Methylated RASSF1a Is the first specific DNA marker for minimal residual disease testing in neuroblastoma

Purpose: PCR-based detection of minimal residual disease (MRD) in neuroblastoma (NB) is presently based on NB-specific transcripts. However, the expression of these targets varies between patients and upon treatment, and only PHOX2B is truly specific. RASSF1a is methylated (RASSF1a _M) in NB, and we investigated whether it can serve as a specific and stable DNA MRD marker. Patients and Methods: The RASSF1a _M-specific quantitative real-time PCR was tested on control bone marrow (BM; n=50), on 71 NB tumors, and on 159 clinical BM samples at diagnosis and at follow-up of 77 patients. Results were compared with a panel of RNA markers and correlated with prognosis. Results: RASSF1a _M was present in all stage 4 and 4s tumors (n = 50) and in 86% stages 1 to 3 tumors (n = 21). The level of methylation in stage 4 NB was correlated with overall survival (P = 0.02). RASSF1a _M-PCR was highly specific (only 1 amplification in 50 control samples tested in triplicate) and had a similar sensitivity as the RNA-based PCRs, as shown on clinical samples. Moreover, RASSF1a _M enabled accurate quantification without need for the original tumor. Conclusions: RASSF1a _M is a novel, highly specific DNA marker for MRD detection in NB, equal to PHOX2B in specificity and sensitivity, and better suitable for MRD quantification. We propose to include RASSF1a _M in further prospective MRD studies in NB alongside RNA MRD markers. In addition, this assay might also be applicable for detection of circulating tumor cells in patients with other cancers with RASSF1a _Msuch as breast or lung cancer.