Collins, Laura C.

PURPOSE Erb-B2 receptor tyrosine kinase 2 ( ERBB2)–positive breast cancer (BC) is particularly common in young women. Genomic features of ERBB2-positive tumors before and after chemotherapy and trastuzumab (chemo + H) have not been described in young women and are important for guiding study of therapeutic resistance in this population. METHODS From a large prospective cohort of women age 40 years or younger with BC, we identified patients with ERBB2-positive BC and tumor tissue available before and after chemo + H. Whole-exome sequencing (WES) was performed on each tumor and on germline DNA from blood. Tumor-normal pairs were analyzed for mutations and copy number (CN) changes. RESULTS Twenty-two women had successful WES on samples from at least one time point; 12 of these had paired sequencing results from before and after chemo + H and 10 had successful sequencing from either time point. TP53 was the only significantly recurrently mutated gene in both pre- and post-treatment samples. MYC gene amplification was observed in four post-treatment tumors. Seven of 12 patients with paired samples showed acquired and/or clonally enriched alterations in cancer-related genes. One patient had an increased clonality putative activating mutation in ERBB2. Another patient acquired a clonal hotspot mutation in TP53. Other genomic changes acquired in post-treatment specimens included alterations in NOTCH2, STIL, PIK3CA, and GATA3. There was no significant change in median ERBB2 CN (20.3 v 22.6; Wilcoxon P = .79) between paired samples. CONCLUSION ERBB2-positive BCs in young women displayed substantial genomic evolution after treatment with chemo + H. Approximately half of patients with paired samples demonstrated acquired and/or clonally enriched genomic changes in cancer genes. ERBB2 CN changes were uncommon. We identified several genes warranting exploration as potential mechanisms of resistance to therapy in this population.

10503 Background: Prior data suggests that the risk of second primary breast cancer (SPBC) is higher among women diagnosed with primary breast cancer (BC) before age 40. There is evidence that risk differs by primary tumor characteristics, cancer treatments, and genetic predisposition. However, studies investigating risk of SPBC by clinical risk factors are often limited by low numbers of young women and lack of complete patient data, particularly regarding germline genetic risk. Methods: Using the Young Women’s BC Study (YWS), a prospective cohort of 1,302 women ≤40 years diagnosed with BC from 2006-2016, we sought to compare clinical and treatment characteristics among women who developed an SPBC vs. those who did not. The analytic cohort included women diagnosed with Stage 0-III BC who did not undergo bilateral mastectomy (N = 685). Demographics, genetic testing and results, clinical events, and treatment data were collected through self-report on baseline and follow-up surveys (every 6 months for 3 years, then annually). Detailed tumor and treatment information, confirmation of genetic testing results, and new and recurrent cancer events were obtained through regular and triggered medical record review. Univariate and multivariate Cox proportional hazards models were used to examine associations of age, race, genetic testing results, hormone receptor status, stage, and grade of primary tumor, and cancer treatments with SPBC. Participants were censored at the time of local recurrence, metastasis, death, or loss of follow-up. Results: At a median follow-up time of 9.9 years [inter-quartile range (IQR) = 6.6-12.0 y], 17 patients developed an SPBC (2.5%), in whom 3 occurred in the ipsilateral breast after lumpectomy as primary BC surgery. Median time from primary BC diagnosis to SPBC diagnosis was 4.2 years (IQR = 3.3-5.6 y). Compared to those who did not develop an SPBC, a greater percentage of women who developed an SPBC had in situ disease as their primary BC (24% v. 6%) and were of Ashkenazi descent (12% v. 5%). In the 577 women who had had genetic testing, 544 (94%) were non-carriers of pathogenic variants. Risk of SPBC was 2.2% (N = 12) in non-carriers and 9.1% (N = 3) in the 33 carriers. In Cox univariate analyses, carriers had a 4.1 times higher risk of SPBC compared to non-carriers (HR = 4.14, 95% confidence interval (CI) = 1.67-14.66, p = 0.03), and primary diagnosis with in situ BC was associated with a 5-fold increased risk of SPBC (HR = 5.02, 95% CI = 1.63-15.41, p = 0.005). Both factors remained strongly associated with SPBC when tested together in a multivariate model. Conclusions: This prospective study demonstrated a low risk of SPBC in the first 10 years after diagnosis of early BC in young women who are not germline pathogenic variant carriers. The finding that risk of SPBC was higher after in situ primary BC at a young age warrants further investigation. These findings have important implications for the treatment and follow-up care of young women with BC.

505 Background: Increased TILs are associated with better prognosis in triple-negative BC, including in patients (pts) age < 40. However, the role of TILs remains unclear in HR+/HER2- BC and little data exist in young pts, in whom immune microenvironment could be altered by age-related host/tumor differences. We assessed the extent and composition of immune infiltration in HR+ tumors of young women and correlated with clinicopathologic features and survival outcomes. Methods: From a prospective cohort study of women with BC diagnosed age ≤40, we identified those with stage I-III HR+/HER2- BC and available pre-treatment (tx) tumor tissue. Multiplexed immunofluorescence was used to quantify cytotoxic T (CD8+), T helper (Th, CD3+CD8-), T regulatory (Tregs, FOXP3+CD3+) and exhausted T (PD1+CD8+) cells in stroma and tumor. Univariate analyses tested associations between clinicopathologic variables and immune markers by high or low expression, divided based on median. High vs. low TILs were evaluated in Cox regression analyses for invasive breast cancer-free survival (iBCFS), distant disease-free survival (DDFS) and overall survival (OS). Results: In 390 pts, median age was 37 years (21-40), most had grade 2 (51%), T1 (65%), N0 (63%) tumors and 67% received adjuvant chemotherapy. Black pts (n = 17) had higher expression of stromal CD8+ ( P= .010), FOXP3+CD3+ ( P= .027) and PD1+CD8+ TILs ( P= .043); intratumoral TILs did not differ by race. Older age (36-40) was associated with high expression of CD8+ ( P= .033) and PD1+CD8+ TILs ( P= .031) within stroma and CD3+CD8- TILs within tumor ( P= .046). Grade 3 tumors had higher stromal and intratumoral expression of CD3+CD8- ( P= .002; P< .001) and FOXP3+CD3+ TILs ( P= .020; P< .001). No differences in TILs were seen according to recency of pregnancy, BRCA1/2 status or T/N stage. Over a median follow up of 8 years, 85 iBCFS events, 64 DDFS events and 37 deaths occurred. High stromal expression of CD3+CD8- TILs was associated with better iBCFS (HR 0.49, P= .002) and DDFS (HR 0.57, P= .046), which remained significant when adjusted for T/N stage, grade and chemotherapy (iBCFS HR 0.41, P< .001; DDFS HR 0.45, P= .008). High stromal expression of CD3+CD8- and FOXP3+CD3+ TILs was associated with better OS (HR 0.47, P= .038) and iBCFS (HR 0.58, P= .018), respectively, on adjusted analyses only. High intratumoral expression of CD3+CD8- and FOXP3+CD3+ TILs was associated with better iBCFS (HR 0.59, P= .025; HR 0.63, P= .043) after adjustment only. Conclusions: The distribution of TIL subtypes in young women’s HR+ BC varied according to race, age and grade. High stromal and intratumoral expression of Th and Tregs was associated with improved BC outcomes. Characterization of immune cell subsets could help refine the prognostic value of TILs in HR+ BC, particularly in young pts who may benefit from individualized escalated/de-escalated tx strategies.

Abstract Purpose: Young women’s breast cancers (YBCs), defined herein as breast cancer (BC) diagnosed at age ≤ 40 years, are etiologically and biologically distinct from postmenopausal BCs, demonstrating a higher proportion of estrogen receptor (ER) negative subtypes and a worse prognosis. Studies have not assessed whether risk factors such as recent parity and body mass index (BMI) are linked to protein expression profiles in YBCs. Accordingly, we examined protein expression assessed with GeoMx® Digital Spatial Profiling (DSP) in 640 YBCs prepared as tissue microarrays (TMAs) in the Young Women’s BC Study (YWBCS). Methods: The YWBCS is a multi-institutional cohort of 1,297 women with incident BC diagnosed at age &lt;40 years identified through medical record review, with risk factor data collected via questionnaires. TMAs including 1,505 cores of 640 YBCs that were collected prior to therapy and were suitable for molecular analysis were evaluated for 72 markers with NanoString GeoMx DSP. Associations of log-transformed biomarker expression with BMI and time from last live birth to BC diagnosis were carried out using linear mixed modeling approaches, accounting for multiple tumor tissue cores per individual. Results: Mean age at BC was 35.5 years (range 17-40). Tumor subtypes based on clinical immunostaining were the following: luminal B 46%; luminal A 36%; triple negative 12% and HER2+ 6%. Of 72 biomarkers tested, 53 (74%) were evaluable after QC and normalization. GeoMx ER and PR protein levels were higher in luminal A and luminal B YBCs than in HER2+ and TNBC (p=2x10-94 and 2x10-52, respectively), supporting the accuracy of the GeoMx assay. Compared to nulliparas or those with most recent birth &gt; 3 years before BC, those who gave birth ≤ 3 years prior to BC had tumors with higher expression of PR (8x10-6) and ER (p=0.006), and lower expression of BCL-2 (p=0.006). In contrast, women with last birth ≥ 6 years prior to BC had higher expression of CD8 (p=0.006) and PD-1 (p=0.001) than nulliparas and those with more recent births. Compared to women with BMI between 18.5 and 29.9, those with higher or lower BMIs had higher expression of CD34 (p=0.0003), CD4 (p=0.0009) and CD14 (p=0.001). BCL-2 expression was higher among women with BMI &lt; 18.5 compared to those with higher values (p=0.005). Women with BMI of 30+ had higher β2-microglobulin levels than those with lower BMIs (p=0.003). Inverse dose-response associations (with higher BMI related to lower levels) were observed for expression of ER (p=0.01), pan cytokeratin (p=0.004) and four proteins in the p13K/AKT signaling pathway (p ≤ 0.01 for each). Conclusions: Preliminarily, we identify differences in hormonal and immune markers in YBCs in relation to timing of births and BMI, suggesting that these factors may contribute to etiologic heterogeneity. Multivariate modeling is ongoing to incorporate molecular subtype and other risk factors. Citation Format: Robert A. Vierkant, Laura M. Pacheco-Spann, Stacey J. Winham, Jennifer M. Kachergus, Ji Shi, Craig Snow, Fergus J. Couch, Janet E. Olson, Chen Wang, Derek C. Radisky, Amy C. Degnim, E. Aubrey Thompson, Laura C. Collins, Kathryn J. Ruddy, Ann H. Partridge, Mark E. Sherman. Associations of parity and body mass index with NanoString digital spatial protein profiling in early onset breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1000.

Abstract Background: Young women with DCIS are at increased risk of recurrence and second BC compared to older women and may derive the greatest benefit from risk reduction strategies. Little is known about treatment decision-making, engagement in preventive behaviors, or fear of recurrence among young DCIS survivors. Thus, we sought to investigate treatment and prevention issues in this population. Methods: Using a multicenter prospective cohort that enrolled 1302 women with stage 0-IV BC at age ≤40, we identified participants with DCIS only who completed surveys in the year following diagnosis (dx). Surveys assessed treatment decisions, lifestyle factors (alcohol, smoking, physical activity [PA]), and fear of recurrence (Lasry Scale). Data was summarized with descriptive statistics and groups were compared with Fisher’s exact test. Results: Among 87 patients (median age 38, range 26-40), 75 had available treatment decision data, 19 (25%) of whom had breast-conserving surgery (BCS), 11 (15%) unilateral mastectomy (UM), and 45 (60%) bilateral mastectomy (BM). Most (77%) indicated their doctor said BCS was an option or recommended, including 34 (59%) who had BM. Of 40 patients who reported a patient-driven surgical decision, 73% had BM, 13% UM, and 15% BCS. Among 15 patients who reported the decision as doctor-driven, none had BM, 27% UM, and 73% BCS. Those who had BM were more likely to be extremely confident about the decision (80% vs. 73% UM vs. 53% BCS, p=0.022). Adjuvant tamoxifen was used by 10/27 (37%) patients with ER+ DCIS who had BCS/UM. Most who took tamoxifen reported this decision was shared with their doctor (90%); 1 reported the decision was made on her own. Of the 17 patients who did not take tamoxifen, 88% indicated their doctor said endocrine therapy was an option or recommended. Among all 87 patients, 31 (36%) were former and 3 (4%) were current smokers at baseline assessment (median 5 months post-dx); 51 (59%) never smoked. Overall, 4 quit within 1 year of dx. Most patients (83%) were current drinkers at baseline, though 79% consumed &lt;5 alcoholic beverages/week; 8% reported lower consumption at 1 year. Regarding PA at 1 year, 73% (58/80) reported ≥150 minutes of moderate-intensity PA/week. Overall, 41% of young survivors were at least moderately concerned about their BC coming back, with concern varying numerically by surgery (65% post-BCS, 50% post-UM, 29% post-BM, p=0.053). Conclusions: BC recurrence concerns were frequent among young DCIS patients, particularly among those with remaining breast tissue. BM decisions were largely patient-driven and made with high confidence, reflecting preferences towards surgical prevention. While adherence to survivorship lifestyle guidelines was high, including not smoking, consumption of ≤7 drinks/week, and engaging in PA, tamoxifen use was low, even when offered by a provider. Young DCIS survivors appear motivated to lower future BC risk through surgery and maintenance of healthy behaviors, and less so with tamoxifen; such values should be considered during treatment and survivorship counseling. Citation Format: Megan E. Tesch, Julia S. Wong, Laura Dominici, Kathryn J. Ruddy, Rulla Tamini, Lidia Schapira, Virginia F. Borges, Ellen Warner, Steven E. Come, Karen Sepucha, Laura C. Collins, Ann H. Partridge, Shoshana M. Rosenberg. Breast cancer (BC) risk reduction in young women with ductal carcinoma in situ (DCIS) [abstract]. In: Proceedings of the AACR Special Conference on Rethinking DCIS: An Opportunity for Prevention?; 2022 Sep 8-11; Philadelphia, PA. Philadelphia (PA): AACR; Can Prev Res 2022;15(12 Suppl_1): Abstract nr PR009.

Objective: We aimed to investigate eligibility for breast conserving surgery (BCS) pre- and post-neoadjuvant systemic therapy (NST), and trends in the surgical treatment of young breast cancer patients. Background: Young women with breast cancer are more likely to present with larger tumors and aggressive phenotypes, and may benefit from NST. Little is known about how response to NST influences surgical decisions in young women. Methods: The Young Women's Breast Cancer Study, a multicenter prospective cohort of women diagnosed with breast cancer at age ≤40, enrolled 1302 patients from 2006 to 2016. Disease characteristics, surgical recommendations, and reasons for choosing mastectomy among BCS-eligible patients were obtained through the medical record. Trends in use of NST, rate of clinical and pathologic complete response, and surgery were also assessed. Results: Of 1117 women with unilateral stage I-III breast cancer, 315 (28%) received NST. Pre-NST, 26% were BCS eligible, 17% were borderline eligible, and 55% were ineligible. After NST, BCS eligibility increased from 26% to 42% (P < 0.0001). Among BCS-eligible patients after NST (n = 133), 41% chose mastectomy with reasons being patient preference (53%), BRCA or TP53 mutation (35%), and family history (5%). From 2006 to 2016, the rates of NST (P = 0.0012), clinical complete response (P < 0.0001), and bilateral mastectomy (P < 0.0001) increased, but the rate of BCS did not increase (P = 0.34). Conclusion: While the proportion of young women eligible for BCS increased after NST, many patients chose mastectomy, suggesting that surgical decisions are often driven by factors beyond extent of disease and treatment response.

514 Background: The 21- gene Breast Recurrence Score predicts benefit from adjuvant chemotherapy in estrogen receptor positive (ER+), HER-2 negative (-) breast cancer (BC). We aimed to examine whether the 21-gene assay predicts response to neoadjuvant chemotherapy (NAC). Methods: We identified patients with stage I-III ER+/HER2− BC treated with NAC from the Young Women's Breast Cancer Study, a prospective cohort of women diagnosed with BC at age ≤ 40 years. The 21-gene assay was performed on tumor specimens removed prior to NAC either as part of clinical care or retrospectively for research. Pathologic complete response (pCR) was defined as no residual invasive tumor (ypT0/is ypN0). The relationship between Recurrence Score result (RS) and pCR was evaluated using logistic regression modeling. Results: 76 women in the cohort had undergone NAC for ER+, HER2- BC and were eligible for this analysis: 5 had undergone clinical 21-gene assay testing, 71 had banked specimens retrospectively tested. Median age at diagnosis was 36.7 (24.3-40). Most tumors were of ductal histology (78%), high grade (51%), progesterone receptor (PgR) positive (86%), ≥T2 (88%), clinically node positive (74%), and anthracycline and taxane containing protocols were administered in 86% of cases. RS ranged between 5-75 with 50% > 25 and only 4 < 11. Mean RS was significantly higher among tumors achieving pCR vs. non-pCR response (51.9 vs. 26.6, pwilcoxon= 0.0005). pCR rate in patients with RS > 25 was 21% (8/38) vs. 5% in patients with RS < 25 (2/38), with both pCRs in the <25 group in patients with RS 21-25. In univariable analysis, PgR negativity (odds ratio (OR) 5.62, 95% confidence interval (CI) 1.27-24.89, p = 0.02), high grade (OR 9.03, 95%CI 1.07-76.32, p = 0.04) and higher RS as a continuous variable (OR 1.08, 95%CI 1.04-1.13, p = 0.0003) were associated with a greater likelihood of pCR. In multivariable analysis only RS remained significantly associated with pCR (OR: 1.07, 95%CI 1.01-1.12, p = 0.01): a 7% increase in the odds of pCR for every 1-point increase in RS. Conclusions: In young women with ER+, HER2- BC who received NAC, higher pretreatment RS was associated with an increased likelihood of pCR. Genomic expression profiling assays may have a role in decision-making in young women in need of neoadjuvant therapy. For women with low likelihood of benefiting from NAC, alternative approaches are clearly warranted. Given the demonstrated efficacy of neoadjuvant endocrine therapy in post-menopausal women, further evaluation in young women should be pursued.

Abstract Background: Breast cancer arising in young women is more likely to have higher risk features than in older women and more likely to be associated with germline BRCA1 or BRCA2 mutations. We previously reported a higher distribution of luminal B-like tumors among young women. Here we present the clinical and pathologic features of invasive disease in a large prospective cohort of young women with breast cancer, and associations between surrogate molecular subtype and BRCA 1/2 mutation status. Methods: Among 1,297 young women (aged ≤40 years) with breast cancer enrolled from 2006-2016, tumor histopathological features were examined by central review, and biomarker status was extracted from pathology reports. Based on histologic grade and biomarker expression, invasive tumors were categorized as luminal A-like (LA=ER/PR+ and HER2-, histologic grade 1 or 2), luminal B-like (LB=ER/PR+ and HER2+, or ER/PR+, HER2-, and grade 3), HER2 enriched (E) (ER/PR- and HER2+) or triple negative (TN=ER-, PR- and HER2-). Tumor stage, clinical characteristics, and BRCA status were retrieved by medical record review (and patient survey for BRCA status if not in medical record). Results: The majority of women presented with early stage invasive disease (8% stage 0, 32% stage 1, 40% stage 2, 15% stage 3; 5% stage 4). Among invasive tumors with evaluable molecular phenotype (n=1,136), 57% were high grade and the distribution of subtypes was: 33% LA, 42% LB, 8% HER2E, and 16% TN (Table 1). Among different age groups (≤ 30, 31-35, and 36-40 years), there were no significant differences in molecular phenotype, tumor stage, tumor grade or histopathological features including presence of tumor necrosis, lymphocytic infiltration, and central fibrotic focus. 96% of women were tested for BRCA1 and BRCA2 germline mutations, and 127 (11%) tested positive: 65% were BRCA1+, and 35% were BRCA2+ (table 2). Among BRCA1+ patients, 2% of cancers were LA, 30% LB, 5% HER2E, and 62% TN; among BRCA2+, 29% were LA, 56% LB, 2% HER2E, and 13% TN. These proportions were relatively consistent across age groups. BRCA1+ patients’ tumors were proportionately more TN and BRCA2+ patients’ tumors more likely to be LB/HER2 negative (p&lt;0.0001), irrespective of age category. Conclusion: In a large cohort of women diagnosed with breast cancer at age ≤40, there was no association between age group (≤ 30, 31-35, vs. 36-40 years) and tumor stage, grade, or tumor molecular phenotype among women with invasive disease. As in older women, tumor molecular phenotype does vary by BRCA1 or BRCA2 mutation status, which has implications for prevention and treatment. Table 1. Subtype by age groupTotal (n=1136)≤30 yrs (n=144)31-35 yrs (n=314)36-40 yrs (n=678)p=0.42Luminal A-like377 (33%)48 (33%)99 (32%)230 (34%)Luminal B-like477 (42%)59 (41%)137 (44%)281 (41%)• ER/PR+, HER2-, grade 3245 (22%)27 (19%)68 (22%)150 (22%)• ER/PR+, HER2+232 (20%)32 (22%)69 (22%)131 (19%)HER2E95 (8%)13 (9%)34 (11%)48 (7%)Triple Negative187 (16%)24 (17%)44 (14%)119 (18%) Table 2. Subtype by BRCA statusTotal(n=1136)BRCA1+(n=82)BRCA2+(n=45)No mutation/not tested(n=1009)p&lt;0.0001Luminal A-like377 (33%)2 (2%)13 (29%)362 (36%)Luminal B-like477 (42%)25 (30%)25 (56%)427 (42%)• ER/PR+, HER2-, grade 3245 (22%)21 (26%)18 (40%)206 (20%)• ER/PR+, HER2+232 (20%)4 (5%)7 (16%)221 (22%)HER2E95 (8%)4 (5%)1 (2%)90 (9%)Triple Negative187 (16%)51 (62%)6 (13%)130 (13%) Citation Format: Yaileen D Guzman-Arocho, Shoshana M. Rosenberg, Philip Poorvu, Kathryn J. Ruddy, Greg Kirkner, Craig Snow, Rulla M. Tamimi, Jeffrey Peppercorn, Lidia Schapira, Virginia F. Borges, Steven E. Come, Elena F. Brachtel, Ellen Warner, Ann H. Partridge, Laura C. Collins. Clinicopathological features and BRCA 1/2 status in a large prospective cohort of young women with breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-07-02.

PURPOSE The 21-gene recurrence score (RS) assay is prognostic among women with early-stage estrogen receptor–positive (ER+) and human epidermal growth factor receptor 2–negative (HER2−) breast cancer and is used to inform recommendations for chemotherapy. Women ≤ 40 years of age represent a minority of patients studied using gene expression profiles. METHODS The Young Women's Breast Cancer Study is a prospective cohort of women diagnosed with breast cancer at age ≤ 40 years and enrolled patients between 2006 and 2016 (N = 1,302). We identified patients with stage I-III ER+/HER2− breast cancer. The RS assay was performed on banked specimens for patients who had not been tested clinically. Distant recurrence-free survival (DRFS) was assessed by TAILORx and traditional RS risk groups among patients with axillary node–negative (N0) and limited node–positive (N1) breast cancer. RESULTS Among eligible women (N = 577), 189 (33%) had undergone RS testing, and 320 (56%) had banked specimens sufficient for testing. Median follow-up was 6.0 years. Median age at diagnosis was 37.2 years; 300 of 509 patients (59%) had N0 breast cancer, of whom 195 (65%) had an RS of 11-25 and fewer than half (86 of 195; 44%) received chemotherapy. Six-year DRFS rates were 94.4% and 92.3% (RS < 11), 96.9% and 85.2% (RS 11-25), and 85.1% and 71.3% (RS ≥ 26) among women with N0 and N1 disease, respectively. CONCLUSION The RS assay is prognostic among young women with node-negative and limited node-positive breast cancer, representing a valuable tool for risk stratification. Disease outcomes with a median follow-up of 6 years among young women with N0 disease and an RS of 0-25, a minority of whom received chemotherapy, and node-positive disease with an RS < 11 were very good, whereas those with N0 disease and an RS ≥ 26 or N1 disease with an RS ≥ 11 experienced substantial risk of early distant recurrence.

554 Background: HER2+ breast cancer (BC) is particularly common in young women. Genomic features of HER2+ tumors before and after H-based therapy have not been described in a population of young women and may point to clinically targetable mechanisms of resistance. Methods: From a large prospective cohort of women diagnosed with BC age ≤40 years, we identified those with HER2+ BC and tumor tissue available for sequencing before and after chemo+H. Whole exome sequencing (WES) was performed on each tumor and on germline DNA from blood. Tumor-normal pairs were analyzed for mutations and copy number (CN) changes. Evolutionary analysis was performed for patients with both pre- and post-treatment (tx) samples. Results: 22 women had successful WES samples from at least one timepoint; 13 of these had paired sequencing results both before and after chemo+H. For the majority of women, post-tx sample was following neoadjuvant chemo + H, though post-tx timepoint for other women represented locoregional or distant metastasis (Table). TP53 was the only gene that was significantly recurrently mutated in both pre- and post-tx samples. Comparison of matched pre-tx and post-tx samples demonstrated that large changes in HER2 CN over the course of tx were uncommon, only 2/13 pts had > 2-fold change in HER2 CN. Other clonal and subclonal genomic alterations were found to be acquired in the post-tx sample compared to the pre-tx sample. One patient acquired a putative activating mutation in ERBB2. Another patient acquired a clonal hotpsot mutation in TP53. MYC gene amplification was observed in 4 post-tx tumors. NOTCH2 alterations were found in post-tx biopsies from 2 different patients, and mutations in STIL were also found in post-tx biopsies from 2 patients, though the function of these mutations is not known. Conclusions: HER2+ breast tumors in young women display genomic evolution following tx with chemo+H. HER2 CN changes are uncommon, but we identified several genes that warrant exploration as potential mechanisms of resistance to therapy in this population.[Table: see text]