Multimodal Cytogenetic and Molecular Approach for the Detection of a Constitutional Balanced Paracentric Inversion Disrupting <i>RB1</i> in an Infant With Bilateral Retinoblastoma

We report a case of hereditary bilateral retinoblastoma due to a de novo germline inversion on chromosome 13, resulting in disruption of the RB1 gene. The patient is a 22-month-old female who initially presented to the emergency room at 11 months of age with an erythematous left eye and leukocoria of the right eye. Computed tomography (CT) of the brain and orbits showed solid internal calcifications arising from the posterior globes concerning for bilateral retinoblastoma. Magnetic resonance imaging (MRI) of the brain and orbits confirmed bilateral retinoblastoma without associated pineal region or suprasellar mass. On initial examination under anesthesia, the right eye showed one tumor in the macula and two tumors in the inferior mid-periphery. Sub-retinal seeding extended to the inferior periphery. The left eye was enucleated and pathology showed leptomeningeal extension along the optic nerve extending to the surgical margin. The patient was treated on a non-protocol treatment plan with five cycles of vincristine, carboplatin, etoposide, cyclophosphamide, and weekly intraventricular topotecan via Ommaya reservoir, followed by autologous stem cell rescue. Tumor analysis showed loss of pRB protein expression by immunohistochemistry and methylation copy number profiling showed several segmental gains and losses, including focal loss of RB1 on 13q. A 123-gene cancer predisposition germline panel using genome and exome sequencing initially did not identify any RB1 single nucleotide variants or insertion/deletions. Subsequent constitutional chromosome analysis for RB1 showed a paracentric inversion between bands 13q14.2 and 13q31. Optical genome mapping (OGM) showed that the proximal breakpoint of the balanced inversion at 13q14.2 was within intron 17 of RB1, while the distal breakpoint at 13q31.3 did not interrupt any known genes of clinical significance. We review the various molecular techniques that aided in diagnosis of this patient and provide a summary of similar RB1-disrupting structural variants reported in the literature.