Genetics and retinoblastoma

The many genes are packaged together; the gene packages are called chromosomes. There are 23 pairs of chromosomes; each chromosome can be seen through a microscope but genes can only be decoded using specialised techniques (DNA analysis or molecular genetics)

The retinoblastoma (Rb) gene is found on chromosome 13. (It is usually called the RB1 gene in scientific literature.) As with nearly all our genes we have two copies of it. It is quite a large gene with 27 different sections. The information it provides allows our bodies to make a substance called a protein, which is then able to carry out a specific function in our bodies and within the cell itself. The protein produced by the retinoblastoma gene plays an important role in ensuring healthy cell division as we grow and develop.

All cells have a life cycle. An important part of that is the period during which the cell replicates and divides. During this process, all the genes within a cell must also be copied so the new cell has all the information it needs to function properly. Certain proteins, such as the one produced by the retinoblastoma gene, perform checks at stages throughout the copying and dividing process to ensure that the cells are dividing correctly and the new cells are healthy, without alterations in the genetic material. If it detects changes, the protein is able to halt the dividing process while repairs are made, so preventing these altered and therefore damaged cells from continuing to grow and divide.

In the first few years of life the retina grows very quickly; this means that our cells must divide quickly. At this fast pace, mistakes are more likely to occur as our genes are copied. As a result of these alterations a retinoblastoma can occur; this can happen in either of two different ways:

1. When a child inherits one altered copy of the Rb gene from a parent. If the normal copy of the gene does not become damaged in a retinal cell no retinoblastoma will develop. If damage occurs to the normal copy, a retinoblastoma can develop. This is the heritable form of retinoblastoma. About 90% of people who inherit an altered Rb gene from a parent will develop retinoblastoma; most will have bilateral disease (i.e. both eyes affected), a few will have multifocal disease (more than one tumour in an eye), and a few will have unilateral disease (one eye affected) with only one tumour. About 10% will not develop a tumour at all. In heritable retinoblastoma all the cells in the body, including the blood cells, will contain the altered copy.

2. When a child inherits one normal copy of the Rb gene from each parent but copying mistakes then affect both copies of the gene in one cell of the retina. Again, as a result of these alterations a retinoblastoma can develop. In these circumstances the child will develop the non-heritable type of retionblastoma, and there will be one tumour in one eye only.