Genetics of hereditary retinoblastoma

Clinical background


Retinoblastoma is a tumor of the developing retina. It is the most common malignant ocular tumor in childhood, affecting approximately 1 in 20 000 live births. In the USA retinoblastoma is the 10th most common pediatric cancer, with an incidence of 10.6 per million children under the age of 4, 1.53 per million in children between the ages of 5 and 9 years and only 0.27 per million in children over the age of 10. Worldwide, retinoblastoma is responsible for 1% of childhood cancer deaths and 5% of childhood blindness. No gender or race predilection and no significant environmental risk factors have been identified. However there may be an association between retinoblastoma and low socioeconomic status worldwide.

Historical development

Knudson in 1971 provided critical insight into the genetic understanding of retinoblastoma by postulating the “two-hit” hypothesis. He proposed that two mutational events were necessary for retinoblastoma tumorigenesis. Comings later expanded the theory by proposing that the mutations were in both alleles of a gene with a tumor-suppressive function. The retinoblastoma gene ( RB1 ) was later localized and cloned on chromosome 13q14, becoming the first tumor suppressor gene identified and laying the groundwork for great advances in the understanding of oncogenesis.

In this chapter we will briefly discuss the clinical presentation and management of retinoblastoma. We will also summarize the current understanding of the molecular pathophysiology and genetics of hereditary retinoblastoma.

Signs and symptoms

Patients usually present in the first year of life (average 7 months) for bilateral cases and at approximately 24 months for unilateral cases. In patients without a family history, when retinoblastoma is not suspected, the most common clinical presentation is leukocoria or white pupil ( Figure 48.1 ). The second most common presenting sign is strabismus, which is usually constant and unilateral and can manifest as an eso- or exodeviation. This is the result of macular involvement and represents an early sign of the disease, with higher survival rates and higher chances of globe preservation. In contrast leukocoria is a late sign and is associated with lower rates of globe salvage. A small proportion of patients have a more atypical presentation with consequent poor prognosis. If the tumor spreads into the anterior chamber, the patient can develop hypopyon, rubeosis, or glaucoma or present with an apparent orbital cellulitis secondary to extensive tumor necrosis. Other presenting signs include unilateral mydriasis, heterochromia, hyphema, uveitis, and nystagmus. Extraocular extension of the disease may present with proptosis. Systemically, patients can present with signs of increased intracranial pressure secondary to an intracranial mass when affected by trilateral retinoblastoma.

Figure 48.1

Leukocoria is the presenting sign of retinoblastoma in 60% of patients, as demonstrated here in a patient with unilateral disease. Strabismus is a presenting sign in 20% of patients with retinoblastoma. The remaining 20% of patients present with atypical clinical presentations like orbital cellulitis.

Diagnostic workup

All patients with any clinical suspicion of retinoblastoma should have a complete ophthalmic exam and a careful family history. In the presence of hypopyon or orbital cellulitis, retinoblastoma should be ruled out before performing any surgical intervention such as a needle tap or biopsy. Exam under anesthesia is required for a thorough examination of the posterior pole and peripheral retina with scleral depression in infants and young children. A wide-angle camera (Ret-Cam) is commonly used and provides 130° imaging of the retina and the anterior segment.

On dilated fundus exam a round creamy yellow-white mass can be identified projecting into the vitreous cavity with large irregular blood vessels on the surface and penetrating the tumor ( Figure 48.2 ). Vitreous or subretinal seeding of tumor cells can be observed. Calcification within the tumor mass is common. The localization of the tumor is variable but related to the age at presentation. Posterior pole masses tend to present at an earlier age. Patients can also present with a retinal detachment covering an underlying tumor mass. An irregular gray plaque on the retinal surface can be seen in the diffuse infiltrating form of this disease. This is an uncommon presentation that is more difficult to diagnose; the presence of a hypopyon can sometimes alert the clinician to this unusual presentation.

Figure 48.2

Fundus photograph showing multiple round elevated white masses projecting into the vitreous cavity.

In the presence of vitreous opacity or a retinal detachment, when visualization of the tumor mass is difficult, B-scan ultrasonography or computed tomography (CT) can identify calcifications ( Figure 48.3 ). These tests should always be performed to exclude retinoblastoma before any surgical intervention is performed in a child with a restricted fundus exam.

Figure 48.3

(A) B-scan ultrasonography demonstrating a high-reflectivity mass with characteristic shadowing. (B) Computed tomography can identify calcified masses.

B-scan ultrasonography characteristically demonstrates a high reflectivity mass with shadowing behind the tumor. Magnetic resonance imaging may be preferred to CT to reduce the risk of radiation-associated cancer in these pediatric patients.

Ultrasound biomicroscopy is useful to detect disease anterior to the ora serrata: this is an indication for immediate enucleation due to the increased risk for systemic metastasis. A biopsy of the tumor or vitreous is contraindicated due to the associated risk for tumor spread outside the eye. Bone marrow aspiration and lumbar puncture should be performed to screen for metastasis in children who present late or with high-risk features of the disease.

Screening examinations of babies with a family history allow early detection of tumors even before they are clinically evident. Currrent recommendations for screening include: initial fundoscopic exam under anesthesia at birth and subsequently every 2–4 weeks for the first several months. The oldest age that a patient with a family history of retinoblastoma has presented is 48 months.

Genetic testing for retinoblastoma

Genetic testing can characterize the specific mutation affecting an individual patient as well as identify the presence of a nonpenetrant mutation in a carrier parent. Karyotypic studies are less useful for clinical diagnosis because only 3–5% of retinoblastoma patients carry large deletions detectable by these methods. Occasionally 13q deletions or translocations are evident through application of these techniques. In these cases, other systemic abnormalities, including severe developmental delay and dysmorphic features (13q deletion syndrome), can be clinically observed.

More sophisticated direct and indirect DNA analysis techniques are needed to detect smaller mutations. These techniques identify the initial germline mutation in approximately 85% of patients. Direct methods involve extracting DNA from a fresh unfixed fragment of the tumor after enucleation. If this is not available, testing can be performed with leukocytes from peripheral blood. These techniques include single-strand conformation polymorphism (SSCP) analysis, gel electrophoretic analysis of synthetically amplified exons, and fluorescent in situ hybridization (FISH). Indirect methods can also be used in cases where the initial mutation cannot be found. These methods involve restriction fragment length polymorphism (RFLP) or variable number of tandem repeats (VNTR) analysis of parental and tumor DNA to detect the presence of a genetic marker that segregates along the retinoblastoma gene ( RB1 ). Indirect techniques require the presence of two or more affected family members and are in general less sensitive than direct techniques.

A recently described multistep testing strategy combines multiplex polymerase chain reaction (PCR) with double exon sequencing and promoter-targeted methylation-sensitive PCR to achieve a sensitivity of 89% in detecting the RB1 mutation. Protein truncation testing has also been shown to be effective in screening for germline mutations. New methodologies using microarray chips and robotic sequencing are currently on the horizon. In the future, knowing the specific gene mutation in a particular patient could be useful to predict disease severity and to provide prognostic and therapeutic guidance.

Genetic testing of affected patients and their families is extremely important, not only because patients with a germline mutation are at risk of developing secondary tumors, but also for genetic counseling. Occasionally, low-penetrance pedigrees can be identified where there is unilateral or even no detectable disease or family history. It is possible to perform preimplantation genetic diagnosis during in vitro fertilization. Therefore, screening for constitutional RB1 mutations should become an integral part of the management of patients with retinoblastoma, irrespective of tumor laterality or family background.

Differential diagnosis

The differential diagnosis of retinoblastoma includes lesions that simulate retinal tumors like Toxocara canis and astrocytic hamartoma, lesions that can cause retinal detachments such as retinopathy of prematurity, Coats disease, and persistent hyperplastic primary vitreous and other conditions like retinal dysplasia and medulloepithelioma (dikytoma).

Retinoma is a benign growth of the retina that is also produced by a mutation in the RB1 gene. It presents as a nonprogressive, elevated gray retinal mass that can have calcification and pigmentation. It may develop when the second mutation occurs in a nearly developed retinal cell and does not acquire the additional necessary mutations for full malignancy.

Treatment ( Box 48.1 )

Management algorithms for retinoblastoma have changed rapidly over the past few decades and continue to evolve. The goals of treatment are cure of the disease, globe salvage, preservation of vision, and early detection and treatment of secondary malignancies. A wide array of systemic and local treatments exists ( Table 48.1 ).

Box 48.1

Treatment key points

  • Enucleation is still the most common form of treatment worldwide. In the developed world it is only used for advanced tumors

  • External-beam radiation is used less commonly today due to associated increased secondary tumor risk and other complications

  • Chemoreduction in combination with focal therapy is a highly effective treatment strategy

  • Ongoing clinical trials will further elucidate the most appropriate treatment strategies

  • New understandings in the pathogenesis of retinoblastoma will likely produce targeted molecular treatments with reduced systemic side-effects

Table 48.1

Treatment of retinoblastoma

Focal Cryotherapy, laser photocoagulation, thermotherapy, brachytherapy, accelerated proton beam radiation
Whole eye Subconjunctival/intravitreal chemotherapy, enucleation, external-beam radiation
Systemic Intravenous chemotherapy

Systemic chemotherapy protocols have replaced enucleation and radiation as the primary treatment for retinoblastoma. Chemotherapy reduces tumor volume (chemoreduction) to permit application of focal techniques such as laser photocoagulation or cryotherapy to ablate remaining tumor mass. The choice of agents, combination, and dosage varies among treatment centers. The most commonly used chemotherapeutic agents include vincristine, carboplatin, etoposide, and teniposide. Ciclosporin A is sometimes used to combat multidrug resistance. Subtenon injections of carboplatin can be used as an adjunct to systemic chemotherapy. Intravenous carboplatin can also be used in conjunction with infrared diode laser radiation applied directly to the tumor through the pupil (transpupillary thermotherapy) because heat increases the permeability of the cellular membrane to antimitotics, reinforcing their cytotoxic effect. For small tumors (diameter less than 3 mm), transpupillary thermotherapy can be used alone, relying on the cytotoxic effect of heat by raising the temperature of the tumor above 45°C. Complete tumor control can be achieved in 85% of appropriately selected patients. Complications include iris atrophy, lens opacities, retinal traction, retinal detachment, and disk edema. Chemotherapy alone does not achieve permanent tumor control. When local therapy is applied in conjunction with chemotherapy, success rates approach 85%. Systemic chemotherapy carries potentially serious systemic adverse effects, including hearing loss, cytopenia, neutropenia, infections, gastrointestinal toxicity, and neurotoxicity.

External-beam radiation is indicated in advanced bilateral cases or in cases of disease relapse. It may also be considered for small tumors located within the macula, because it offers a better chance for useful vision when compared to other focal treatments. Radiation increases the risk of secondary nonocular malignancies. Side-effects include cataract formation, dry eye, retinopathy, vitreous hemorrhage, growth retardation of the orbit, and resultant midface hypoplasia.

Brachytherapy uses radioactive plaques, like iodine-125 or ruthenium-106, on the sclera over the base of the tumor. The total dose of radiation is ~4000 cGy delivered at a rate of 1000 cGy daily. It can be used for medium-sized tumors (4–10 disk diameters) for consolidation or as a secondary method after treatment failure with localized relapse. Tumors involving the macula or optic disk are not optimal candidates for this treatment modality. The recurrence rate for this treatment is 12% at 1 year if used as primary treatment and 8–34% if used as salvage therapy after failure of other methods. Radiation retinopathy, cataract, and neovascular glaucoma are reported complications of brachytherapy. Accelerated proton beam irradiation uses tantalum rings sutured to the sclera to mark the tumor edges in order to deliver an accelerated particle beam to active intraocular tumor. It can also be applied as an adjunct to enucleation with positive tumor margins in the optic nerve stump.

Focal treatments include transpupillary argon laser photocoagulation and transscleral cryotherapy. Small tumors (less than 3 mm in diameter and 2 mm in thickness) without vitreous seeding are good candidates for photocoagulation. Complications include retinal detachment, fibrosis, and vascular occlusions. Alternatively, cryotherapy can be used if a small tumor is located peripherally. Cryotherapy causes intracellular ice crystal formation, protein denaturation, pH changes, and cell membrane rupture. Circulation to the tumor is also disrupted. Cryotherapy can also be applied as a secondary treatment for tumor previously treated with laser, transpupillary thermotherapy, or external-beam radiation. Tumor destruction is usually achieved after one or two sessions of triple-freeze therapy at 1-month intervals. Complete destruction is achieved in 90% of tumors. Complications include pain, intraocular inflammation, chemosis, lid edema, vitreous hemorrhage, and retinal detachment.

In the USA, enucleation is reserved for advanced cases of retinoblastoma with massive involvement of the retina and vitreous, rubeosis, glaucoma, or tumor invasion into the anterior segment or optic nerve. It remains the most common form of treatment worldwide. Fortunately, enucleation is effective, achieving total cure in 99% of patients with no extraocular involvement. During enucleation it is of utmost importance to avoid inadvertent perforation of the globe, since this carries a very high risk for extraocular tumor seeding. A long section of the optic nerve should be obtained for histopathological analysis. Enucleation is the only treatment modality that allows genetic analysis of fresh tissue. High-risk characteristics of enucleated specimens include massive choroidal infiltration, anterior-chamber seeding, tumor invasion beyond the lamina cribrosa, and scleral invasion. Children with these high-risk features are recommended to have adjunctive chemotherapy (Children’s Oncology Group, Group E Prospective Trial). Tumor at the surgical margin of the optic nerve has an associated mortality rate of 50–81%. High-dose chemotherapy with autologous marrow transplantation is therefore indicated to prevent metastases in patients with tumor extending beyond the cut end of the optic nerve (Children’s Oncology Group, Group F Concept Proposal).

Novel treatment modalities under investigation include subconjunctival ( Figure 48.4 ) and intravitreal delivery of chemotherapeutic agents, injection of photosensitizing agents followed by selective laser treatment, gene therapy, and ophthalmic artery injection of chemotherapeutic agents. New understanding in the genetic alterations in retinoblastoma tumorigenesis will likely produce novel molecular targets to treat the tumors directly with reduced systemic side-effects.

Aug 26, 2019 | Posted by in OPHTHALMOLOGY | Comments Off on Genetics of hereditary retinoblastoma

Full access? Get Clinical Tree

Get Clinical Tree app for offline access