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Flow cytometry analysis and sorting of chromosomes following hybridization with fluorescent probes that target specific DNA repeat sequences Brind'Amour, Julie

Abstract

Traditional cytogenetic approaches allow analysis of the chromosomal composition (karyotype) of mitotic cells fixed on slides cells by microscopy. The combination of karyotyping and Fluorescence In Situ Hybridization (FISH) enables the detection of specific target sequences on individual chromosomes. Disadvantages are that traditional cytogenetic approaches are very labor and time consuming and that chromosome specific information from only a few dozen cells has poor statistical power. An alternative is flow karyotyping, a method to analyze chromosomes in suspension by flow cytometry. For flow karyotyping, the DNA composition of specific chromosomes in suspension is measured based on the DNA-specific dyes Hoechst 33258 (HO) and Chromomycin A3 (CA3). My thesis work has focused on the development of a new method to analyze and sort chromosomes using FISH with labeled peptide nucleic acid (PNA) probes on chromosomes in suspension. I found that, following FISH, flow karyotyping can be used to detect and quantify repetitive DNA sequences within individual chromosomes. Using chromosome flow FISH (CFF), chromosomes isolated from cells of various species were hybridized to PNA probes and analyzed by flow cytometry. CFF was used to detect a variety of repeats; interstitial telomeric sequences in Chinese Hamster chromosomes, major satellite in mouse chromosomes and D18Z1 alpha satellite repeats in human chromosomes. Quantitative measurements of repeat length by CFF were validated by comparison with measurements obtained using Q-FISH. We found that parental homologs of human chromosome 18 with different D18Z1 satellite repeat array size could be purified using CFF and Fluorescence Activated Cell Sorting (FACS). Illumina short read sequencing of libraries built from these purified chromosomes enabled us to determine, with a high resolution, the allelic phasing of each homolog over the entire chromosome 18. Finally, CFF was modified to study sister chromatids separately. Using a cell model with inducible separation of sister chromatids, flow karyograms were generated. Using chromosome orientation FISH (CO-FISH) in suspension, we could identify sister chromatids according to the presence of DNA template strands. We anticipate that this approach will allow the purification of sister chromatids to study epigenetic differences between sister chromatids defined on the basis of DNA template strands.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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