High rate of mutation reporter gene inactivation during human T cell proliferation

Longitudinal study of t-cell somatic mutations conferring glycosylphosphatidylinositol-anchor deficiency in gulf war I veterans exposed to depleted uranium

Fifty Veterans of the first Gulf War in 1991 exposed to depleted uranium (DU) were studied for glycosylphosphatidylinositol-anchor (GPIa) deficient T-cell mutants on three occasions during the years 2009, 2011, and 2013. GPIa deficiency was determined in two ways: cloning assays employing aerolysin selection and cytometry using the FLAER reagent for positive staining of GPIa cell surface proteins. Subsequent molecular analyses of deficient isolates recovered from cloning assays (Nicklas JA et al. [2019]: Environ Mol Mutagen) revealed apparent incomplete selection in some cloning assays, necessitating correction of original data to afford a more realistic estimate of GPIa deficient mutant frequency (MF) values. GPIa deficient variant frequencies (VFs) determined by cytometry were determined in the years 2011 and 2013. A positive but nonsignificant association was observed between MF and VF values determined on the same blood samples during 2013. Exposure to DU had no effect on either GPIa deficient MF or VFs. Environ. Mol. Mutagen. 60:494-504, 2019. © 2019 Wiley Periodicals, Inc.

The development of an in vitro Pig-a assay in L5178Y cells

A recent flow cytometry-based in vivo mutagenicity assay involves the hemizygous phosphatidylinositol class A (Pig-a) gene. Pig-a forms the catalytic subunit of N-acetylglucosaminyltransferase required for glycophosphatidylinositol (GPI) anchor biosynthesis. Mutations in Pig-a prevent GPI-anchor synthesis resulting in loss of cell-surface GPI-linked proteins. The aim of the current study was to develop and validate an in vitro Pig-a assay in L5178Y mouse lymphoma cells. Ethyl methanesulfonate (EMS)-treated cells (186.24-558.72 µg/ml; 24 h) were used for method development and antibodies against GPI-linked CD90.2 and stably expressed CD45 were used to determine GPI-status by flow cytometry. Antibody concentration and incubation times were optimised (0.18 µg/ml, 30 min, 4 °C) and Zombie Violet™ (viability marker; 0.5%, 30 min, RT) was included. The optimum phenotypic expression period was 8 days. The low background mutation frequency of GPI-deficiency [GPI(-)] in L5178Y cells (0.1%) constitutes a rare event, thus flow cytometry acquisition parameters were optimised; 10⁴ cells were measured at medium flow rate to ensure a CV ≤ 30%. Spiking known numbers of GPI(-) cells into a wild-type population gave high correlation between measured and spiked numbers (R² 0.999). We applied the in vitro Pig-a assay to a selection of well-validated genotoxic and non-genotoxic compounds. EMS, N-ethyl-N-nitrosourea and 4-nitroquinoline-N-oxide dose dependently increased numbers of GPI(-) cells, while etoposide, mitomycin C, and a bacterial-specific mutagen did not. Cycloheximide and sodium chloride were negative. Sanger sequencing revealed Pig-a mutations in the GPI(-) clones. In conclusion, this in vitro Pig-a assay could complement the in vivo version, and follow up weak Ames positives and late-stage human metabolites or impurities.

Both PIGA and PIGL mutations cause GPI-a deficient isolates in the Tk6 cell line

Molecular analysis of proaerolysin selected glycosylphosphatidylinositol anchor (GPI-a) deficient isolates in the TK6 cell line was performed. Initial studies found that the expected X-linked PIGA mutations were rare among the spontaneous isolates but did increase modestly after ethyl methane sulfate (EMS) treatment (but to only 50% of isolates). To determine the molecular bases of the remaining GPI-a deficient isolates, real-time analysis for all the 25 autosomal GPI-a pathway genes was performed on the isolates without PIGA mutations, determining that PIGL mRNA was absent for many. Further analysis determined these isolates had several different homozygous deletions of the 5' region of PIGL (17p12-p22) extending 5' (telomeric) through NCOR1 and some into the TTC19 gene (total deletion >250,000 bp). It was determined that the TK6 parent had a hemizygous deletion in 17p12-p22 (275,712 bp) extending from PIGL intron 2 into TTC19 intron 7. Second hit deletions in the other allele in the GPI-a deficient isolates led to the detected homozygous deletions. Several of the deletion breakpoints including the original first hit deletion were sequenced. As strong support for TK6 having a deletion, a number of the isolates without PIGA mutations nor homozygous PIGL deletions had point mutations in the PIGL gene. These studies show that the GPI-a mutation studies using TK6 cell line could be a valuable assay detecting point and deletion mutations in two genes simultaneously.

Mutagenicity monitoring following battlefield exposures: Longitudinal study of HPRT mutations in Gulf War I veterans exposed to depleted uranium

A total of 70 military Veterans have been monitored for HPRT T-cell mutations in five separate studies at 2-year intervals over an 8-year period. Systemic depleted uranium (DU) levels were measured at the time of each study by determining urinary uranium (uU) excretion. Each HPRT study included 30-40 Veterans, several with retained DU-containing shrapnel. Forty-nine Veterans were evaluated in multiple studies, including 14 who were in all five studies. This permitted a characterization of the HPRT mutation assay over time to assess the effects of age, smoking and non-selected cloning efficiencies, as well as the inter- and intra-individual variability across time points. Molecular analyses identified the HPRT mutation and T-cell receptor (TCR) gene rearrangement in 1,377 mutant isolates. An unexpected finding was that in vivo clones of HPRT mutant T-cells were present in some Veterans, and could persist over several years of the study. The calculated HPRT mutant frequencies (MFs) were repeatedly elevated in replicate studies in three outlier Veterans with elevated urinary uranium excretion levels. However, these three outlier Veterans also harbored large and persistent in vivo HPRT mutant T-cell clones, each of which was represented by a single founder mutation. Correction for in vivo clonality allowed calculation of HPRT T-cell mutation frequencies (MutFs). Despite earlier reports of DU associated increases in HPRT MFs in some Veterans, the results presented here demonstrate that HPRT mutations are not increased by systemic DU exposure. Additional battlefield exposures were also evaluated for associations with HPRT mutations and none were found.

Changes in chromosome organization during PHA-activation of resting human lymphocytes measured by cryo-FISH

During interphase, chromosomes are arranged into territories within a highly organized nuclear space containing several compartments. It is becoming clear that this complex nuclear arrangement is important for gene regulation and therefore expression. The study of chromosome organization in interphase requires high-resolution imaging methods that at the same time allow for flexible labelling strategies and preserve nuclear structure. Tokuyasu cryosections of cells or tissues provide a simple, high-resolution platform for performing immunolabelling and fluorescence in situ hybridization (FISH) on well-preserved samples. Here we show how FISH performed on thin cryosections (cryo-FISH) can be used for the study of chromosome organization at high resolution and in a quantitative manner. We have measured chromosome intermingling, volume and radial position, in resting and activated human lymphocytes, and observed chromosome-specific differences between the two cellular states. These differences are in part related to the nuclear expansion that occurs during activation, but are also likely to be tied to their different transcriptional profiles. Extrapolation of our dataset to the whole genome suggests that activated cells contain a lower amount of chromatin involved in intermingling than resting cells.