Monosomy of chromosome 17 in breast cancer during interpretation of HER2 gene amplification

Abstract

Monosomy of chromosome 17 may affect the assessment of HER2 amplification. Notably, the prevalence ranges from 1% up to 49% due to lack of consensus in recognition. We sought to investigate the impact of monosomy of chromosome 17 to interpretation of HER2 gene status. 201 breast carcinoma were reviewed for HER2 gene amplification and chromosome 17 status. FISH analysis was performed by using double probes (LSI/CEP). Absolute gene copy number was also scored per each probe. HER2 FISH test was repeated on serial tissue sections, ranging in thickness from 3 to 20 µm. Ratio was scored and subsequently corrected by monosomy after gold control test using the aCGH method to overcome false interpretation due to artefactual nuclear truncation. HER2 immunotests was performed on all cases. 26/201 cases were amplified (13%). Single signals per CEP17 were revealed in 7/201 (3.5%) cases. Five out of 7 cases appeared monosomic with aCGH (overall, 5/201, 2.5%) and evidenced single signals in >60% of nuclei after second-look on FISH when matching both techniques. Among 5, one case showed amplification with a pattern 7/1 (HER2/CEP17>2) of copies (3+ at immunotest); three cases revealed single signals per both probes (LSI/CEP=1) and one case revealed a 3:1 ratio; all last 4 cases showed 0/1+ immunoscore. We concluded that: 1) monosomy of chromosome 17 may be observed in 2.5% of breast carcinoma; 2) monosomy of chromosome 17 due to biological reasons rather than nuclear truncation was observed when using the cut-off of 60% of nuclei harboring single signals; 3) the skewing of the ratio due to single centromeric 17 probe may lead to false positive evaluation; 4) breast carcinomas showing a 3:1 ratio (HER2/CEP17) usually show negative 0/1+ immunoscore and <6 gene copy number at FISH.

Keywords: Breast carcinoma; FISH analysis; HER 2 amplification; aCGH method; chromosome 17; double probes; false positive; monosomy; ratio (HER2/CEP 17); single signals.

Figure 1

Breast carcinoma with monosomy of chromosome 17. Ductal phenotype, luminal A young age (2/5 cases <38 years) (A); triple negative breast carcinoma (2/5 cases) (B); a case of bilateral breast carcinoma (1/5) (C). Four out of five displayed a negative (0 or 1+) HER2 immunoscore (D).

Figure 2

Breast carcinoma with monosomy of chromosome 17. The tumor harbors single signals in >60% of neoplastic nuclei per both LSI Her-2/neu and CEP17 loci at FISH analysis and reveals loss of whole chromosome 17 (type 1 monosomy) after aCGH molecular profiling.

Figure 3

Breast carcinoma with partial monosomy of chromosome 17. The tumor harbors single signals per both LSI Her-2/neu and CEP17 probes in >60% of neoplastic nuclei and evidenced losses of chromosome 17p, CEP17 and the partial chromosome 17q12-21 (type 2 monosomy).

Figure 4

Recommendations’ on Her-2/neu FISH testing when single signals is observed at FISH analysis; when monosomy of chromosome 17 is observed (>60% nuclei with single CEP17 probe), use the single absolute Her-2/neu gene copy number (>6) when distinguishing a case of breast carcinoma with amplification versus not amplified. Type 1 chr17 monosomy reveals loss of whole chromosome 17 whereas type 2 chr17 monosomy show partial losses of chromosome 17 (i.e. loss of 17q, CEP17 and part of 17p).