In this retrospective study, the gene mutation landscape in metastatic breast cancer was compared across two different populations. The results of the mutational analysis and prevalence varied for particular alterations between the two Ethnic groups. Currently, cfDNA-derived gene mutational assays have gained much attention in breast cancer (Fig. 3A and B). In prior work, the cfDNA-derived mutations were detected using next-generation sequencing (NGS) platforms in almost all of the studies. However, the means of extracting, processing and analyzing cfDNA samples were different, which can lead to a limited ability to compare differences across populations.
This retrospective, multicenter study is the first proof-of-concept study to report on the use of a single harmonized ctDNA assay in clinically uniform populations of HR + MBC from two institutions in the US and CN. The populations analyzed included newly recurred and de-novo MBC patients who had not been exposed to treatment in the advanced setting, but who had received adjuvant endocrine therapy in similar proportion following standard-of-care guidelines. The study demonstrates that cfDNA analysis can provide a reliable real-world assessment of the molecular landscape of luminal breast cancer and identify differences in genomic abnormalities between patients in the US and CN.
For cancer variants that were known to interfere with endocrine therapy combined with or without PI3K/AKT/mTOR or CDK4/6 inhibitors, such as PTEN gene deletion (i.e., copy number loss) or ESR1 activating mutations and copy number gain were evaluated in the present study. Previous studies have shown that PIK3CA was mutated in 18–40% BC patients [25]. Some studies using cfDNA showed that PIK3CA mutations were present in 43.3%[12] and 25%[26] MBC patients in different studies. A recent study with cfDNA on a small cohort in CN observed that PIK3CA mutations were present in 29.4% (5/17) MBC patients [17]. In this study, we found significant difference of prevalence for AKT1 (18.5% vs. 3.1%, P = 0.008) between US and CN patients, respectively. While the percentage was all within the previously reported ranges in other studies, the difference in PIK3CA detection in our study may have been secondary to differences in features of the study populations and raise the fashinating hypothesis of differential molecular pathways of endocrine resistance when comparing Ethnic gropus treated with similar therapies.
In prior work, a mutational frequency of 0.4% was observed for ATM in 7,675 BRCA1 & 2-negative breast cancer patients in a Chinese population [27, 28] and a mutational frequency range from 0.45 to 1.0% was found in the US and Europe [29, 30]. However, no available data regarding ctDNA based comparison between US and CN have been reported. Here, we found that ATM loss was more frequently observed in CN patients with HR+/HER2- mBC as compared to US (16.9% vs. 0.0%, P = 0.017). Previous studies showed that the mutational prevalence of CDH1 detected using plasma of HR+/HER2- patients was 12.2% for US [31] and 5.0% for CN [21]. In contrast, our data showed that the CDH1 mutation occurred more frequently in CN as compared to US (44.6% vs. 22.9%, P = 0.021). Our data also showed the CNV mutation prevalence of FGFR1 gain was higher in CN patients when comparing to US (24.6% vs. 7.4%, P = 0.048). This result was in line with previous reports found in HR+/HER2- cohort of US (1.0 ~ 8.0%) and CN (13.0%) [12, 21, 31].
PTEN exerts its function in multiple ways including repressing tumor cell growth and cell survival. The nuclear PTEN exhibit phosphatase-independent tumor suppressive function such as regulation of chromosome stability, DNA repair, and apoptosis [32, 33]. PTEN loss has been identified in cfDNA of 25% mBC patients [26]. Here, we found that the difference of PTEN mutation frequency between the US and CN did not reach statistical significance (11.1% vs. 3.1%, P = 0.15). However, PTEN deletion, an agonist of PIK3CA/AKT pathway, was associated with shorter PFS of HR + MBC patients in CN (P = 0.03), but not in US, a finding that may have be limited by the smaller US sample size (P = 0.65) .
On the other hand, circulating ESR1 mutation is a poor prognostic factor in ER + MBC, Chandarlapaty et al reported that cfDNA ESR1 mutations were associated with shorter OS of MBC patients from the BOLERO-2 study [10]. A recent meta-analysis by Zhang et al. demonstrated that plasma ESR1 mutation carriers had significantly worse PFS compared to wild-type ESR1[34]. In the present study, the ESR1 mutation prevalence between the US and CN was not significantly different. Patients with cfDNA-based ESR1 copy number gain or mutation had shorter PFS compared to the other patients in the first-line treatment after relapse in the CN patient cohort (P = 0.023). Only two CN patients with ESR1 were received letrozole. However, this finding was not observed in US patients (P = 0.62). The results mentioned above may partly due to the limited number of patients included in the US cohort. Future sequencing efforts and clinical trials should include patients of diverse ethnic backgrounds to explore the impact of differences in genomic landscape on probability of benefit from treatments. Interestingly, there is a trend that patients with liver or lung meatstasis tend to have shorter PFS (P = 0.18) in CN cohort, which is not found in US cohort (P = 0.80). This may partly due to the massive application of CDK4/6 inhibitor in US patients, and this hypothesis need to be futher demostared in larger cohort. Besides, we also noted that lung metastatic rate was higher in CN group as compared to US group (P = 0.021). previous study demonstrated that HR-posive patients with non-visceral metastases had a better prognosis than those with visceral metastases[35]. Thus we believe the higher lung metastatic rate of CN group may contributes to the shorten PFS.