Evaluation of assay performance
First, to confirm the accuracy of the NSG-based deep sequencing, we checked whether this method could distinguish the true existence of low-abundance mutants from background errors arising from polymerase chain reaction (PCR) or sequencing process. We constructed a TP53 mutant (NM_000546.6: c.844C>A) as a reference sample; then we utilized this TP53 mutant with serial concentrations of 100%, 10%, 1%, and 0.1% to test whether the experimental method could detect these mutants at these concentrations (Supplementary Methods). The results demonstrated that the signal from 0.1% mutant was significantly higher than background errors (Supplementary Figure S1A), suggesting NGS testing accurately detected mutants present at 0.1%. In addition, the mutation level could be measured with a linear fashion (R2 = 0.9997, Supplementary Figure S1B).
Second, in deep cell-free analyses, another source of variants that make it hard to distinguish cancer mutations is clonal hematopoiesis of indeterminate potential (CHIP)[27-29]. Although CHIP mutations mostly occur in the DNMT3A, TET2, ASXL1 and TP53 genes, pathogenic variants of TP53 are the main mutations in breast cancer tissue[30]. We examined the concordance of TP53 genetic alterations between ctDNA and DNA from diagnostic tumor biopsies, and 100% of them were compatible (Supplementary Figure S2), suggesting that genetic alterations of ctDNA originated from tumors.
Patients
A total of 95 patients were enrolled in this study. The median age was 50.0 years old. Forty-one patients had ER(+) Her2(-) breast cancer, 29 patients had Her2(+) breast cancer, and 25 patients had triple-negative breast cancer (TNBC). Before NAT, tumors with T1, T2 and T3-4 size classifications were found in three, 54 and 38 patients of each population, respectively. Eighty-two patients had positive axillary lymph nodes. According to standard clinical practice, ER(+) Her2(-) breast cancer patients with large tumors were treated with NAT. Out of the 95 patients, 77 patients received anthracycline while 80 patients received taxane in their NAT regimens. All Her2(+) patients received only trastuzumab or trastuzumab in combination with pertuzumab. After NAT, 13 patients achieved a pCR of their primary breast tumors; 82 patients had a non-pCR status. Among the 13 pCR patients, there was one ER(+) Her2(-), six Her2(+) and six TNBC patients. The frequency of pCR was significantly higher in patients with Her2(+) breast cancer or TNBC than ER(+)Her2(-) patients (p = 0.002). CtDNA was detected in 60 patients before NAT and 31 patients after NAT. All of the clinical and pathologic characteristics are shown in Table 1.
Genetic alterations in tumor ctDNA
Among the 95 patients, 19 patients were found to have ctDNA before and after NAT; 41 patients had ctDNA only before NAT, 12 patients had ctDNA only after NAT, and 23 patients had ctDNA neither before nor after NAT (Supplementary Table S1). The most common genetic variants were in the TP53 (n = 28), followed by PIK3CA (n = 16), CDH1 (n = 15), and Her2 (n = 7) genes. Eighteen patients had altered CNVs in their ctDNA, including of AKT1, CCND1, CDH1, c-MYC, Her2, PIK3CA, S100A, and ZNF703, either before or after NAT (Supplementary Table S1 and Figure 1). Before NAT, Patient #73 (Figure 1A) and Patient #24 (Figure 1B) exhibited copy number gains of the S100A and Her2 genes in ctDNA, respectively; after NAT, the copy numbers of these genes in ctDNA returned to normal levels. Patient #3 (Figure 1C) had a new copy loss of the PTEN gene after NAT. We observed gains of Her2 and c-MYC in patient #27 (Figure 1D) before NAT that were only partially resolved after NAT.
Association between ctDNA and clinical characteristics
Patients in whom ctDNA was detected before NAT tended to have a larger tumor size than those in whom ctDNA was not detected (mean 5.0 cm vs. 4.3 cm, p = 0.104, Supplementary Figure S3). However, the presence of ctDNA after NAT did not correlate with the tumor size or LN numbers after NAT. Although the difference was not significant, patients with pCR had a lower proportion of patients with detected ctDNA after NAT than patients with no pCR (patients with pCR vs. non-pCR: 15.4% vs. 35.4%, p = 0.132). (patients with pCR vs. non-pCR: 15.4% vs. 35.4%, p = 0.132). Additionally, the presence of ctDNA was not correlated with the molecular type of breast cancer.
Impact of clinical factors and ctDNA on RFS
The median follow-up time of the entire cohort was 5.1 years, and the 5-year recurrence-free survival (RFS) was 58% (95% CI 48.0 – 68.0%). For clinical factors, Kaplan-Meier analysis showed that the residual tumor size after NAT and N classification after NAT were prognostic factors for RFS; patients who achieved a pCR tended to have a better RFS than patients who did not achieve a pCR (Figure 2A – C and Table 2). On the other hand, patients with ctDNA after NAT had significantly inferior RFS (p < 0.001, Figure 2D). Other factors, such as age, ctDNA detection before NAT, molecular type, initial tumor size before NAT and N classification before NAT, did not influence RFS. RFS was similar between patients with and without TP53, PIK3CA and CDH1 mutations (Table 2).
We then analyzed the clinical and pathologic characteristics of patients with and without ctDNA after NAT, and no difference was found between the two patient groups (Supplementary Table S2). After incorporating the residual tumor size, N classification after NAT, pCR and ctDNA after NAT, multivariate analysis showed that an N3 classification and ctDNA positivity after NAT were independent risk factors that predicted tumor recurrence (N3, hazard ratio (HR) 3.352, 95% CI 1.267 – 8.870, p = 0.015; ctDNA, HR 4.135, 95% CI 2.014 – 8.491, p < 0.0001). Other factors did not significantly impact RFS (Table 2).
Next, we analyzed the 72 patients who had detected ctDNA, either before or after NAT. Patients with ctDNA positivity after NAT had a significantly inferior RFS compared to those without detectable ctDNA (Supplementary Figure S4, p<0.001). After adjusting for tumor size (after NAT), N classification (after NAT) and pCR, multivariate analysis with the Cox model revealed that ctDNA positivity after NAT was the most significant risk factor that predicted tumor recurrence (HR 8.02, 95% CI 3.24 – 19.86, p < 0.0001) (Supplementary Table S3).
The impact of ctDNA on disease recurrence in different molecular types of breast cancer
The median RFS of all the patients with ctDNA positivity after NAT was 1.19 years. When stratified by the molecular type, ctDNA positivity after NAT was associated with a significantly inferior RFS for ER(+) breast cancer or TNBC patients and a trend of higher recurrence rates for patients with the Her2 subtype (Figure 3A – 3C). The median RFS of ER(+) breast cancer, Her2 (+) breast cancer and TNBC patients with ctDNA positivity after NAT were 0.90, 2.52 and 0.74 years, respectively.
The impact of ctDNA on disease recurrence in patients with and without a pCR
For the entire cohort, the presence of ctDNA after NAT was a significant risk factor associated with recurrence in both pCR and non-pCR patients (Figure 3D and 3E, all p < 0.001). Because pCR was previously reported as a surrogate marker for survival in patients with Her2(+) and TNBC[7], we analyzed these patient subgroups. Between the two patient populations, pCR was related to a trend of improved survival than non-pCR (HR 3.328, 95% CI 0.777 – 14.243, p = 0.105, Supplementary Table S4). Multivariate analysis showed that advanced nodal status and ctDNA after NAT were independently correlated with high risk (N2-3, HR 3.753, 95% CI 1.146–12.297, p = 0.029; ctDNA, HR 3.123, 95% CI. 1.139 – 8.564, p = 0.027), and pCR status did show a not significant correlation with recurrence (Table 3). A potential reason for this phenomenon is that pCR only represents the therapeutic efficacy of local breast tumor and the ctDNA may indicate that an occult lesion is present that is not effectively treated with NAT. In our study, 13 patients achieved a pCR after NAT, and among those patients, two exhibited ctDNA positivity after NAT. One patient (case #50) had TNBC and received neoadjuvant docetaxel/epirubicin (four cycles) and achieved a pCR for their primary breast and axillary tumors. However, she had hepatic metastases at 6 months after mastectomy (Supplementary Figure S5). The other patient (case #5) had Her2-positive breast cancer and received neoadjuvant docetaxel/trastuzumab (four cycles) and epirubicin/cyclophosphamide (four cycles). The pathology showed no residual tumors. Trastuzumab was continuously maintained for one year. At the end of trastuzumab treatment (13 months after mastectomy), a cerebellar metastasis was found. The other 11 patients who achieved a pCR did not have ctDNA after NAT nor did they experience recurrence or metastasis.