BSC 422 Lecture Notes - Lecture 13: Tumor Suppressor Gene, Epistasis, Homologous Chromosome
22 May 2018
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Tumor Suppressor Continued
The first suppressor gene to be identified is involvement in hereditary
retinoblastoma, a rare type of eye cancer that develops in children who have a
family history of the disease. Such children inherit a chromosomal deletion in a
specific region of one copy of chromosome 13.
Although the deletion occurs in all cells, only a few in the retina actually become
malignant because the initial deletion in chromosome 13 does not cause cancer
by itself; for cancer to develop, a subsequent mutation must also occur in the
same region of the homologous chromosome 13.
It has, therefore, been concluded that chromosome 13 contains a gene on
homologous chromosome of a normal diploid cell where such gene normally
functions to inhibit retinoblastomas. In inherited retinoblastoma one defected
copy of gene is genetically transmitted.
The loss of this single copy of gene is compensated by the identical second copy of
the gene present on the same region of the second copy of chromosome 13.
Loss of a single copy of gene is not by itself sufl5cient to trigger tumor
development, but retinoblastoma almost always develops in these individuals as a
result of a second somatic mutation leading to further loss of the function of the
remaining second copy of normal gene.
The gene lost in hereditary retinoblastoma is called RBI. It is a tumor suppressor
gene that codes for the nuclear protein pRB that inhibits expression of a group of
genes whose products are needed for uncontrolled cell proliferation. In
hereditary retinoblastoma a defective or copy of the RBI gene is inherited from
the affected person.
Hence a lack of pRB resulting from loss of both copies of RBI (one due to deletion
and other due to a second somatic mutation) can lead to uncontrolled
proliferation which ultimately causes the development of retinoblastoma. In
nonhereditary cases, two normal RBI genes are inherited and retinoblastoma
develops only if two somatic mutations in adult inactivate both copies of RBI in
the same cell.
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Document Summary
The first suppressor gene to be identified is involvement in hereditary retinoblastoma, a rare type of eye cancer that develops in children who have a family history of the disease. Such children inherit a chromosomal deletion in a specific region of one copy of chromosome 13. It has, therefore, been concluded that chromosome 13 contains a gene on homologous chromosome of a normal diploid cell where such gene normally functions to inhibit retinoblastomas. In inherited retinoblastoma one defected copy of gene is genetically transmitted. The loss of this single copy of gene is compensated by the identical second copy of the gene present on the same region of the second copy of chromosome 13. The gene lost in hereditary retinoblastoma is called rbi. It is a tumor suppressor gene that codes for the nuclear protein prb that inhibits expression of a group of genes whose products are needed for uncontrolled cell proliferation.
