Coordinated Co-amplification of 8p11 and 11q13 Genomic Loci Establishes a Closed Autocrine Oncogenic Circuit in Luminal Breast Cancer with Neuroendocrine Differentiation: A Hypothesis Report with Liquid Biopsy Corroboration

Version 2.0 — 2026-05-04 This release supersedes v1.0 with substantial scientific and editorial revisions following independent peer-style audits. Key changes: New clinical data: Two-platform circulating tumor DNA analysis (April 2026) confirms persistence of CCND1 and FGFR1 amplifications, identifies three RB1 loss-of-function alterations, and detects an emerging ESR1 D538G subclone after progression on second-line fulvestrant plus abemaciclib. Reference corrections: Citations 21 (Gupta et al., APOBEC3), 22 (Kalinsky et al., postMONARCH), and 23 (Bidard et al., SERENA-6) corrected to canonical titles and author lists. Issue and page numbers verified for references 2, 3, 4, 8, and 20. Ormandy et al. (2003) added. Biological framing: β-Klotho described as "required under physiological conditions" rather than "obligate co-receptor", reflecting reported FGFR1-amplified context exceptions. Therapeutic table: Internal contradiction between RB1 loss and ESR1-directed CDK4/6 inhibitor recommendation resolved through clonal-context conditioning. Anonymity: Exact biopsy dates generalized to early/mid-April 2026; institutional references neutralized. Editorial: Frequency rounding harmonized across the manuscript; UGT1A1 *28/*28 markdown escaping fixed; en-dash usage standardized; FOENIX-MBC2 reframed as a single-arm phase 2 efficacy signal rather than an empirical anchor. This version is intended as the reference record. Earlier version (v1.0) remains accessible through Zenodo's version history. The simultaneous high-level amplification of two distinct genomic loci — 8p11 (FGFR1) and 11q13 (CCND1, FGF3, FGF4, FGF19) — in a case of stage IV hormone receptor-positive (HR+), HER2-negative breast cancer with extensive neuroendocrine differentiation (BC-NED) prompted the hypothesis that these co-amplified elements form a closed autocrine oncogenic circuit: FGF ligands (FGF3/4/19) co-amplified with their cognate receptor (FGFR1/FGFR4) and a key downstream effector (CCND1/cyclin D1) create a self-sufficient mitogenic loop that operates independently of upstream hormonal input, thereby explaining observed resistance to sequential CDK4/6 inhibitor-based regimens. This report integrates: (1) published mechanistic data on FGFR1-mediated PI3K/AKT activation, FGF19–FGFR4 autocrine signaling in breast cancer, and the prognostic impact of 8p11/11q13 co-amplification; (2) population-level data from cBioPortal (17,758 breast cancer samples across 36 studies) confirming statistically significant co-occurrence of these amplifications and their association with reduced overall survival; and (3) serial liquid biopsy data (Guardant360 CDx + an independent orthogonal liquid biopsy assay, April 2026) demonstrating persistence of both amplicons in circulating tumor DNA at disease progression, co-occurring with three independent RB1 loss-of-function mutations and an emergent ESR1 D538G variant — a polyclonal resistance landscape consistent with, and extending, the proposed circuit model. To our knowledge, the conceptualization of the co-amplified FGF ligand cluster + cognate receptor + downstream effector as a structurally closed autocrine circuit has not been previously described in the BC-NED context. This framing generates specific, testable predictions and informs a rational therapeutic strategy centered on pan-FGFR1–4 inhibition.