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552 Background: Extensive efforts by The Cancer Genome Atlas (TCGA) Network had provided much of our current understanding of the molecular profile of various solid tumors including breast tumors; however, Asian patients were underrepresented in this cohort. In this study, we aimed to elucidate the comprehensive genetic alteration profile of early-stage breast tumors among Chinese patients. Methods: Surgical tissue samples from 589 Chinese women with stage I-III breast cancer with various histology and molecular subtype consecutively diagnosed at Guangdong Provincial People’s Hospital were sequenced using a panel targeting 520 cancer-related genes spanning 1.64Mb of the human genome. Clinical and genomic data from our cohort was compared with publicly-available data from 1,046 stage I-III breast cancer patients from the TCGA dataset. Results: Based on the analysis of the genetic alteration profile from our cohort, at least one genetic alteration was observed from 98% of the tumor samples, with TP53 (47%), PIK3CA (45%), ERBB2 (30%), and CDK12 (18%) as the most commonly altered genes. The most common genetic alteration types were copy number amplification (43.6%) and missense mutations (36.8%). As compared with the TCGA dataset, our cohort is mostly composed of women 50 years and younger (59.1% vs. 30.4%, P< 0.001), with significantly fewer patients with lobular carcinoma histology (2.4% vs. 19.0%, P< 0.001), and significantly more patients with pathologic stage I tumors (23.3% vs. 17.3%, P= 0.012). Consistently, genetic alterations detected from our cohort affected genes involved in PI3K (63% vs. 56%, P= 0.009) and cell cycle (23% vs. 35%, P< 0.001) pathways, with statistically different genetic alteration rates as compared with the TCGA dataset. Comparison of genetic alteration profile between the two cohorts revealed that our cohort had more frequent genetic alterations in genes including PIK3CA ( P< 0.001), TP53, particularly in hotspot mutations Q192* ( P< 0.001) and A307V/del ( P= 0.02), and ERBB2 amplification ( P< 0.001). Conclusions: Our study contributes to the understanding of the key pathways and specific genetic alterations harbored by Chinese patients with early-stage breast cancer that could potentially be developed as markers of treatment response to targeted therapeutics.

553 Background: HER2 expression or amplification qualify patients to receive targeted therapeutics against HER2; however, traditional methods of quantifying HER2 amplification using fluorescence in situ hybridization (FISH) do not include a reliable definition for low level amplification. With the promising response rate of patients with low HER2 amplified-metastatic breast cancer to subsequent-line trastuzumab deruxtecan (DS-8201a) therapy, there is a need to improve the existing criteria to accurately identify patients with low HER2. In our study, we investigate whether HER2 amplification quantified by NGS could provide a method to stratify patients into subgroups. Methods: A total of 774 patients diagnosed with breast cancer from Guangdong Provincial People's Hospital (GDPH) who underwent targeted NGS using 520 or 33 cancer-related genes and had their HER2 status evaluated with either FISH or IHC were included in this study. HER2 status were defined as per 2018 ASCO/ACP guidelines. Results: Our results demonstrate that NGS could quantify HER2 amplification with high sensitivity and specificity, with area under the curve of 0.990 [95%CI: 0.982-0.999]. The receiver operating curve indicated an optimal cut-off of 2.62 copy number (CN) for identifying IHC/FISH HER2-negative status with 97.8% specificity. Meanwhile, the cut-off of ≥ 3.62 CN identified patients with IHC/FISH HER2-positive status with 99.8% specificity. Among the 774 patients, 65.8% (n = 509) had HER2 CN of ≤ 2.62 and were classified as HER2 non-amplified, while 25.8% (n = 199) had HER2 CN of ≥ 3.62, classified as HER2-amplified. The remaining 66 patients (8.5%) had HER2 CN between 2.62 and 3.62, and were the patients with heterogeneous IHC/FISH results, classified using NGS as HER2 low-amplified. Patients with low-amplified (49.0% vs. 38.8%, P < 0.001) and amplified (50.3% vs. 38.8%, P < 0.001) HER2 had significantly more number of copy number amplifications in other gene, including CDK12, RARA, and SPOP (P < 0.001, P < 0.001) than patients with HER2 non-amplified, indicating distinct mutation profile. Conclusions: Our results demonstrate that NGS could provide a more accurate stratification of patients based on their HER2 amplification levels. Patients with low levels of HER2 amplification has a distinct mutation profile, suggesting that NGS could serve as a robust tool to identify patients with HER2 amplification, whether high or low, who could benefit treatment with targeted agents designed against heterogeneous HER2 expression.

3537 Background: Pathologic(p)stage I lung adenocarcinoma (LUAD) patients exhibit high levels of genetic heterogeneity and the association between the genomic characteristics of (p)stage I LUADs and tumor recurrence remains poorly understood. Circulating tumor DNA (ctDNA) monitoring after resection represents a useful tool to predict response to therapy and tumor recurrence but its application in (p)stage I LUAD patients remains controversial. In addition, it is of great clinical interest to decipher the difference of genetic features between ground-glass opacity (GGO) and solid nodules (non-GGO) subgroups. Methods: Tumor tissues and matched post-operative plasma samples were collected from a total of 86 Chinese (p)stage I LUAD patients who were enrolled in a clinical study (NCT03172156). Comprehensive genomic profiling was performed using capture-based hybrid next generation sequencing by targeting 422 cancer relevant genes. Results: EGFR and TP53 represent commonly mutated genes in this cohort of (p)stage I lung adenocarcinoma, followed by alterations in ALK, PIK3CA, STK11and MYC. For a median follow up period of 21.54 months after surgical resection, we observed that ctDNA positivity significantly correlated with an increased probability of early tumor recurrence or metastasis ( P= 0.03, HR = 7.9), and in particular, the EGFR mutation status of ctDNA samples rather than that of primary tumor samples significantly correlated with shorter disease-free survival (DFS). Further comparison between GGO and non-GGO subgroups indicated that the frequency of TP53 mutations in non-GGO was markedly higher than that in GGO (48% vs 20%, P< 0.05). In addition, pathway analysis showed that the epigenetic regulation pathway was more frequently affected in the GGO subgroup, while impaired apoptosis/cell cycle pathway was more enriched in the non-GGO LUADs. Conclusions: Our data show that ctDNA positivity, including the EGFR mutation status, significantly correlated with early relapse or metastasis after surgery, representing a useful tool to predict treatment response and tumor relapse in (p)stage I LUAD patients. Mutated TP53 was more abundant in non-GGO comparing to GGO (p)stage I LUADs that may act as potential oncogenic driver in LUAD development and/or disease progression. Clinical trial information: NCT03172156 .

554 Background: Limited studies have investigated the molecular underpinnings driving breast cancer development in Chinese younger women. Based from our previous data, more Chinese women are diagnosed with early-onset breast cancer than in the West. In our study, we aim to investigate the comprehensive mutational profile of Chinese women 35 years old and younger (≤35y) diagnosed with breast cancer. Methods: Targeted sequencing was performed on surgically-removed tumor tissues and blood samples collected from 589 women diagnosed with stage I-III breast cancer of various molecular subtypes at the Guangdong Provincial People’s Hospital (GPDH) using a gene panel interrogating 520 cancer-related genes. We compared the data of 53 women aged ≤35y from our cohort to the data from 33 breast cancer patients aged ≤35y included in The Cancer Genome Atlas (TCGA) dataset. Results: Among the women aged ≤35y with early-stage breast cancer from both cohorts, our cohort had more number of hormone receptor-positive (HR+) patients (GPDH, 72% vs. TCGA, 61%, P< 0.001). Analysis revealed an overall mutation detection rate of 98% in our cohort, with mutations affecting genes involved in the PI3K pathway (47%) and cell cycle pathway (23%). TP53 and PIK3CA were the most commonly mutated genes, with mutation rates of 51% and 25% from our cohort. No statistical difference in mutation profile was found between GPDH and TCGA cohorts. Moreover, germline mutations considered as pathogenic and likely pathogenic (P/LP) in breast cancer susceptibility genes including BRCA1 (n = 4), BRCA2 (n = 2), PALB2 (n = 1), PMS2 (n = 1), PTEN (n = 1), and ATM (n = 1) were detected from 18.9% (10/53) of the patients from our cohort. Women aged ≤35y had significantly more germline BRCA1 mutations than patients > 35y from our cohort (7.5%, 4/53 vs. 2.1%, 11/536 P= 0.049). Conclusions: Our study has identified the involvement of PI3K and cell cycle as the two key pathways in the early development of breast tumors in younger women. In addition, our results also support the role of P/LP germline mutations in breast oncogenesis in Chinese patients with early-onset breast cancer, indicating the need to include a more comprehensive germline mutation screening in our population.

543 Background: HER2 targeted therapy has revolutionized the survival outcomes of early and advanced HER2+ breast cancer (BC). However, among HER2+ patients, the therapeutic response to HER2 inhibitors vary. To understand the molecular mechanism of the variability in therapeutic efficacies, the mutational landscape of HER2+ tumors need to be elucidated. Methods: 107 HER2+ Chinese stage I-III BC patients were included in the study, including 64 HR+ and 43 HR- patients. A majority of the patients were diagnosed with infiltrating ductal carcinoma (99/107). Capture-based targeted sequencing was performed using a panel consisting of 520 cancer-related genes spanning 1.64 MB of the human genome. Results: 1,119 alterations were detected, including 478 single nucleotide variants (SNVs), 14 insertions or deletions, 29 fusions, 593 copy number amplifications (CNA), 2 large genomic rearrangements and 3 CN deletions in 267 genes. Alterations in 99 genes were shared between HR+/HER2+ and HR-/HER2+ tumors; while 123 and 45 genes were only detected in either HR+/HER2+ or HR-/HER2+ tumors, respectively. CNA, splice site and frameshift mutations were significantly more in HR+/HER2+ patients ( p= 0.017). Specifically, CNA in SPOP, CCND1, FGF19, FGF3, FGF4, RNF43, RAD51C, ADGRA4 and MDM4 and various alterations in GATA3 were significantly more among HR+/HER2+ tumors ( p< 0.05). In addition to HER2 amplifications, concurrent fusions in ERBB2 (67%, 4/6), SNVs in ERBB3 (100%, 3/3) and ERBB4 (100%, 1/1) were more likely to be detected in HR+/HER2+ tumors, while concurrent EGFR amplifications were exclusively detected in HR-/HER2+ tumors. The trend of concurrent mutations was consistent with mutation types detected in HER2- tumors based on HR status, wherein EGFR amplifications were more frequent in HR-/HER2- tumors, while SNVs in EGFR, ERBB2, ERBB3 and ERBB4 were more predominant in HR+/HER2- tumors. Based on KEGG pathway analysis, HR+/HER2+ tumors had more frequent alterations in TGFb ( p= 0.007), WNT ( p= 0.002) and homologous recombination ( p= 0.004) pathways than HR-/HER2+ tumors. Furthermore, our data revealed that HR+/HER2+ and HR-/HER2+ patients had comparable TMB ( p= 0.24), with a median TMB of 4.0 mutations/Mb for both. Conclusions: Our study revealed genetic heterogeneity between HR+ and HR- HER2+ tumors. The distinct genetic alterations are potentially relevant in the development of optimal treatment strategies for such patients.

579 Background: The standard management of early stage human epidermal growth factor receptor 2 (HER2) positive (+) breast cancer (BC) involves neoadjuvant therapy with combination of chemotherapy and HER2-targeted therapy followed by surgery. However, diverse pathologic responses were observed. We interrogated whether baseline genomic heterogeneity contributes to the varied therapeutic responses. Methods: Capture-based targeted sequencing using a panel consisting of 520 cancer-related genes, spanning 1.6MB of human genome, was performed on tissue biopsy samples, obtained prior to neoadjuvant therapy, of 33 HER2+ women with stage I-III BC. The median age of the cohort was 53. The correlation between genomic alterations and pathologic response were analyzed by multivariate analysis. Results: A majority of them was diagnosed with stage II (67%, 22/33), while 30% (10/33) had stage III and 3% (1/33) had stage I disease. 58% (19/33) were HR+ and 42% (14/33) were HR-. Mutation profiling of baseline samples revealed 349 mutations spanning 145 genes, with TP53, CDK12 and PIK3CA being the top 3 most frequently mutated genes. Neoadjuvant regimen was comprised of trastuzumab and HER2 inhibitor (i.e. pertuzumab or lapatinib). 15 patients used single HER2 inhibitor;18 used dual HER2 inhibitors. Endocrine therapy was also administered to HR+ patients (19/33) in combination with trastuzumab and HER2 inhibitor. Complete pathologic response (pCR) was observed in 45.5% (15/33) of patients. Interestingly, ROS1 copy number amplifications (CANs) were only identified in patients achieved pCR (p = 0.033). In contrast, missense mutations in PIK3CA and CNAs in CCND1, FGF19, FGF3, FGF4, SPOP, HNF1B and BRIP1 showed a trend of being less likely to mutate in pCR patients (p values between 0.05-0.1). Previous reports have suggested that pCR rates in HER2+ patients are associated with HR status. However, our data revealed comparable pathologic response of patients based on either HR status or neoadjuvant regimen. Conclusions: Our data revealed a distinct mutational profile between patients achieved pCR vs patients did not. Further studies with a larger cohort are required to confirm these findings.