Deciphering the Role of Phosphatidylinositol 3-Kinase Mutations in Human Epidermal Growth Factor Receptor 2–Positive Breast Cancer

Targeted therapies directed against human epidermal growth factor receptor 2 (HER2) have transformed treatment outcomes for patients with HER2-positive breast cancer. Recurrence rates for patients with operable disease have declined substantially, and the median survival of those with metastatic disease now approaches 5 years. Running against the tide of this progress, however, 15% of patients with early-stage disease still develop a recurrence, and those with metastatic disease almost invariably succumb after progressing through several lines of therapy. Working alongside clinical researchers, translational scientists have also made great progress through their efforts to understand the biology of HER2-positive breast cancer. If these efforts have taught us but one lesson, it is that HER2-positive cancers are biologically heterogeneous, with diversity in their levels of HER2 and hormone receptor expression, their immune milieu, and both genomic and transcriptomic profiles. These two features—variability in outcomes and variability in biology—provoke an important question: Are there particular biologic characteristics within a HER2-positive tumor that will predict its response to HER2-targeted therapies, such as the monoclonal antibody trastuzumab? Several candidate markers have been proposed. These include truncations or mutations of HER2, enhanced signaling through alternate growth factor receptors, heightened activity of cell cycle machinery, genomic polymorphisms affecting host immune function, the degree of stromal lymphocytic infiltrate, and distinct patterns of gene expression. Although each of these biomarkers has a solid biologic rationale, none has been reproducibly validated in specimens from randomized trials, the gold standard for defining a predictive marker. Morethananyofthesefactors,molecularalterationsinthephosphatidylinositol 3-kinase (PI3K) pathway are the best-characterized predictors of resistance to HER2 pathway blockade, at least at a preclinical level. Phosphorylation and activation of the HER2 tyrosine kinase stimulates a complex network of downstream effectors, including the lipid kinase PI3K (Figure). By phosphorylating phosphatidylinositol (4,5)bisphosphate (PIP2), PI3K increases the amount of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) at the plasma membrane. This leads to recruitment of the AKT kinase to the membrane through its direct interaction with PIP3. AKT is then activated through two distinct mechanisms: the Thr308 residue within its activation loop is phosphorylated by PDK1, and the Ser473 residue at its C-terminus is phosphorylated by the mTORC2 complex. Ultimately, AKT activation results in phosphorylation of a wide range of downstream targets that regulate cell proliferation, survival, growth, and metabolism. This entire pathway is negatively regulated by the lipid phosphatase PTEN, which dephosphorylates PIP3. PI3K has four distinct catalytic isoforms—p110 , p110 , p110 , and p110 —each encoded by a separate gene. Although the , , and isoforms are all expressed in epithelial cells, p110 (encoded by the gene PIK3CA) is the most important isoform for mediating HER2 signaling. This critical role for p110 is evidenced by observations that mice lacking p110 are resistant to HER2 transgene–mediated tumor formation and, conversely, mice bearing activating mutations in PIK3CA (the most commonly mutated isoform in HER2-positive breast cancers) develop HER2-driven breast cancers more quickly. Given that HER2-directed therapies exert their effects (at least in part) by inhibiting p110 signaling downstream of HER2, it seems logical that molecular alterations that constitutively activate p110 could mediate resistance to these agents (including the monoclonal antibodies trastuzumabandpertuzumabandthekinaseinhibitorlapatinib).Preclinicaldatashowthatactivatingmutationsinthehelicalorkinasedomainsof PIK3CA render HER2-positive cell lines resistant to growth inhibition by trastuzumab, and that the same mutations promote trastuzumab resistance in xenograft models. Loss of PTEN has similar effects. The preclinical data are thus consistent with the hypothesis that HER2positivebreastcancerswithactivatingmutations inPIK3CAorPTENloss (which are each seen in approximately 25% of HER2-positive breast cancers), should respond poorly to HER2 pathway blockade. In the accompanying article, Pogue-Geile et al use tissue specimens from the NSABP (National Surgical Adjuvant Breast and Bowel Project) B-31 trial, which randomly assigned patients with HER2positive early-stage breast cancers to adjuvant chemotherapy with or without trastuzumab, to determine whether activating mutations in PIK3CA are associated with resistance to trastuzumab therapy. On the basis of the preclinical data, the authors hypothesized that those patientswhose tumorsweremutant forPIK3CAwouldderive lessbenefit from trastuzumab than those whose tumors were wild-type. After sequencing six mutational hotspots in PIK3CA from 672 tumors in the JOURNAL OF CLINICAL ONCOLOGY U N D E R S T A N D I N G T H E P A T H W A Y VOLUME 33 NUMBER 12 APRIL 2