Analysis of CLLU1 expression levels before and after therapy in patients with chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is incurable, but therapy leading to eradication of minimal residual disease (MRD) in CLL is associated with improved clinical outcomes. CLL upregulated gene 1 (CLLU1) is solely upregulated in CLL patient samples. We hypothesized that CLLU1 could be used to monitor for residual disease in CLL patient samples after therapy.

We examined whether the CLLU1 real-time quantitative PCR (RQ-PCR) could detect small numbers of CLL cells in mixtures of normal peripheral blood mononuclear (PBMC) cells. We then performed a retrospective analysis on time-matched cryo-preserved specimens from patients who achieved MRD-negative remissions that underwent serial marrow biopsies for evaluation of residual disease by 4-color flow cytometry. RNA from PBMC samples collected at the time of the marrow assessments was analyzed for CLLU1. Nine patients underwent a total of 46 paired blood and marrow evaluations (median 5 assessments per patient).

CLLU1 RQ-PCR on PBMCs of healthy donors reconstituted with varying amounts of CLL cells demonstrated leukemia cells could be reliably detected with high sensitivities depending on the CLLU1 expression level. Analysis of time-matched samples assessed for CLLU1 levels in the blood by RQ-PCR and residual disease of the marrow determined by 4-color flow cytometry revealed a correlation coefficient of 0.96 (P < 0.0001).

The CLLU1 RQ-PCR is a sensitive and specific assay for detecting residual CLL cells after therapy. Assessment of blood CLLU1 levels can be used as a reliable marker of tumor burden and has the potential to complement currently used techniques for MRD monitoring in patients with CLL.