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

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. Metastatic lungs were resected 2 weeks after primary tumor removal, 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 3000/4000 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 metastatic lungs of mice bearing MDA-MB-231 hypoxia fate-mapping tumors.