reported a higher frequency of
TP53,
PIK3CA, and
ERBB2 mutations in IBC than in non-IBC [
9,
10], but these studies were performed in a small series and need further study to draw any conclusions.
Therefore, there is a need to extensively describe the genomic alterations in IBC to identify pathways involved in metastatic processes and drug resistance and to generate new treatment strategies for IBC patients. We have designed a breast cancer and targeted treatment-associated gene panel and performed targeted next-generation sequencing (NGS) in a large cohort of 156 IBC samples. Using the clinicopathological data and long-term survival follow-up, the association of the IBC mutational landscape with clinical outcomes was studied.
Discussion
A key purpose of precision cancer medicine is to tailor clinical management based on the specific events that are relevant to tumor development and progression. The high frequency of clinically relevant genomic alterations in IBC when sequenced with a targeted NGS raises the possibility that targeted therapies may be developed for patients with this highly aggressive form of breast cancer. First, a heavy mutation burden was found in IBC tumors. This could be the hallmark of increased genomic instability correlating with tumor aggressiveness.
TP53 was the most frequently mutated gene, in accordance with previous studies on IBC [
7,
10,
20]. There were 12 genes with more than 5% mutation frequencies in IBC:
TP53,
PIK3CA,
MYH9,
NOTCH2,
BRCA2,
ERBB4,
FGFR3,
POLE,
LAMA2,
ARID1A,
NOTCH4, and
ROS1. For
TP53,
PIK3CA, and
BRCA2, high mutation rates in IBC have been also reported by other groups. In contrast, we did not detect higher frequent mutation of
ERBB2,
RB1, or
NOTCH1 [
7,
9,
10].
Comparative analysis of biology pathways between IBC and non-IBC revealed high mutation frequencies of genes in DNA repair, NOTCH, and RTK/RAS/MAPK pathways that could be clinically relevant. The alteration of
BRCA1/BRCA2/POLE genes of the DNA repair pathway was independent of molecular subtypes, so PARP inhibitor may be especially evaluated in IBC [
21]. To the best of our knowledge, this is the first time that
POLE has been detected frequently mutated in IBC. The correlation of
POLE mutation with PD1/PD-L1 immunotherapy in colorectal cancer and endometrial cancer [
22‐
24] leads us towards further research to explore whether there is a treatment option for immunotherapy in
POLE-mutated IBC. Note that POLE-detected variants are not hotspots pathogenic variants that have already been described.
NOTCH1/2/4 and
FBWX7 genes were more frequently mutated in each subgroup of IBC compared with non-IBC. A preclinical study in IBC showed that a gamma secretase inhibitor, RO4929097, was able to block the Notch signaling and to attenuate the stem-like phenotype of IBC cells and regulate the inflammatory environment [
25]. Targeting the Notch pathway might be an option for IBC treatment. Receptor tyrosine kinases (RTKs) are frequently activated in cancer cells and therefore have become the target of numerous treatments. The BreastCurie gene panel included most targetable RTK genes and we found that IBC carried higher frequencies of unknown pathogenic variants of RTKs than non-IBC. Higher gene instability due to DNA repair dysfunction may promote variants of unknown significance in IBC but we cannot exclude that other unknown mechanisms are also implicated. Activation of downstream pathways of RTKs, such as ERBB2, EGFR, and IGF1R, has been proven to be related to tumor cell anoikis resistance, and IBC cells have been associated with more evasion of anoikis [
26,
27] which is consistent with our findings. We now aim to explore whether the frequent unknown pathogenic mutations of RTKs on
ERBB4,
FGFR3,
EGFR, and
ERBB2 reported in the present study are potential therapeutic targets.
Compared with the breast cancer literature [
10,
12,
13,
28], we did not find IBC-specific amplified genes with our gene panel. Unfortunately, some frequent CNAs of breast cancer (e.g.,
MYC,
CCND1 amplification) reported previously were not included in our gene panel. We detected
STAG2 deletion, a tumor suppressor gene coding cohesion protein, in 3.7% of IBC.
STAG2 loss of function was reported in different cancers but not in IBC [
29]. However, ONCOCNV did not compute allele frequencies, which may affect the precision of the method in admixed data [
15].
Our study demonstrates that
PIK3CA gene mutations and PIK3CA/AKT/mTOR pathway alteration were very common events in IBC.
PIK3CA gene mutations were especially observed in luminal and HER2-positive subtypes, and mainly located in hotspots of the helical domain and the catalytic domain, similar to non-IBC in previous reports [
13,
30]. Recently, a large pooled analysis of more than 10,000 early-stage breast cancer patients reported that PIK3CA-mutated tumors are associated with a better prognosis [
31]. However, this good prognostic effect was observed in HR
+/HER2
– and TNBC subtypes, but not in the HER2
+ subtype where
PIK3CA mutations were associated with a worse overall survival. Interestingly, in our IBC cohort,
PIK3CA mutation was a poor prognostic factor for MFS for HER2
+ and TNBC subtypes, whereas no prognostic value was found in the HR
+/HER2
– subtype. Of note, the prognostic effect was weak in the TNBC subtype of IBC since
PIK3CA mutations were rare in this subtype and our TNBC cohort was small. For the HER2
+ subtype, previous studies reported that
PIK3CA mutations were associated with adverse prognosis in non-IBC, but results were not conclusive [
31‐
34]. As
PIK3CA mutation could lead to resistance to anti-HER2 treatments [
34,
35], we checked that the percentage of patients in our IBC cohort receiving trastuzumab combined with chemotherapy was balanced in both
PIK3CA genotypes (71.4% in mutant type, 75.9% in wild type). Therefore, the association between
PIK3CA mutation and worse MFS in IBC may be reliable. For the HR
+/HER2
– subtype, the prognostic difference regarding
PIK3CA genotype between IBC and non-IBC may reflect the influence of the PI3K pathway in the two distinct biological environments of IBC and non-IBC, and we presume interactions between ER and PI3K pathways are different. We know that PI3K inhibitors have been investigated in many breast cancer trials and have shown promising results in ER-positive endocrine therapy-refractory breast cancer [
36], but no clinical trials have been performed specifically in IBC to date. The association of
PIK3CA mutations with worse MFS in IBC should draw our attention to the role of the PI3K pathway in this aggressive and treatment-refractory form of breast cancer. Further experimental research to explore the PI3K pathway in IBC is therefore required.