Among patients with breast cancer, luminal subtypes are the most common. Differences in prognosis have been demonstrated between the luminal subtypes, ranging from patients with an excellent prognosis to those who require much more aggressive treatment. Thus, the identification of new prognostic biomarkers and potential treatment targets within this large subgroup of breast cancer patients is of clinical importance. In this study, the authors used fluorescence in situ hybridization (FISH) to investigate the frequency of the Zinc finger protein 703 gene (ZNF703) in a cohort of women with breast cancer. ZNF703 is located on 8p12 and is part of the A1 amplicon. It has been identified as the most likely driver gene in the A1 amplicon due to its role in estrogen receptor (ER) signaling and cell cycle progression. Overexpression of ZNF703 is associated with increased proliferation in breast cancer, reduced survival in luminal tumors, and reduced response to endocrine therapy in ER positive breast.

Using FISH, mean ZNF703 copy number was determined and compared to mean CEP8 copy number (chromosome 8 enumeration probe, which tags the chromosome 8 centromere, outside of the A1 amplicon). There was an increase in ZNF703 copy number in 14% of cases, with half of those cases having ZNF703 copy number ≥4<6 and the other half having ZNF703 copy number ≥6. Among the same cases, only 4% had an increase in CEP8 copy number. This suggests amplification of the ZNF703 gene in these tumor cell nuclei.

ZNF703 amplification was found within all molecular subtypes but was most strongly associated with the Luminal B subtype (p < 0.001). High ZNF703 copy number was also associated with measures of increased proliferation including Ki ≥ 15% (p < 0.01) and high mitotic counts (p < 0.01). After ten years of follow up, patients with ZNF703 copy number ≥6 had a hgher cumulative risk of death from breast cancer than patients with ZNF703 copy number <4 (48% vs. 32%, respectively; p = 0.04). This result was consistent even when adjustments were made for age, stage, grade, Ki67 and HER2 status.

The authors also used immunohistochemistry (IHC) to quantify expression of ZNF703 and investigate potential links between ZNF703 copy number and protein expression, as well as ZNF703 protein levels and prognosis. The proportion of tumor cells with nuclear ZNF703 immunostaining was assessed by two pathologists and cases were then divided into two groups (ZNF703 <50% and ZNF703 ≥50%). Positive nuclear staining (≥50%) was seen in all molecular subtypes, but was most frequent among the luminal subtypes, with the highest proportion seen among Luminal B tumors (p < 0.001). There was a significant association between increased ZNF703 copy number and positive IHC staining (p < 0.01). However, despite the earlier association between ZNF703 copy number and poor prognosis, and the association between ZNF703 copy number and positive IHC staining, there was no significant association between positive ZNF703 IHC staining and prognosis (p=0.2). The authors noted that the proportion of tumors with positive IHC staining was much higher than the proportion of cases with ZNF703 copy number increase. Further study will be needed to elucidate the relationship between ZNF703 amplification, ZNF703 protein levels, and prognosis.

Klæstad, E., Sawicka, J. E., Engstrøm, M. J., Ytterhus, B., Valla, M., & Bofin, A. M. (2021). ZNF703 gene copy number and protein expression in breast cancer; associations with proliferation, prognosis and luminal subtypes. Breast Cancer Research and Treatment, 186(1), 65–77. doi:10.1007/s10549-020-06035-0.