Of 111 participants analyzed, 26 – the “paired cohort” – had samples taken prior to and after ≥ 6 weeks of TKI treatment; 30 – the “post-TKI cohort” – had samples collected only after ≥ 6 weeks TKI treatment; and 55 – the “pre-TKI cohort” – had samples collected only prior to ≥ 6 weeks TKI treatment (Fig. 1A). All participants across the three cohorts had HER2-positive breast cancer per American Society of Clinical Oncology - College of American Pathology guidelines at time of metastatic diagnosis.7 All paired cohort participants were HER2-positive at time of primary diagnosis except for one participant who did not undergo HER2 testing due to lack of availability of HER2 testing at the time. Detailed primary and metastatic receptor status for the paired cohort is included in Supplementary Tables 1 and 2, respectively. Median age at metastatic diagnosis for the paired cohort was 48 years. Most participants initially presented with stage I-III disease (66%) with the remaining presenting as de novo metastatic (34%). Participants were followed for a median of 4.9 years from metastatic diagnosis and the majority were female (96%) with hormone receptor-positive (HR-positive) disease (62%). Most participants had visceral disease (69%), and 65% of participants had central nervous system (CNS) involvement. Most participants in the paired cohort received lapatinib (92%) and a median number of 6 (range 1–10) lines of metastatic therapy. Median overall survival in the paired cohort was 4.51 years from metastatic diagnosis. A median number of 3 samples (2–7) per participant were collected, with a median tumor fraction of 13.2% (2.8%-53.2%). The post-TKI and pre-TKI cohorts were comparable to the paired cohort for median age at diagnosis, overall survival, follow-up time, gender, hormone receptor (HR) status, and race (Supplementary Table 3).
The genomic landscape of post-TKI HER2-positive advanced breast cancer
To investigate mechanisms underlying resistance to HER2-targeted TKIs, we performed whole exome sequencing (WES) on 192 tumor and cfDNA samples from 111 participants with acquired or intrinsic resistance to anti-HER2 TKIs. Alterations in previously characterized genes that could explain treatment resistance were labeled as known mechanisms, including kinase domain mutations in ERBB2 (L755S, D769Y, V777L) and activating PIK3CA mutations.5,6,8 Potential mechanisms were identified in genes with mutations known to cause resistance in other cancer types or therapies (Fig. 1A).
In the paired cohort, we found that eleven (42%) participants had at least one known alteration leading to resistance and 38% had no identified mechanism of resistance (Fig. 1B). We then assessed the frequency of alterations in canonical oncogenic pathways, as well as a gene set containing ESR1 and its transcriptional regulators (Supplementary Table 4), to determine whether any pathways were significantly recurrently mutated in the cohort.9,10 The full list of genes altered in each pathway can be found in Supplementary Table 5 with variant classification for each mutation. RTK-RAS and TP53 were the most frequently mutated pathways with 16 (62%) participants having an alteration, followed by the WNT pathway (11 participants, 42%) and the HIPPO, NOTCH, and ESR1 and regulators pathways (each 10 participants, 38%). The TP53 and ESR1 and regulators pathways were both significantly mutated (q < 0.05) above the expected background mutation rate. We analyzed known mutational signatures and found the most frequently represented signatures were clock-like, APOBEC and capecitabine (Fig. 1B).11 We identified no other statistically significant differences at a pathway level between cohorts (Fig. 1C).
In the pre-TKI cohort, RTK-RAS was the most frequently mutated pathway with 31 (56%) participants having an alteration, followed by the TP53 pathway (30 participants, 55%) and PI3K pathway (25 participants, 45%) (Supplementary Fig. 1A). In the post-TKI cohort, TP53 was the most frequently mutated pathway with 23 (77%) participants having an alteration, followed by the PI3K pathway (19 participants, 63%) and RTK-RAS pathway (18 participants, 60%) (Supplementary Fig. 1B). The TP53 pathway was significantly mutated (q < 0.05) in both the pre- and post-TKI cohorts, and the PI3K pathway was significantly mutated (q < 0.05) in the post-TKI cohort only.
Mutation frequencies in the most frequently altered genes were similar between the paired cohort and the pre- and post-TKI cohorts. TP53, the most frequently altered gene in all three cohorts, was altered in 58% of paired participants, 67% of post-TKI participants, and 53% of pre-TKI participants. PIK3CA was altered in 31% of paired participants, 50% of post-TKI participants, and 35% of pre-TKI participants. ERBB2 was altered in 19% of paired participants, 17% of post-TKI participants, and 13% of pre-TKI participants. ESR1 mutations were enriched in the paired cohort (19%) compared to both the post-TKI cohort (10%) and pre-TKI cohort (5%) (Supplementary Table 6).
Next, we evaluated copy number alterations. The most frequently altered genes were ERBB2—as expected—followed by TP53, PTK2 and MYC, respectively. Six of eight participants in the paired acquired resistance cohort had focal high amplification of ERBB2, compared to only eight of the eighteen participants in the paired intrinsic cohort (p = 0.22, Fisher’s Exact test). TP53 biallelic inactivation—i.e., loss-of-function point mutation of one allele and deletion of the other allele—was observed across the paired cohort and occurred in 14 (54%) participants. Seven participants showed MYC amplifications, with one participant demonstrating focal high amplification. Five participants with intrinsic resistance did not have ERBB2 amplification (Fig. 2A), and lower pre-TKI ERBB2 copy number was associated with intrinsic resistance (p = 0.0074, Wilcoxon rank-sum test) (Fig. 2C). All five participants had clinical HER2-positive breast cancer confirmed on metastatic biopsy, a median of 842 days prior to assessed sample collection. The ERBB2 copy number of a sample was not associated with its genomic purity or tumor fraction (Spearman’s rho=-0.082, p = 0.46), suggesting that absence of ERBB2 amplification was not a result of low purity or tumor fraction. Within the paired cohort, ERBB2 copy number did not significantly change at a within-participant level during TKI treatment, and within-participant change did not differ significantly by HR status (Fig. 2B). When stratifying the paired cohort by HR status, participants with HR-positive MBC had lower ERBB2 copy number than HR-negative participants in both baseline and end-of-TKI treatment samples (p = 0.031 and p = 0.041, Wilcoxon rank-sum test), as has previously been shown (Supplementary Fig. 2).12
ESR1 alterations and ERBB2 copy number in HR-positive/HER2-positive MBC
Five out of the sixteen paired cohort participants with HR-positive disease had at least one activating ESR1 hotspot mutation (D538G or Y537S). Two of these five participants had never received an aromatase inhibitor (AI) or prior hormone therapy in either the early or metastatic setting. An activating ESR1 mutation presented in a growing subclone in four out of five participants. Three participants with an activating ESR1 mutation did not show ERBB2 copy number amplification per our analysis (Fig. 2A). Participants with an ESR1 mutation had significantly lower ERBB2 copy number relative to ESR1 wildtype cases in pre- and post-TKI paired samples (p = 0.024 and p = 0.0096, Wilcoxon rank-sum test) (Supplementary Fig. 3). When stratifying by HR status in an integrated analysis of the paired cohort and the pre- and post-TKI cohorts, participants with an activating ESR1 mutation had significantly lower ERBB2 copy number than both HR-negative and HR-positive ESR1 wildtype participants in both pre- and post-TKI samples (Fig. 2D).
Evolution of Clonal Dynamics in TKI resistance
To assess mechanisms of resistance we performed deeper evolutionary analysis on six participants with at least three serial samples collected (Fig. 3A). Known drivers of resistance were observed in multiple clones and included ERBB2 and PIK3CA mutations (Figs. 3B, 3C, 3D). Potential other mechanisms of resistance were identified as mutations in ESR1, FGFR2, and FGFR4 (Figs. 3B, 3E, 3F).
In participant 1, we observed two competing ESR1 mutations in separate clones with the clone harboring a Y537S mutation, a potential driver of resistance, initially predominant during lapatinib treatment. Upon treatment switch to capecitabine with trastuzumab and eventually trastuzumab with letrozole, we observed the clone containing the ESR1 D538G mutation outcompeting the Y537S-containing clone (Fig. 3B). One participant with a PIK3CA mutation also had several missense mutations, including a truncal ESR1 regulator pathway mutation present in PGR, a missense mutation in FOXA1, and a subclone with a NOTCH2 mutation growing during neratinib treatment (Fig. 3C). In another participant, growing subclones were detected with missense mutations in RTK-RAS, NOTCH, and WNT pathway genes such as SPRED2, EP300, and TLE2, respectively (Fig. 3G). However, for this participant, we did not observe any known or potential mechanisms of resistance, even though all participants were HER2 amplified (biopsy confirmed within 30 days of initial metastatic diagnosis) (Fig. 3G).
WNT pathway enrichment in HER2-positive brain metastases
Thirteen (50%) participants in the paired cohort developed brain metastases. These participants more often had HR-negative breast cancer (54% of participants who developed brain metastases vs. 23% of participants who did not; p = 0.23, Fisher’s Exact test). There were no significant differences in pathway mutation frequencies between participants who developed brain metastases and those who did not. However, mutations occurring specifically in the tumor suppressor genes of the WNT pathway were significantly enriched in participants with brain metastases (p = 0.0077, Boschloo’s test), with altered genes including APC, CHD8, LZTR1, TLE1, TLE2, and ZNRF3. Three of the five participants presented with subclonal WNT alterations in a cfDNA, breast or bone sample, with a median time of 903 days before the identification of any brain metastases (Fig. 4A, 4B). Evolutionary analysis for Patient 8, with HR-negative breast cancer brain metastasis, illustrates WNT pathway involvement in the growing subclone containing a missense mutation in the tumor suppressor gene TLE2 (Fig. 3G). In a combined analysis of the mutually exclusive pre- and post-TKI cohorts, mutations in WNT tumor suppressor genes remained significantly enriched in participants who ever developed brain metastases (p = 0.037, Boschloo’s test) (Supplementary Fig. 4). Of the eight participants with WNT tumor suppressor gene mutations in the combined pre- and post-TKI cohorts, two had the TSG alteration present before presentation of any brain metastases (median time of 1528 days before).