This project report details the characteristics of human protein “MSH6” highlighting its significance, sequence and homologues. Expasy, Blast, OMIM etc are some of the online databases, which have been used to collect data. Out of the several organisms, Xenopus has been selected as model organism for studying the Cancer disease associated with MSH6 protein. Xenopus are bizarre-looking frog species with distinct features like small heads, absent eye lids etc. The sequence of the MSH6 protein closest matching homologue “Xenopus Laevis” protein is then illustrated, followed by comparison of amino acids of both the proteins. Moving ahead, the MSH6 gene is discussed along with its mapping with chromosomes, mutation and the associated diseases by laying focus on Colon Cancer.
The MSH6 Protein
The MSH6 protein is a Homo sapiens (Human) protein which is also known as “MutS-alpha 160kDa subunit” or “G/T mismatch-binding protein” or “p160”. It forms a heterodimer known as hMutSalpha with MSH2 protein, which binds base-base mismatches and small insertion/deletion loops in extracts of human cells and thus initiating DNA repair (Laccarino et al, 1998). It is a component of the post-replicative DNA mismatch repair system (MMR). Read more at: http://www.essaywriter.co.uk/human-molecular-genetics.aspx?id=HWox1o4pj4gLw
Homologues Protein in Xenopus Organism
Xenopus, which is considered as a specialized and primitive representation of Amphibia or Anura, is identified and selected as one of the most appropriate model organism for this biological study. Dawid & Sargent (1988) state that Xenopus is one of the species which is easily bred and maintained. Further, it exhibits a highly derived biology as compared to other frogs (Cannatella & Sa, 1993). Also, it is best-known amphibian and vertebrates with features like small heads, flattened pyriform bodies, dorsally pointing eyes, absent eye-lids, muscular hind limbs etc. Blast search was then performed to find the homologues protein as MSH6 protein in Xenopus organism. As a result, hypothetical protein LOC734294 (Xenopus Laevis) was discovered as having sequence closely matching with MSH6. In 1930s and 1940s, Xenopus Laevis protein was basically used for physiological research and pregnancy diagnosis but now it plays a significant role in molecular biology studies as well (Cannatella & Sa, 1993).
Identical and Simalar Amino Acids in MSH6 and Xenopus Laevis Protein
Amino acids are building blocks of proteins and act as intermediates in metabolism. The 20 amino acids exhibit enormous chemical flexibility and establish the biological activity of the protein they exists in. Alanine, glycine, Tyrosine and Valine are some of the examples of amino acids. Even a very small and simple protein is comprised of several amino acids characteristics. Further, proteins constitute within their amino acid sequences the information describing the way protein will unfold into a three dimensional structure and stability of the resulting structure. It is extremely important to study amino acid structure and properties before understanding protein structure and properties ( http://www.biology.arizona.edu/biochemistry/problem_sets/aa/aa.html).
In this experimental study, the amino acids of both MSH6 and its homologue Xenopus Laevis protein have been compared by aligning the sequences of both the proteins obtained by blast search (NCBI database) using Clustalw and Boxshade databases.
The MSH6 Gene
The MSH6 gene encodes a protein identical to the MutS protein. In E. coli, the MutS protein also helps in the recognition of mismatched nucleotides, prior to their repair. A highly conserved region of approximately 150 aa, called the Walker-A adenine nucleotide binding motif, exists in MutS homologs. The encoded protein of this gene combines with MSH2 to form a mismatch recognition complex that functions as a bidirectional molecular switch that exchanges ADP and ATP as DNA mismatches are bound and dissociated. Refer Appendix 2 for MSH6 mutS homolog 6 Sequence Alignment.
The MSH6 Gene Mapping
Papadopoulos et al. (1995) discovered that the MSH6 gene maps to chromosome 2p16 by PCR evaluation of a panel of somatic cell hybrids composing segments of chromosome 2. Further, Papadopoulos et al. (1995) also found out that MSH6 resides in the same YAC as MSH2.
The Diseases Associated with MSH6 Gene
A couple of diseases are associated with the MSH6 gene as stated below:
- Cancer suspectibility
- Colorectal cancer
- Endometrial cancer
- Endometrial carcinoma
- Mismatch repair cancer syndrome
- Ovarian cancer (endometrial type)
- Ovarian cancer (endometrioid type)
Focussing specifically on Colon cancer, several scholars have observed its existence. Wu et al. (2001) found two patients with Hereditary Non Polyposis Colon Cancer (HNPCC), who had the same mutation in the MLH3 gene and two mutations in the MSH6 gene. One was a val878-to-ala mutation and the other was an insertion of a T at nucleotide position 650. Huang et al. (2001) described a 2-bp deletion (CT) type Colon cancer at nucleotide 3052 in exon 4 of the MSH6 gene in a large family that met Amsterdam I criteria and had late-onset HNPCC. Moving ahead, Plaschke et al.(2003) identified that a patient with HNPCC with loss of MSH6 expression in tumors and no germline mutations, an Alu repeat-mediated deletion of 13 kb affecting the promoter region, exon 1 and exon 2 of the MSH6 gene. They also identified a duplication of 4.9 kb of the MSH6 gene containing 1.6 kb of the 3-prime end of exon 4 and exon 5, integrated into intron 5. Suchy et al. (2002) examined a Polish family and diagnosed a 49 years old woman with bilateral ovarian cancer of the endometrioid type and who had a positive family history for both colon cancer and endometrial cancer. The 3311-3312delTT mutation created a termination codon at 1106 and removed a C-terminal MSH2 interaction region and a nucleotide-binding region.
Mutation in MSH6
Mutation is defined as a permanent change in the DNA sequence of a gene, which can even alter the amino acid sequence of the protein encoded by the gene. Mutation differs in size from a single DNA building block to a large segment of a chromosome (http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/genemutation). Mutations can occur in the following ways:
- Hereditary or germline mutation which is mutation inherited by child from a parent. Mutation of this type is present throughout a person’s life in almost every cell in the body.
- New mutation which occur just after the fertilization and are only present in an egg or sperm cell. No family history of disorder is related with this kind of mutation.
- Acquired Mutation, which occurs in the DNA of individual cells mainly due to environmental factors like ultraviolet rays from sun. This type of mutation occurs at some time during a person’s life and cannot be passed to the next generation.
As mutation occurs during growth and development, the individuals usually have some cells with mutation and some cells without genetic change. Further, some genetic changes are very rare and are present in about 1 percent of the population and are known as polymorphisms. Polymorphism is the reason behind different eye colours, hair colours, blood type etc. Having said this, there are even those polymorphisms which involve high risk of developing certain disorders.