*2.5.4 Low-penetrant retinoblastoma*

In a typical "null" germline mutation, there is a 90% chance that patient will develop RB. In a few families, however, the penetrance is far less than 90% with subsequent reduced expressivity and an increased proportion of unilateral RB. Some patients will be carriers with no tumors [73]. These low-penetrant

mutations are usually missense mutations. Moreover, a low level of RB protein production due to mutations in the promoter region can occur without total absence of the protein [33].

#### **2.6 Genetic testing**

### *2.6.1 Clinical context*

Heritability of retinoblastoma is confirmed if a proband with retinoblastoma has a family history of retinoblastoma. In the absence of family history of RB, genetic testing is required to identify heterozygous germline RB1 pathogenic variants. This will allow early diagnosis and identification of potential carriers of the heritable RB1 mutations which eventually will improve disease management and family counseling.

The following staging system has been proposed to facilitate description of one's genetic risk of possessing germline RB1 pathogenic variant: [74, 75]


#### *2.6.2 Single-gene testing*

For bilateral, unilateral familial, and unilateral multifocal retinoblastoma, peripheral blood DNA should be tested initially using sequence analysis and genetargeted deletion/duplication analysis of RB1. If blood testing did not reveal any mutation, molecular analysis of tumor DNA (if either eye was enucleated) should subsequently be done. Mosaicism can be assumed if germline mutation was not found in this group of patients [76].

For unilateral unifocal non-familial retinoblastoma, the chance of carrying RB1 germline mutation is 15%. And, with the high sensitivity of RB1 mutation detection techniques which reach up to (95%), testing blood from unilateral patients, extracted at the time of the first examination under anesthesia may confirm the diagnosis of germline RB1 mutation. If blood DNA testing did not reveal any mutation, molecular analysis of tumor DNA (if the eye was enucleated) should subsequently be done. If pathogenic variants were detected in the tissue, the blood DNA should be re-tested looking for these specific variants or any large rearrangements within the RB1 gene. If tumor tissue was unavailable, a negative blood result will reduce the chance of possessing a mutation to less than 1%. With such very low risk of RB development, the proband needs no additional exam under anesthesia and frequent examination in the clinic is sufficient. The same conclusion also applies to the offspring.

It has been previously reported by Rushlow et al. that retinoblastoma is not exclusively caused by mutations in RB1 gene; mutations in MYCN oncogene has been predicted to be responsible for 18% of cases diagnosed with non-familial unilateral RB in children less than 6 months of age. They reported group of patients who harbored mutations in MYCN amplification and no RB1 mutations. An additional 1.5% of patients with unilateral non-familial RB were found to have normal RB1 and MYCN genes [77].

**9**

*History and Genetics of Retinoblastoma DOI: http://dx.doi.org/10.5772/intechopen.89035*

**2.7 Genetic counseling**

*2.6.3 Chromosomal microarray (CMA)*

should be offered to the relatives at risk.

vision-threatening tumor [82, 83].

postnatal period [83, 85].

*2.7.1 Prenatal screening*

delay, and/or other congenital anomalies [79, 80].

Chromosomal microarray analysis (CMA) is a technology used for the detection of clinically significant microdeletions or duplications and genetic rearrangement (including RB1), with a high sensitivity for submicroscopic aberrations [78]. It can be utilized in retinoblastoma patients with dysmorphic features, developmental

Genetic testing and counseling are essential parts of RB disease management; they help delineate heritable RB for non-heritable ones. The increased knowledge of molecular basis of RB allowed for better understanding and management of the disease. Genetic evaluation and counseling should be done in collaboration with a geneticist and genetic counselor [81]. The aim of the geneticist is to implement the proper test to detect the underlying disease causing mutation and communicate with the genetic counselor who will educate patients and their families about their condition in general, discuss anticipated risk of developing subsequent tumors, and construct the appropriate screening plan for the affected individual and their relatives. At first encounter of a patient and family of retinoblastoma, detailed family history should be obtained and family pedigree should be drawn with special attention to relatives at risk. Then, genetic testing is offered after discussion about purpose, possible outcome, and limitations. Specimens are then collected and sent to a specialized laboratory for analysis. Another counseling session is arranged to convey and interpret the reported results. Accordingly, additional testing and counseling

In the presence of family history of retinoblastoma and when a specific RB1 mutation is detected, a pregnancy at risk can be tested by chorionic villus sampling or amniocentesis. Prenatal testing and preimplantation genetic diagnosis are indicated as approximately 30% of newborns with an RB1 mutation will harbor a

Amniocentesis can be used to screen the fetus for carrying *RB1* gene using the above-mentioned methods. If an *RB1* pathogenic variant was identified, then fetal ultrasonography may be used to identify intraocular tumor as small as 2–3 mm in size [84]. If tumors are present, preterm delivery to allow early treatment may be indicated. Alternatively, early term delivery (i.e., 36–38 weeks gestation) can be induced even if no intraocular tumor can be detected as this was found to improve the visual outcome and reduce the need for invasive tests and therapies in the

Prenatal testing and management may raise several ethical issues especially if it involved termination of pregnancy. A generally accepted approach is to discuss all

Genetic testing should be performed carefully especially when the propand's blood is found negative for pathogenic mutations. It remains possible that large genetic rearrangement is being missed or they are mosaicism for RB1 mutation. Carriers of RB1 pathogenic variants should be frequently examined for development of new tumors whether under anesthesia or in the outpatient setting until they

options with the parents and leave them to decide on further steps.

*2.7.2 Genetic screening and counseling after birth*
