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Analysis of RNA quantitation in formaldehyde-fixed cells

ABSTRACT: Fluorescence-activated cell sorting (FACS) is a sensitive and valuable technique to characterize cellular subpopulations and great advances have been made using this approach. Cells are often fixed with formaldehyde prior to the sorting process to preserve cell morphology and maintain the expression of surface molecules, as well as to ensure safety in the sorting of infected cells. It is widely recognized that formaldehyde fixation alters RNA and DNA structure and integrity, thus analyzing gene expression in these cells has been difficult. We therefore examined the effects of formaldehyde fixation on the stability and quantitation of nucleic acids in cell lines, primary leukocytes and also cells isolated from SIV-infected pigtailed macaques. We developed a method to extract RNA from fixed cells that yielded the same amount of RNA as our common method of RNA isolation from fresh cells. Quantitation of RNA by RT-qPCR in fixed cells was not always comparable with that in unfixed cells. In comparison, when RNA was measured by the probe-based NanoString system, there was no significant difference in RNA quantitation. In addition, we demonstrated that quantitation of proviral DNA in fixed cells by qPCR is comparable to that in unfixed cells when normalized by a single-copy cellular gene. These results provide a systematic procedure to quantitate gene expression in cells that have been fixed with formaldehyde and sorted by FACS.

ORGANISM(S): Homo sapiens

PROVIDER: GSE50690 | GEO | 2013/09/09

SECONDARY ACCESSION(S): PRJNA218535

REPOSITORIES: GEO

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Project description:We show that high quality microarray gene expression profiles can be obtained following FACS sorting of cells using combinations of transcription factors. We use this transcription factor FACS (tfFACS) methodology to perform a genomic analysis of hESC-derived endodermal lineages marked by combinations of SOX17, GATA4, and CXCR4, and find that triple positive cells have a much stronger definitive endoderm signature than other combinations of these markers. Additionally, SOX17+GATA4+ cells can be obtained at a much earlier stage of differentiation, prior to expression of CXCR4+ cells, providing an important new tool to isolate this earlier definitive endoderm subtype. Overall, tfFACS represents an advancement in FACS technology which broadly crosses multiple disciplines, most notably in regenerative medicine to redefine cellular populations. 21 Total samples were analyzed. Samples collected after 5 days of hESC differentiation included SOX17+GATA4+ CXCR4+ cells, unfixed CXCR4+ cells, and unsorted fixed cells. Samples collected after 3 days of differentiation included SOX17+GATA4+ cells, SOX17-GATA- cells, and unsorted fixed cells. As controls, we analyzed both unfixed hESCs and fixed hESCs. All samples contained biological duplicates, triplicates or quadriplicates. We performed hierarchical clustering to demonstrate whether cellular fixation alone could change gene expression. We based this analysis on 1647 transcript clusters with coefficient of variation > 0.5 across the samples and expression values >= 500 in at least 2 out of the 21 samples. We found that fixation and staining steps did not cause distortions in gene expression measurements. This is supported by the fact that fixed and unfixed cells cluster together based upon differentiation stage, not based upon degree of fixation. Furthermore, using GSEA analysis, we found that the SOX17+GATA4+CXCR4+ day 5 cells and day 3 SOX17+GATA4+ were more enriched for compiled gold-standard endodermal genes than the CXCR4+ day 5 population.