Some Aggressive Retinoblastomas Lack RB1 Mutations

Ricki Lewis, PhD

March 19, 2013

Most retinoblastomas are driven by loss of both copies of the RB1 gene. Now researchers report that nearly 1 in 5 children with aggressive, very-early-onset retinoblastoma have normal RB1 genes; their tumors are driven by amplification of the MYCN oncogene, according to a report published online March 13 in Lancet Oncology.

Retinoblastoma (RB) inspired Alfred Knudson's 2-hit model of cancer origin, and in 1971 RB1 became the first-described tumor suppressor gene (Proc Natl Acad Sci U S A 1971;68:820-823).

A germline mutation in RB1 coupled with somatic mutation in a retinal cell lies behind the 40% of RB cases that are bilateral and familial. The remaining 60% of cases are unilateral. Of those, only 15% include an inherited mutation; the other 85% have 2 somatic mutations in the original cell and its descendants.

Challenging the Dogma

The newly published work by Diane E. Rushlow, BSc, from the Toronto Western Hospital Research Institute, Ontario, Canada, and colleagues challenges the dogma that all RB cases result from 2 hits to RB1.

"Over the last decade of molecular testing, RB1 mutations were found in more than 95% of cases. The new MYCN form of sporadic, nonfamilial RB has never been included in linkage studies," Timothy Corson, PhD, assistant professor of ophthalmology, biochemistry, and molecular biology at the Indiana University School of Medicine, Indianapolis, and a coauthor of the report, told Medscape Medical News.

The investigators found that of 441 unilateral RB tumors, 7 (1.6%) had no mutations, deletions, or promoter methylation in RB1. To identify candidate genes, they used quantitative multiplex polymerase chain reaction to measure copy numbers of genes lost or gained in RB. "Unexpectedly, five of seven RB1+/+ tumours showed striking amplification of the MYCN oncogene," they write. In other words, MYCN amplification causes retinoblastoma.

To validate the finding of amplified MYCN, the researchers collected 1068 cases from RB clinical laboratories in New Zealand, France, Germany, and the Netherlands. "It was only as a result of a large international collaborative study, led by Brenda Gallie, that we could look at more than 1000 tumors," Dr. Corson said. The new subtype of the cancer is called RB +/+ MYCN A , or MYCN RB for short.

For the 1068 cases, the researchers assessed RB1 gene expression and protein function, MYCN copy number, clinical data, and retinal gene expression, comparing patients with 2 wild-type RB1 alleles to the majority whose tumors had 2 mutant alleles. A minority (29 of the patients, or 2.7%) did not have RB1 mutations, and 27 of those had elevated MYCN copy numbers, 16 of them with very high copy numbers (28 to 121).

MYCN RB is an early-onset, aggressive disease. Although it accounted for only 1.4% of the total 1068 cases, the researchers identified it in 18% of children diagnosed at less than 6 months of age. Median age at diagnosis for MYCN RB was 4.5 months, compared with 24 months for 79 children who had nonfamilial unilateral disease with 2 RB1 mutations.

Additionally, the tumors in the newly recognized subtype are histologically distinct. They have markers of embryonic retina, so they are true retinoblastomas. The cells are undifferentiated, with large multiple nucleoli and indications of necrosis and apoptosis. They resemble neuroblastoma cells and lack the nuclear molding and Flexner-Wintersteiner rosettes seen in classic RB. Tumors are large and invasive.

Clinical Implications

A combination of factors — MYCN amplification in the presence of functional RB1 protein, early onset, distinctive histologic features, and lack of copy number amplification in other genes — supports the hypothesis that amplification of just one copy of MYCN is sufficient to drive the cancer, making it "one-hit." Therefore, the condition is no more likely to repeat in siblings, or to cause another cancer in the patient, than in the general population. Classic RB, in contrast, often causes other cancers later in life, such as osteosarcoma.

Diagnosis with MYCN RB can save patients from invasive follow-up. "An infant must be put under general anesthesia to be still enough to carefully evaluate the eyes for tumors, so it's a considerable burden these patients are relieved of," Dr. Corson said.

Identifying this form of RB could change surgical management, Joan O'Brien, MD, chair of ophthalmology at the University of Pennsylvania School of Medicine, Philadelphia, told Medscape Medical News. "A child who has early-onset RB is often considered to have the heritable form of the disease. The other eye may become involved. For this reason, specialists may avoid removing the [first] eye and give chemotherapy instead. In the case of the MYCN-amplified disease, this study suggests that since the disease is particularly aggressive, and the other eye will not become involved, surgical removal of the diseased eye is the correct therapeutic course."

Shalom Kieval, MD, MBA, chief of ophthalmology at Albany Memorial Hospital, New York, adds, "Treatment of retinoblastoma is one of the great success stories in pediatric oncology, with cure rates approaching 95%. This discovery may help us do even better."

Dr. Gallie is medical director of Impact Genetics. The other authors and the commentators have disclosed no relevant financial relationships.

Lancet Oncol. Published online March 13, 2013. Abstract