Liver x receptor splice variants are prognostic indicators in triple negative breast cancer patients


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Priscilia Lianto1, J. Bernadette Moore1, Thomas A Hughes1, James L Thorne1
1University of Leeds

Abstract

Background

The liver x receptors (LXRs) are ligand-responsive transcription factors that link lipid metabolism with cancer pathophysiology. LXR activity is elevated in triple negative breast cancer (TNBC) relative to other breast cancer (BCa) subtypes, driving gene signatures associated with drug resistance and metastasis. Splicing of the LXR pre-mRNAs produces several transcript and protein variants but their relevance to BCa has not previously been investigated.

Method

The expression of LXR transcript variants was assessed in the TCGA splicing variant database and linked to survival in 803 oestrogen receptor (ER)-positive and 101 TNBC patients. LXR splice variants were then measured by immunoblot in 38 primary TNBC tumours in duplicate. Median follow-up for disease-free survival (DFS) was 36 months. Identity of variants was confirmed using both PCR targeting unique exon-exon junctions, and mass-spectrometry of protein lysates.

Results

Five LXRα and two LXRβ isoforms were detected in TNBC tumours. Three have not been reported in the literature before and five were prognostic indicators of DFS at protein level. High expression of LXRα1 protein and its extended N-terminus splice variant LXRα4, were associated with reduced DFS (LXRα1: p=0.0005; LXRα4: p=0.0079). Conversely, high levels of LXRα5, which harbours deletions in the activation function and ligand binding domains, and LXRβ variants, were associated with longer DFS (LXRα5: p=0.04; LXRβ1: p=0.0023; LXRβ4: p=0.037). LXRα1 was the predominant LXRα splice variant in TNBC samples. LXRα5 was absent or low in TNBC samples but highly expressed in ER-positive samples. LXRα2 and LXRα3 proteins were not associated with DFS (p>0.05). However, the α3 transcript, which codes for intact LBD but harbouring a small deletion in the AF1 domain was, like LXRα1, associated with shorter DFS (p=0.022); the α2 transcript, which codes for a deletion in the ligand binding domain was, like α5, associated with longer DFS (p=0.031).

Conclusion

These data support the hypothesis that LXR plays a role in TNBC tumour pathophysiology. 

Impact statement

LXR splicing may be an important parameter for stratification of TNBC patients. Detection of transcript splice variants in fresh/frozen biopsy is rapid, reproducible and accurate and offers additional prognostic value in this hard to cure group of cancer patients.