Aqueous humor as a surrogate biomarker for retinoblastoma tumor tissue

Purpose: To demonstrate the feasibility of identifying a germline RB1 pathogenic variant in retinoblastoma (RB) from an aqueous humor (AH) sample.

Methods: In this pilot case series, peripheral blood, fresh tumor tissue, and AH were obtained from 3 eyes of 3 RB patients who underwent enucleation at a tertiary eye care institute. After isolation of the cell-free DNA (cfDNA), sequence analysis of the RB1 core promoter and of exons 1 through 27, including nearby flanking intronic regions, was performed using a custom targeted hybridization protocol, followed by high-throughput sequencing.

Results: The study cohort included 3 enucleated eyes with advanced RB (group E [n = 2], group D [n = 1]). In case 1, deletion of the RB1 promoter to exon 23 (delP->23) on both alleles was identified from tumor as well as AH samples and absent in the blood sample, indicative of absence of a germline RB1 pathogenic variant. In case 2, two heterozygous RB1 nonsense variants, c.610G>T p.(Glu204Ter) and c.751C>T p.(Arg251Ter), were identified in tumor and AH samples (allele frequency of 49% and 45%, resp.) and were absent in the blood sample, indicative of absence of a germline RB1 pathogenic variant. In case 3, a heterozygous c.2326-14T>A substitution on allele 1 and loss of heterozygosity on allele 2 were identified in the tumor and AH (allele frequency of 97%), with the same heterozygous mutation in the blood sample, indicating presence of a germline RB1 pathogenic variant.

Conclusions: The pathogenic RB1 variant results from AH in all 3 eyes were concordant with direct tumor DNA sampling, suggesting that AH can serve as a surrogate for tumor tissue. Because the AH can be accessed during treatment, specific testing can be performed even in the absence of enucleation.