The cone-rod homeobox transcription factor (CRX) mRNA as a molecular marker in metastatic retinoblastoma

Abstract

Purpose: Disseminated retinoblastoma is still the major cause of mortality for this tumor worldwide. Research on the dissemination of this neoplasm has been hindered by the rarity of extraocular cases in developed countries. However, it would be of interest for developing countries. Using a prospective evaluation of a diagnostic clinical test, we evaluated the cone-rod homeobox transcription factor (CRX) as a new lineage-specific molecular marker for metastatic retinoblastoma and its usefulness as a tool for improving diagnostic accuracy and assessing the response to treatment and the patterns of disease dissemination in different clinical scenarios by the evaluation of minimal dissemination (MD) in extra-ocular sites.

Methods: To validate CRX mRNA as a marker, we evaluated its expression in 17 retinoblastoma primary tumors, two cell lines, and 47 specimens from other malignancies as negative controls. Seventeen consecutive patients with metastatic retinoblastoma (9 at diagnosis, 8 at relapse) were included. CRX mRNA was evaluated by retrotranscription followed by real-time polymerase chain reaction (RT-qPCR) in bone marrow (BM), peripheral blood (PB), and cerebrospinal fluid (CSF) at diagnosis, after induction chemotherapy, and during follow-up after autologous BM transplantation.

Results: With a sensitivity of 1 in 10⁷ cells, CRX mRNA was expressed in all tumors and cell lines studied but it was negative in all control samples. BM metastatic cells showed expression of CRX in all nine children presenting with metastasis. After induction chemotherapy, no MD was evident in any of the eight responding children. In the CSF of children who had a metastatic relapse, CRX mRNA detection was positive in all 11 cases studied. MD in the CSF heralded a clinical relapse in three cases. No concomitant MD was evident in the BM in any case.

Conclusions: CRX mRNA is a novel marker for retinoblastoma at extraocular sites. In patients with BM metastasis, there is quick, complete and sustained molecular response after induction chemotherapy. In all patients with secondary metastasis, CSF relapse occurs independently from the BM suggesting a sanctuary site.