PCR-Fluo-ASXL1-FA: A fast, sensitive and inexpensive complementary method to detect ASXL1 mutations in haematological malignancies

INTRODUCTION

The additional sex combs like 1 (ASXL1) gene is frequently mutated in a number of haematological neoplasms. The c.1934dupG, known to be the most common alteration in ASXL1, is associated with poor clinical outcome. A systematic determination of ASXL1 mutational status in myeloid malignancies is therefore necessary for prognostic stratification.

METHODS

Because direct sequencing is not sensitive and next-generation sequencing (NGS) is time-consuming, expensive and sometimes does not allow the detection of the c.1934dupG, we have developed a fragment analysis assay, complementary to NGS, that allows the detection of c.1934dupG mutation in addition to other nearby insertions/deletions of ASXL1 located close to it. We called this assay the "PCR-Fluo-ASXL1-FA."

RESULTS

First, we evaluated the efficiency of our approach compared to NGS and Sanger. We showed that "PCR-Fluo-ASXL1-FA" could detect all insertional mutations of ASXL1 located on its area, with a high sensitivity (1.5%). Then, we have illustrated the interest of this technique by three concrete cases.

DISCUSSION

In summary, we have established a fragment analysis approach, which can detect most ASXL1 mutations, in particular the c.1934dupG, in a sensitive, fast and inexpensive manner. We therefore recommend the synchronous use of this method with NGS, to ensure complete detection of all clinically relevant ASXL1 mutations in patients suffering with myeloid neoplasms.

Plasmacytoid dendritic cells proliferation associated with acute myeloid leukemia: phenotype profile and mutation landscape

Neoplasms involving plasmacytoid Dendritic Cells (pDCs) include Blastic pDC Neoplasms (BPDCN) and other pDC proliferations, where pDCs are associated with myeloid malignancies: most frequently Chronic MyeloMonocytic Leukemia (CMML) but also Acute Myeloid Leukemia (AML), hereafter named pDC-AML. We aimed to determine the reactive or neoplastic origin of pDCs in pDC-AML, and their link with the CD34+ blasts, monocytes or conventional DCs (cDCs) associated in the same sample, by phenotypic and molecular analyses (targeted NGS, 70 genes). We compared 15 pDC-AML at diagnosis with 21 BPDCN and 11 normal pDCs from healthy donors. CD45low CD34+ blasts were found in all cases (10-80% of medullar cells), associated with pDCs (4-36%), monocytes in 14 cases (1-10%) and cDCs (2 cases, 4.8-19%). pDCs in pDC-AML harbor a clearly different phenotype from BPDCN: CD4+ CD56- in 100% of cases, most frequently CD303+, CD304+ and CD34+; lower expression of cTCL1 and CD123 with isolated lymphoid markers (CD22/CD7/CD5) in some cases, suggesting a pre-pDC stage. In all cases, pDCs, monocytes and cDC are neoplastic since they harbor the same mutations as CD34+ blasts. RUNX1 is the most commonly mutated gene: detected in all AML with minimal differentiation (M0-AML) but not in the other cases. Despite low number of cases, the systematic association between M0-AML, RUNX1 mutations and an excess of pDC is puzzling. Further evaluation in a larger cohort is required to confirm RUNX1 mutations in pDC-AML with minimal differentiation and to investigate whether it represents a proliferation of blasts with macrophage and DC progenitor potential.

OS1.3 Tumor cell-free DNA detection in CSF for primary CNS lymphoma diagnosis

BACKGROUND

Primary CNS lymphoma (PCNSL) is a rare disease accounting for around 3% of the primary brain tumors. In the vast majority of cases, PCNSL are classified as diffuse large B-cell lymphoma according to the WHO classification. The diagnosis is based on cranial MRI, in combination with a brain biopsy. In case of classical MRI findings, the identification of lymphoma cells in the cerebrospinal (CSF) or vitreous fluid by cytology and flow cytometry might obviate brain biopsy. The presence of the somatic mutation p.Leu265Pro (L265P) in MYD88 is detectable in 50 to 80% of PCNSL, and might also be helpful to confirm the diagnosis. The aim of our study was to evaluate the contribution of a highly sensitive digital droplet PCR, targeting the mutation L265P MYD88, for the detection of tumoral circulating DNA from CSF supernatant.

MATERIAL AND METHODS

We identified 9 PCNSL expressing the L265P mutation at diagnosis or relapse. The mutation was found by an allele specific PCR technique either on biopsy or in CSF cells. Circulating DNA was isolated from CSF supernatant with the « QiaAmp® circulating nucleic acid» kit. The quantity of DNA collected was estimated by quantitative PCR for a reference gene (albumine) with 7900HT (Life technologies™) device. Subsequently, the L265P MYD88 mutation was quantified by digital droplet PCR Biorad™: The droplets generated were amplified by PCR, detected with the QX200 Reader, and analyzed with the QuantaSoft™ software.

RESULTS

The circulating DNA concentration was low, varying between 0 and 2.2 ng/mL of CSF. However, the mutation was detected in the circulating DNA from CSF supernatant in 6 out of 9 cases (66%). The fractional abundance varied from 2.6 to 85%. In 3 cases, the mutation was detected even though cytology and flow cytometry did not reveal leptomeningeal disease. For 3 other cases, the mutation was not detected: The genome copy number was below 1 copy/µL, indicating a low analytical sensitivity for theses samples.

CONCLUSION

This study shows that circulating DNA is present in low concentration in CSF and can be amplified by a sensitive digital PCR for the L265P MYD88 mutation. The detection of circulating PCNSL DNA in CSF is possible and might be used to improve the non-invasive diagnosis of PCNSL. It might also help to select patients for targeted therapies.