Genetics of Cancer

The incidences of cancer started shooting up in the year 1991. Since a decade the cancer cases increased by a factor of three. There is simply no complete cure to this dreadful condition. It involves changes in the genome leading to uncontrolled cellular proliferation, transformation, invasion, metastasis, apoptosis suppression, and angiogenesis. The environmental factors such as chemicals, radiations, viruses, microbes, and hormones cause cancer. Apart from the above reasons, there are more factors involved. The cell follows a cyclical pattern of division involving different phases. It includes the G1, S, G2, and the M phases respectively. The transition of one phase from the other involves checkpoints. The CDK/cyclin complexes mainly control the cell cycle. The checkpoints determine the damaged DNA. They also help in checking the problems in the cell cycle machinery. Hence, they play a crucial role in permitting normal cells to continue. Problems in the cell cycle checkpoints trigger cancerous conditions. Certain viruses such as retroviruses cause cancer. They increase the oncogenic products. Also, for the normal cells, apoptosis plays a crucial role. The cancer cells do not undergo apoptosis.

Image 1: Cancer cells and normal cells

Cell cycle and cancer:

A cell cycle involves six main checkpoints such as the restriction point, the G1/S DNA damage checkpoint, the S phase DNA damage checkpoint, G2/M checkpoint, centrosome duplication checkpoint, and mitotic checkpoint. The restriction point occurs between the mid to late G1 phase. This point ensures the cell to enter into the S phase after receiving the appropriate signals. The G1/S damage checkpoint occurs at the G1 phase transition. It senses the DNA damage. The S phase DNA damage checkpoint arrests the cell cycle in the later part. It detects the DNA damage or an incomplete replication of DNA. The G2/M checkpoint also detects the damaged DNA. The centrosome duplication checkpoint detects the defects in the centrosome duplication process. This checkpoint also detects centrosome segregation defects. The mitotic checkpoint occurs in the M phase. It checks the formation of mitotic spindles.

The CDK/cyclin complexes control the cell cycle. The cyclin-dependent kinases belong to the class of kinases. The cyclins are known as the regulatory subunits. The CDKs are known as catalytic subunits. The cell cycle checkpoints involve a genetic control. The genes participating in the multiple cell cycle checkpoints are known as gatekeeper genes. These genes prevent the cell cycle progression until the damaged DNA gets repaired.

Phases of the cell cycle	Cyclin-CDK complexes
G1 phase	Cyclin D, CDK-4 Cyclin D, CDK-6
Late G1 phase	Cyclin E, CDK-2
S phase	Cyclin A, CDK-2
G2 phase	Cyclin A, cdc 2
M phase	Cyclin B, cdc 2

Table: All phases of the cell cycle and the associated cyclin-CDK complexes

1.     G1 Phase:

Alterations in the signaling pathways associated with the cyclin-dependent kinases lead to the uncontrolled cell proliferation. Retinoblastoma involves tumor in the retina. It occurs in childhood. The gene responsible for getting mutated and causing the disease is known as the RB gene. It is present on the q arm of the thirteenth chromosome. Deletion or inactivation of both the copies of the RB gene leads to retinoblastoma. The cell loses the protein product pRb.

This phase requires a regulatory protein. It is known as pRb. It gets phosphorylated by cyclin/CDK complex. The pRb binds to the E2F transcription factor and prevents the cell’s entry into the S phase. After the phosphorylation of pRb, it gets inactivated and releases the E2F. Now the cell safely enters the S phase. However, in the retinoblastoma, the cell loses the pRb protein due to RB gene mutation. Hence, the cell enters into the S phase without checking any damaged DNA. Thus, it leads to an unrestrained tumor formation.

2.     G1/S checkpoint:

The tumor suppressor gene known as TP53 gene plays a crucial role in cell cycle arrest and DNA repair. This checkpoint gets invoked due to dsDNA breaks and damage. The product of the TP53 gene is a protein. It is known as p53. It helps in arresting the cell cycle in the G1 phase or the G1/S phase. After the repair of the DNA, the cycle resumes back. However, failure to get repaired leads to apoptosis or cell death. It occurs in the normal cells where p53 gets activated. In the cancer cells, the p53 is not present. Hence, there is no cell cycle arrest and repair of damaged DNA. Thus, the cells form tumors.

3.     G2/M checkpoint:

It is a DNA damage checkpoint. It helps in progressing the cell from the G2 phase to mitosis phase. It maintains the cdc2/ cyclin B1 in an inactive state. The protein p53 also plays a crucial role here.

Image 2: Cell cycle

Cellular proliferation:

Signal transduction involves extracellular growth factors. They regulate cell growth and differentiation. The genes encoding the growth factors or the growth factor receptors may get mutated. Hence, they lead to oncogenic properties. A gene encodes for the signal transducing protein. It is known as ras gene. The transcription factor gets encoded by another gene. It is known as the Myc gene. Mutations in both the genes also cause cancer.

Genes, Viruses, and Cancer:

Cancer involves mutations in three main gene classes. They include proto-oncogenes, tumor suppressor genes, and mutator genes. The products of proto-oncogenes stimulate cell proliferation. The mutant ones are known as oncogenes. They are the active forms of cancer genes. The oncogenes stimulate unregulated cellular proliferation. The RNA viruses also replicate via DNA intermediate. These viruses are known as retroviruses. Upon the retroviral infection, the RNA genome of the viral particle synthesizes a kind of cDNA. It is known as proviral DNA. The viruses also have oncogenes. They are known as viral oncogenes. When they occur in the host cell, these genes are known as cellular oncogenes. The host DNA sequences homologous to that of the virus are known as proto-oncogenes. These genes get activated to oncogenes. Three main methods do this. The first method involves increasing the amount of proto-oncogene product. The second method involves mutations in the coding sequences. Chromosomal translocation also leads to activation of oncogenes.

Apoptosis and Cancer:

The cell death or apoptosis gets triggered in the case of unrepaired damaged DNA or any other unwanted cellular conditions. The failure of the checkpoints in stopping the cell cycle progression also triggers cancer. Cancer also occurs due to the activation of anti-apoptotic genes such as Bcl_2. Thus, many such factors contribute to cancer. 

References:
[1] Human Genetics, 3/e, Gangane
[2] Molecular Genetics of Cancer, John Cowell
[3] The Genetics of Cancer: Genes Associated with Cancer Invasion, Metastasis, Gajanan V. Sherbet, M. S. Lakshmi
[4] API Textbook of Medicine, Ninth Edition, Two Volume Set, Y P Munjal, Surendra K Sharma

© Copyright, 2018 All Rights Reserved.

Mitotic recombination

Crossing over occurs in meiosis as well as mitosis. However, the process of crossing over occurring in the mitosis is rare. Homologous recombination in the meiosis involves the exchange of material between the homologous segments of two DNA molecules. Similarly, studies do mention mitotic recombination. A stage in mitotic recombination resembles the four-strand stage in meiosis. The progeny cells obtained from mitosis include a combination of genes differing from the diploid parent cell.

Mitotic recombination in Drosophila:

Curt Stern studied mitotic recombination in Drosophila strains. These strains had recessive sex-linked mutations. Two varieties of strains included bristle type and body color. One of the strains had short twisty bristles instead of normal long curves. The other type included yellow body versus normal body color. The experimenters considered a cross between grey bodied females with singed bristles and yellow bodied males with normal bristles. The locus for the singed bristles is known as sn locus. The F1 progeny had the majority of females with wild-type phenotype. However, few of them had yellow patches and singed bristles. First, the experimenters thought the reason behind this progeny to be chromosomal non-disjunction or chromosomal loss. Then they identified females with twin spots. Hence, the mitotic crossing over came into the picture.

Image 1: Mitotic recombination in Drosophila leading to different kinds of spots.

Consider a crossing over between the sn locus and centromere. The segregation of chromatids takes place. The first and the third chromatids segregate to one daughter nucleus. The second and the forth chromatids segregate to the other daughter nucleus. Upon the division of these cells, the progeny strains have twin spots. They produce a yellow patch of tissues and a singed patch of tissues. The remaining area shows wild-type phenotype. The other kind of orientation involves segregation of the first and the forth chromatids to one nucleus and second and the third chromatid to the other nucleus.

Mitotic recombination in Aspergillus nidulans:

Aspergillus nidulans is a kind of fungus. The studies of mitotic recombination in this fungus help to construct the genetic maps. The fungus does not involve controlled crosses. Hence, it is not possible to study meiotic recombination. It is a mycelial fungus. The uninucleate asexual spores of the fungus give greenish colonies. The genotype of the nucleus determines the phenotype of the asexual spores. Mixing of the two haploid strains helps them fuse together. The mitotic recombination studies require heterozygous genes. The two haploid strains differing in the phenotypes get fused together. They give rise to a mycelium having two nuclear types. Both the nuclear types divide mitotically in the same cytoplasm. These cells are known as heterokaryons since they have different nuclei.

The parasexual cycle:

It involves the genetic systems involving genetic recombination through the processes other than the meiosis and fertilization. The parasexual cycle is a typical feature of fungi such as Aspergillus. The first step of the cycle involves mycelial fusion. It gives rise to a heterokaryon (having two different nuclei). Then the two haploid nuclei fuse. They form a diploid nucleus. The next step involves a mitotic crossing over within the diploid nucleus. The diploid nucleus, later on, undergoes haploidization.

Image 2: Mitotic crossing over between the pro and the paba loci.

Consider the two haploid strains:

The first strain consists of genotype w ad⁺ pro paba⁺ y⁺ bi.

The second strain consists of genotype w+ ad pro⁺ paba y bi⁺.

The alleles such as ad, pro, paba, and bi show recessive phenotype. They depict adenine, proline, para-aminobenzoic acid, and biotin, respectively. The growth medium must have all the above nutrients for the survival of the mutant strains. The parental strains require the above growth supplements. However, the heterokaryon does not require them, since it consists of all the four genes capable of synthesizing them. The recessive w and y alleles control the color of the asexual spores, thereby giving rise to the colony color. The “w⁺y⁺” strain shows green coloration. The “wy⁺” strain shows white coloration. The “w⁺y” strain shows yellow coloration. The “wy” strain shows white coloration. The heterokaryon of the first and the second strains is not green. The heterokaryon shows a mixture of yellow and white spores. It has a mottled appearance. Diploidization is a relatively rare case. The two haploid nuclei fuse together. They produce a diploid nucleus in such cases. Hence, the spores in these diploid cells would also be diploid. These cells do not require growth supplements since they show genotype with wild-type alleles. Mostly, they give rise to green colonies on a growth medium or a solid medium.

Haploidization produces the haploid sectors. It leads to the formation of the haploid nuclei from the diploid nucleus. These nuclei divide by mitosis. The haploid progeny nuclei are known as haploid segregants. Consider the haploid white sectors. Half of them show a genotype of w ad+ pro paba+ y+ bi. The remaining half strains have a genotype of w⁺ ad pro⁺ paba y bi⁺. It involves a 50:50 segregation ratio indicating the location of genes to be on different chromosomes. The six gene loci are present on the two nonhomologous chromosomes. The white gene is present on one chromosome. The other five genes are present on the other one. The next step involves the determination of the gene order and map distances. The occurrence of the diploid segregants is rare. Hence, consider only the single-crossover segregants. The genes distal to the crossing over point become homozygous due to the crossing over in the mitosis. Thus, the recessive alleles with heterozygous traits turn to become homozygous and exhibit recessiveness.

The production diploid yellow sector in a green diploid:

Case 1: Mitotic crossing over between the pro and paba loci

This type of mitotic crossing over produces a homozygous segregant for the y allele giving yellow coloration. It also produces a twin spot having homozygosity. However, the overall colony color includes green coloration which masks the yellow coloration. The crossing over makes the genes distal to that point homozygous, thereby making the yellow segregant dependent on the para-aminobenzoic acid for the growth.

Case 2: Mitotic crossing over between the paba and y loci

The yellow sector shows heterozygosity for the para-aminobenzoic acid. The yellow sectors consist of y and paba genotypes. The possible results reveal bi phenotype (present in the diploid green sector), and y phenotype (present in the diploid yellow sector).

Mitotic Recombination and Retinoblastoma:

It is ophthalmic cancer occurring in childhood. The RB gene mutations are the culprits. Mitotic recombination in the RB gene leads to the loss of heterozygosity for the wild-type allele. The recombination occurs between the non-sister chromatids of the thirteenth chromosome. The recombination mainly occurs between the RB gene and the centromere. 

© Copyright, 2018 All Rights Reserved.