Amplification of the HER2 gene in breast cancers testing 2+ weak positive by HercepTest immunohistochemistry: false-positive or false-negative immunohistochemistry?
BACKGROUND
The majority of cases of breast cancer scoring HER2 weak positive (2+) on immunohistochemistry (IHC) using the HercepTest are not associated with amplification of the HER2/neu gene.


OBJECTIVE
To examine the reproducibility of IHC in cases scoring 2+ subsequently shown to have gene amplification by fluorescence in-situ hybridisation (FISH).


METHODS
A retrospective analysis of 153 cases referred for FISH confirmation of a weak positive HercepTest (2+) result was performed. Repeat IHC was undertaken in cases with weak positive (2+) referral IHC and amplification of the HER2 gene by FISH.


RESULTS
Amplification of the HER2 gene was confirmed in 29/153 cases (19%) scoring 2+ on IHC. Repeat IHC was carried out on 25 IHC 2+ cases: 7 (28%) scored 2+ on repeat IHC, 18 (72%) scored 3+ and were reclassified as strong positive. A heterogeneous expression pattern was present in 3/17 cases scoring 3+.


CONCLUSIONS
The majority of HercepTest 2+ results are not accompanied by gene amplification and represent "false positive" IHC in terms of prognostic or therapeutic relevance. A small proportion of HercepTest 2+ scores represent true 2+ IHC positive cases accompanied by gene amplification: a category probably biologically related to 3+ IHC cases. The remainder of cases of HercepTest 2+ accompanied by gene amplification represent a category of referral IHC 2+ weak positive, FISH amplified, repeat IHC 3+ strong positive, best described as "false negative 2+ IHC". This has implications for selection of cases for FISH analysis where weak positive (2+) IHC score is used as a triage for FISH testing, and for testing strategies in referral laboratories undertaking FISH analysis.
Barrett
Ciara
C
Department of Cellular Pathology, The Adelaide and Meath Hospital, Tallaght, Dublin 24, Ireland.
Magee
Hilary
H
O'Toole
Denise
D
Daly
Sinead
S
Jeffers
Michael
M
D014408:Biomarkers, Tumor / Q000378:metabolism*; D001943:Breast Neoplasms / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D005188:False Negative Reactions; D005189:False Positive Reactions; D005260:Female; D018734:Genes, erbB-2; D006801:Humans; D007150:Immunohistochemistry; D017404:In Situ Hybridization, Fluorescence; D009361:Neoplasm Invasiveness; D009363:Neoplasm Proteins / Q000378:metabolism; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000378:metabolism*; D012189:Retrospective Studies
D014408:Biomarkers, Tumor; D009363:Neoplasm Proteins; D018719:Erb-b2 Receptor Tyrosine Kinases