HER2 in situ hybridization in breast cancer: clinical implications of polysomy 17 and genetic heterogeneity

Abstract

Trastuzumab-containing therapy is a standard of care for patients with HER2 þ breast cancer. HER2 status is routinely assigned using in situ hybridization to assess HER2 gene amplification, but interpretation of in situ hybridization results may be challenging in tumors with chromosome 17 polysomy or intratumoral genetic heterogeneity. Apparent chromosome 17 polysomy, defined by increased chromosome enumeration probe 17 (CEP17) signal number, is a common genetic aberration in breast cancer and represents an alternative mechanism for increasing HER2 copy number. Some studies have linked elevated CEP17 count ('polysomy') with adverse clinicopathologic features and HER2 overexpression, although there are numerous discrepancies in the literature. There is evidence that elevated CEP17 ('polysomy') count might account for trastuzumab response in tumors with normal HER2:CEP17 ratios. Nonetheless, recent studies establish that apparent 'polysomy' (CEP17 increase) is usually related to focal pericentromeric gains rather than true polysomy. Assigning HER2 status may also be complex where multiple cell subclones with distinct HER2 amplification characteristics coexist within the same tumor. Such genetic heterogeneity affects up to 40% of breast cancers when assessed according to a College of American Pathologists guideline, although other definitions have been proposed. Recent data have associated heterogeneity with unfavorable clinicopathologic variables and poor prognosis. Genetically heterogeneous tumors harboring HER2-amplified subclones have the potential to benefit from trastuzumab, but this has yet to be evaluated in clinical studies. In this review, we discuss the implications of apparent polysomy 17 and genetic heterogeneity for assigning HER2 status in clinical practice. Among our recommendations, we support the use of mean HER2 copy number rather than HER2:CEP17 ratio to define HER2 positivity in cases where coamplification of the centromere might mask HER2 amplification. We also highlight a need to harmonize in situ hybridization scoring methodology to support accurate HER2 status determination, particularly where there is evidence of heterogeneity.

Next-Generation Assessment of Human Epidermal Growth Factor Receptor 2 (ERBB2) Amplification Status

“…In breast tumors, there is no clear relationship of polysomy 17 to HER2 protein status or benefit from HER2-targeted therapy at this time. 45,46 Loss or gain of the pericentromeric region of chromosome 17 is more commonly observed and can result in alterations in the HER2/CEP17 ratio and falsepositive or false-negative results. With NGS assays, not only can information of simultaneous CNAs be assessed but also prognostic information and an overall better understanding of the molecular mechanisms.…”

“…Using the definition of >3 CEP17 copies per cell for polysomy, the reported prevalence rates in breast tumors range between 3% and 46%. 45 There are several reasons for variation in the CEP17 CN during FISH analysis, including section thickness, cell division, and chromosomal instability, and true polysomy is actually rare. In breast tumors, there is no clear relationship of polysomy 17 to HER2 protein status or benefit from HER2-targeted therapy at this time.…”

Topoisomerase‐1 gene copy aberrations are frequent in patients with breast cancer

“…This has led to the suggestion that mean copy number of the HER-2 gene should be used in combination with the HER-2/ CEN-17 ratio. 24 We investigated the TOP1/CEN-20 ratio and found a significant correlation between TOP1 and CEN-20 suggesting that increased TOP1 gene levels is often due to gain of 20q or the whole chromosome. By applying a reference CEN-2 probe, we aimed to investigate if gene amplification could be distinguished from gain caused by other mechanisms.…”

HER2 gene amplification in patients with prostate cancer: Evaluating a CISH-based method

“…Apparent chromosome 17 polysomy, defined by increased CEP17 signal number, is a common genetic aberration in breast cancer and represents an alternative mechanism for increasing HER2 copy number. Elevated CEP17 count (polysomy) has been linked with adverse clinicopathologic features and HER2 overexpression, although there are numerous discrepancies in the literature (37). HER2 overexpression and/or amplification are recurrently reported in numerous tumor types, and have been shown to have significant therapeutic implications in patients with cancer (33).…”

“…Apparent chromosome 17 polysomy, defined by increased chromosome enumeration probe 17 (CEP17) signal number, is a common genetic aberration in breast cancer and represents an alternative mechanism for increasing HER2 copy number (37). However, the prognostic value of HER2/neu amplification in PCa remains controversial (38).…”