Chromosome 6p amplification detected in blood cell-free DNA in advanced intraocular retinoblastoma.

ABSTRACT

BACKGROUND: In patients with retinoblastoma, gains of chromosome 6p have been associated with less differentiated tumors. In cell-free DNA from the aqueous humor (AH), 6p gain has been associated with an increased risk of enucleation. While the identification of somatic copy number alterations (SCNAs) via the AH has been well established, these alterations are not routinely identified in the blood due to low tumor fraction. MATERIALS AND METHODS: SCNAs were considered positive at 20% deflection from the baseline. Somatic RB1 pathogenic variants were identified with targeted sequencing using a panel including all RB1 exons. RESULTS: A 24-month-old patient presented with unilateral retinoblastoma (Group D/AJCC Stage cT2B) and was treated with primary enucleation. In the peripheral blood, a heterozygous mutation (c.3920T>A) in the APC gene was reported. Genomic analysis of the tumor and AH revealed two novel somatic RB1 mutations (c.1589_1590del and c.2330dupC). Both also demonstrated highly recurrent RB-related SCNAs. Chromosome 6p gain was detected in the blood with an amplitude suggesting approximately 12% tumor fraction. At a follow-up of 24 months, there has been no evidence of metastatic disease. CONCLUSIONS: To our knowledge, this is the first time an SCNA has been detected in the blood of an RB patient, suggesting in some advanced eyes there may be a high enough tumor fraction to detect these alterations (>5% needed). It remains unclear whether 6p gain or increased tumor fraction in the blood is indicative of increased risk of metastatic disease or new primary cancer; studies to address this are ongoing.