RB1 gene
The RB1 gene is a crucial anti-oncogene that plays a significant role in tumor suppression. First identified in patients with hereditary retinoblastoma, it is located on the long arm of chromosome 13 and spans over 180 kilobase pairs, including 27 exons. The RB1 gene's primary function is to produce the RB protein, which regulates the cell cycle by mediating the transition from the G1 phase to the S phase, preventing uncontrolled cell division. Mutations in the RB1 gene can disrupt this function, leading to an increased risk of retinoblastoma, a malignant tumor of the retina. The "two-hit" model of cancer, proposed by Alfred Knudsen, explains that individuals with hereditary forms of retinoblastoma often carry one mutated allele from birth, requiring only a second mutation for tumor development, while nonhereditary cases necessitate two mutations. With approximately 90% of RB1 mutations detectable by current methods, the implications of these mutations extend to a higher risk of other cancers in survivors. Understanding the RB1 gene has been pivotal in advancing cancer research, diagnostics, and treatment strategies for inherited cancers.
RB1 gene
SIGNIFICANCE:RB1, the retinoblastoma susceptibility gene, was the first tumor-suppressor gene to be discovered as well as the first hereditary cancer gene to be cloned. Study of RB1 has led to a greater understanding of cancer genes and the development of the “two-hit” model for antioncogene-associated cancers, as a result of retinoblastoma research.
Genetics Summary and Location
The RB1 gene is an anti-oncogene (tumor suppressor) and was first recognized and identified in patients with hereditary retinoblastoma by Thaddeus Dryja’s laboratory at the Massachusetts Eye and Ear Infirmary at Harvard Medical School, originally isolated using the technique called chromosome walking. The cytogenic location of RB1 is 13q14.2, which means it is on the long arm (q) of chromosome 13 at position 14.2, specifically located from base pair 47,775,911 to base pair 47,954,022. RB1 is a very large gene, extending over 180 kilobase pairs, containing 27 exons. The region of RB1 gene is distinctive because it contains binding motifs for transcription factors Sp1 and ATF but no TATA or CAAT box.
The sequence for RB1 product is more than 75 percent identical for humans, rats, mice, and frogs. RB1 exists in all vertebrates, which may indicate that it has a critical biological role that has been maintained through evolution.
RB Protein and Tumor Suppression
RB protein plays an integral role in regulation. It controls auxiliary proteins, mediating their processes. During the G1 (resting phase) phase of a cell cycle, RB protein binds to E2F factor and blocks the transcription of S-phase (phase in which DNA synthesis occurs) genes, mediating the G1-to-S phase checkpoint. A loss of RB protein, due to RB1 mutation, causes elimination of G1-to-S phase checkpoint, increasing the odds of unregulated cellular production or cancer.
There are two distinct types of cancer genes, protooncogenes and anti-oncogenes. Proto-oncogenes are easier to identify and involve gene mutations that result in a gain-of-function, motivating cells to become cancerous. Anti-oncogenes are the most frequently mutated genes (except in leukemia and lymphoma) and cause a decreased expression, leading to cancer. RB1 was the very first of only a few tumor-suppressing anti-oncogenes, that have been discovered and studied, and thus, RB1 has served as an important model for study of anti-oncogene-associated cancers.
RB1 Mutations
In 1971 Alfred Knudsen described the “two-hit” theory, a mutation model based on retinoblastoma observations and Mendelian genetics, which can be applied to inherited cancers. The theory states that in hereditary cancers, the predisposed individuals already carry one allele mutation and therefore, oncogenesis only requires one more mutation to occur. In nonhereditary cases, two or more mutational events are required. This hypothesis was validated with the first cloning of the RB1 gene. Genotyping of retinoblastoma tumors concluded that mutations in both alleles of the RB1 gene were required for tumor development to begin.
The majority of mutations of RB1 occur between internal exons 2 and 25. There are more than 930 reported mutations, identified in more than 1000 families. The band of RB1 mutations is very diverse, and does not demonstrate any distinct hotspots, although certain discrete hotspots of recurrent mutations have been characterized. The majority of these recurrent mutations are associated with C to T transitions in CGA codons. Normally in genetic mutations of large genes, the specific location of the stop will influence the phenotypic expression. This is not the case with RB1; the precise site of the mutation does not alter the phenotype.
The method of mutation analysis depends on the patient’s family history and the specimens (blood or tumor) that are available. The mutations of RB1 are distributed along almost the entire length of the gene, which suggests that no single analysis method will be fully sensitive. Approximately 90 percent of all RB1 mutations can be detected by current molecular analysis procedures whereas 10 percent cannot.
Retinoblastoma
Retinoblastoma, in its hereditary or nonhereditary form, is a malignant tumor, or tumors, which develop from the immature retina. In order for tumorgenesis to occur, mutations in both alleles of the RB1 gene are required. According to the Cleveland Clinic (2023), retinoblastoma typically occurred in early childhood and affected approximately 3.3 in 1 million children under age 20.
Hereditary retinoblastoma is responsible for approximately 40 percent of all cases. It is transmitted in an autosomal dominant fashion and is characterized by multiple tumors in both eyes. Patients with the hereditary form are for a mutation of RB1 that has been inherited from one of their parents or has occurred in embryonic development (RB1+/RB1-). Tumors are formed when a single mutation in the functioning RB1 allele occurs, creating a loss of heterozygosity at the RB1 locus.
The nonhereditary form involves only one eye and a single tumor. These patients are homozygotes (RB1+/RB1+) and require two distinct mutational events to occur, causing a loss of function at both RB1 loci in order for tumor formation to begin. The incidence of two mutational events occurring is rather low, in contrast to the single mutational event required for heterozygotes in order for tumor formation to be initiated.
If left untreated, retinoblastoma is consistently fatal, but with early diagnosis—preferably before age 2—and current treatment methods, the survival rate was 95 percent, according to 2023 information from the Cleveland Clinic. Survivors of hereditary retinoblastoma have a vulnerability toward other cancers, notably osteosarcoma (37 percent) and soft-tissue sarcomas (16.8 percent), melanomas (7.4 percent), brain tumors (4.5 percent), and leukemia (2.4 percent). The incidence of retinoblastoma appears to be increasing as early diagnosis and improved treatment options have improved the survival of patients who have gone on to transmit this autosomal dominant trait to the next generation.
Impact
The discovery and characterization of the RB1 gene, its mutations, and protein product have been fundamental in the understanding of the mechanisms of inherited cancers. Knudsen’s “two-hit” theory established a critical model for tumorgenesis, uncovering new mechanisms for cancer origination. Research has concentrated on the structure and function of the RB1 gene, as well as the regulation of the transcription and translational anti-oncogenic activity of the RB1 protein. Numerous mutational categories have been established and aligned with phenotypic expression. These findings are significant as they expand the ability of physicians to offer improved diagnostic, prognostic, and to patients, as well as more specialized therapeutic interventions.
Key terms
- alleleone of two or more different genes containing specific inheritable characteristics that occupy corresponding positions (loci) on paired chromosomes
- anti-oncogenetumor-suppressor gene
- chromosomea linear strand of DNA that carries genetic information
- deletionthe loss of a portion of a chromosome
- exonone of the coding regions of the DNA of genes
- expressionuse of a gene to create the corresponding protein
- hotspotDNA positions where genetic recombination occurs with above-average frequency
- locusthe position of a gene on a chromosome
- mutationa change in gene, potentially capable of being transmitted to offspring
- retinoblastomamost common intraocular malignancy (eye cancer) in children
Bibliography
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