A novel fate-mapping approach allows intratumoral profiling of hypoxic cells

We designed a novel approach to fate-map hypoxic cells in order to determine their cellular response to physiological O2 gradients. Our system causes a change in the expressing fluorescent protein upon hypoxic exposure (DsRed -> GFP). We generated hypoxia fate-mapping tumors using MDA-MB-231 cells expressing our system. Tumors were resected 2 weeks post-implantation, mechanically disrupted and digested with collagenase to obtain a cell suspension. The cell suspension was enriched using magnetic-activated cell sorting (MACS) and DsRed+ cells were isolated from GFP+ cells by fluorescence-activated cell sorting (FACS) directly into Tris Reagent (Zymo Research). Total RNA was extracted from cells using TRIzol (Invitrogen) and purified using Direct-zol RNA mini kit (Zymo Research) with DNase I treatment. After RNA purification, samples were confirmed to have a RIN value > 9.0 when measured on an Agilent Bioanalyzer. Libraries for RNA-Seq were prepared with KAPA Stranded RNA-Seq Kit. The workflow consisted of mRNA enrichment, cDNA generation, end repair to generate blunt ends, A-tailing, adaptor ligation and 12 cycles of PCR amplification. Unique adaptors were used for each sample in order to multiplex samples into several lanes. Sequencing was performed on Illumina Hiseq 4000 with a 150bp pair-end run. A data quality check was done on Illumina SAV. Demultiplexing was performed with Illumina Bcl2fastq2 v 2.17 program. In order to permanently mark hypoxic cells upon exposure to hypoxia, we generated a dual-vector hypoxia fate-mapping system delivered by a lentiviral approcah. Vector 1 expresses a red fluorescent reporter protein (DsRed) with a stop codon flanked by tandem loxP sites (“floxed”) and located in front of a gene encoding a green fluorescent protein (GFP). Vector 2 contains an altered Cre gene modified by the addition of an oxygen-dependent degradation domain (ODD) that is under the transcriptional control of a synthetic HIF-DNA binding sequence (HRE). HIF stabilization causes the activation of vector 2 by binding to hypoxia-dependent DNA response elements (HREs). Vector 2 activation causes the production of a genetically modified Cre protein that is only stable under hypoxia, leading to the cleavage of DsRed and permanent GFP expression. DsRed+ and GFP+ cells were sorted from MDA-MB-231 hypoxia fate-mapping tumors.