Retinoblastoma via the RB1 Gene

Retinoblastoma (RB) is the most common intraocular cancer in childhood, with onset usually before five years of age. Incidence is one case per 15,000-20,000 livebirths worldwide. The most common presenting sign of RB is a white pupillary reflex (leukocoria), which reflects light (e.g., photographic flash) and blocks view of the red retina. Heritable retinoblastoma presents at a younger age than does nonheritable disease (Dimaras et al. Lancet 379(9824):1436-46, 2012). Approximately 40% of patients affected with RB are affected unilaterally and 60% of patients bilaterally. Children with germline RB1 variants are also at a risk of pinealomas which occur in the pineal gland of the brain. Other cancers which typically occur later adolescent or adult life are osteosarcomas, soft tissue sarcomas, or melanomas (Lohmann and Gallie. GeneReviews. 2010).

Retinoblastoma is caused by variants in the RB1 gene, which is a tumor suppressor that is involved in cell cycle regulation (G1 to S phase; Lohmann and Gallie. GeneReviews. 2010). The pRB protein is phosphorylated by a cyclin-dependent kinase complex. Upon phosphorylation, the binding activity of the pocket domain is lost, resulting in the release of cellular proteins involved in cell cycle progression. Most variants lead to complete nonfunctional pRB activity leading to faulty cell cycle arrest upon cellular stresses; however, some variants have residual activity which can lead to low-penetrance retinoblastoma (Bremner et al. Am J Hum Genet 61:556–70, 1997). Most variants are specific to families, suggesting a high rate of variants at the RB1 locus and usually through the paternal germline (Dimaras et al. Lancet 379(9824):1436-46, 2012). An inherited mutated copy of RB1 predisposes a child to ocular cancer and loss of the other allele is involved in the progression to retinoblastoma along with other cancer pathways.

Overall clinical sensitivity using molecular analysis of the RB1 gene is approximately 95% (Rushlow et al. Hum Mutat 30: 842–51, 2009). About 75% of cases are caused by single nucleotide changes and small indels, which can readily be detected by sequencing. About 10% of cases are due to hypermethylation of the RB1 promoter and will not be detected by sequencing (Lohmann and Gallie. GeneReviews. 2010). Gross deletions and duplications occur in up to 16% of RB1 variants. Approximately 10-12% variants are due to hypermethylation of the RB1 promoter and will not be detected by deletion and duplication analysis (Lohmann and Gallie. GeneReviews. 2010).

This test provides full coverage of all coding exons of the RB1 gene, plus ~10 bases of flanking noncoding DNA. We define full coverage as >20X NGS reads or Sanger sequencing.