Anyone who has had retinoblastoma, and their blood relatives, can benefit from genetic testing.
Retinoblastoma is caused by changes to the RB1 tumour suppressor gene or MYCN oncogene.
Genetic testing looks for these changes in tumour from removed eyes and / or blood. Results define risk for the other eye of a unilaterally affected child, and to other family members.
Who Can Be Tested?
Testing must begin with the person who has retinoblastoma (the proband). Testing can usually be done with blood alone for bilateral probands.
A range of processes are used to identify RB1 or MYCN mutations in the proband. When a constitutional RB1 mutation has been identified, only one test is needed to look for it in the blood of an unaffected relative.
Testing Unilateral Probands
In unilateral probands, both alleles (copies of RB1) are first identified in tumour if a sample is available. This will not be possible if the eye was not removed, or if it was stored in formalin or paraffin (which break down DNA). Tumour samples should be flash frozen or sent fresh directly to the genetics lab, to aid effective testing.
When both RB1 mutations are identified, DNA is isolated from blood to see if the tumour mutations are present. Testing can be done on blood alone if tumour is not available, but the value of testing will depend on the lab’s test sensitivity.
If tumour analysis finds that both copies of the RB1 gene are intact, further testing can examine the MYCN gene to identify if the child’s cancer was caused by too many copies of this gene.
RB1 Genetics Laboratories
RB1 genetic testing is technically difficult because the gene is large. Misprints can happen anywhere, and are often unique to individual families. Only a handful of laboratories around the world offer precise RB1 genetic testing.
Most labs cannot test well for unilateral probands because they cannot test for mosaicism. If no mutation is found in a unilateral proband, the residual risk of an undetected mutation can only be defined if the lab rigorously monitors test sensitivity.
RB1+/+MYCNA retinoblastoma was first described in March 2013. Testing for this type of retinoblastoma is currently available in only a handful of labs worldwide.
Calculating Residual Risk
To calculate residual risk requires three inputs:
Sensitivity: The probability that the lab identifies a heritable mutation when there is truly a heritable mutation (true positive).
Specificity: The probability that the lab DOES NOT find a heritable mutation when there is NO heritable mutation (true negative).
Prevalence: The proportion of people with unilateral retinoblastoma who carry a heritable mutation.
A careful count of the lab’s success rate in diagnosing mutations is required to determine test sensitivity. The lab may be very successful in identifying mutations, but unless they provide a verified detection rate, it is impossible to calculate residual risk for the proband or blood relative.
Success rates differ between labs, and they don’t give equally useful results. The higher a lab’s test sensitivity, the more a “no mutation found” report reduces residual risk.
Several labs can detect very low level mosaic RB1 mutations, and can also test from blood alone for unilateral probands. This testing is a more complex process than when testing begins with a tumour sample.