Background/Purpose: Human SLE patients have an abnormal population of neutrophils called low density granulocytes (LDGs). LDGs express the surface markers of mature neutrophils, yet their nuclear morphology resembles an immature cell.  A similar discrepancy in maturation status is observed in myelodysplasias, suggesting a disruption in neutrophil development in the LDGs.  Because disruption of neutrophil development is frequently associated with genomic alterations, we compared genomic DNA isolated from autologous pairs of LDGs and normal-density neutrophils for genetic changes.

Methods: Somatic alterations were detected by cytogenetic microarray analysis of genomic DNA from LDGs and neutrophils from 13 female SLE patients, as well as neutrophil samples from 9 healthy female donors.  Autologous pairs of SLE LDGs and neutrophils were processed and analyzed together to minimize variability. Variations present in both samples are consistent with inheritance, while alterations found exclusively in the LDG sample suggest somatic alterations.  While copy number alterations are associated with repair of DNA strand breaks, a different type of genomic alteration is associated with defects in the correction of DNA replication errors.  Microsatellite instability (MSI) is a characteristic of replication error-prone cells.  MSI was assessed at 6 mononucleotide markers.

Results: Control neutrophils had an average of 7.76 chromosomal alterations per genome, similar to the average of the SLE neutrophils value of 8.77 alterations per genome, in contrast, the autologous SLE LDGs had 18.77 alterations per genome.  The increased frequency of copy number alterations was comprised of both an elevation in the number of deletions and duplications. Copy number alterations were prevalent in 6 of the 13 patients and occurred preferentially on certain chromosomes with the majority of the alterations restricted to the X chromosome, and chromosomes 19, 17, and 8. These elevations in copy number alterations were accompanied with an increase in large genome-wide losses of heterozygosity in these same SLE patients.  The enhanced level of copy number alterations and losses of heterozygosity are consistent with DNA strand break repair in LDGs.Microsatellite instability (MSI) was also detected in the LDGs of three patients, suggesting impaired correction of replication errors.  Two patients displayed MSI in greater than one marker, and one patient had MSI and increased copy number alterations.  No correlations between genomic instability or MSI and immunosuppressive drugs, disease activity or disease manifestations were apparent.

Conclusion: LDGs had elevated levels of genetic alterations consistent with accumulated genetic damage or genomic instability.  Copy number alterations were more prevalent on certain chromosomes and restricted to discrete chromosomal intervals, suggesting an increased propensity for damage within these intervals and/or that these alterations confer a selective advantage. The pro-inflammatory environment in SLE patients may promote accumulation of these genetic alterations, alternatively a genetic predisposition toward impaired mechanisms of DNA repair may facilitate the apparent genomic instability in the LDGs.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/genetic-alterations-in-abnormal-neutrophils-isolated-from-human-systemic-lupus-erythematosus-patients/