Albinism and It's Relation to Point Mutations and Proteins
By: Tatiana Canaran, Jessie Fraser, Sarah Garisto and Maria Reyes
Where is the gene located?
Albinism is a point mutation, therefore it is caused by insertion, deletion or inversion of a chromosome, depending on the case. People with albinism are either partially or completely lacking pigment in the eyes, skin and or hair. This is caused by a mutation in tyrosinase gene located on chromosome 11 (Oetting, 2002). When mutated tyrosinase is present, it halts the conversion between the amino acid tyrosine and melanin. Melanin determines a person’s skin colour, eye colour and hair colour; therefore with out it, a person’s genes are unable to produce pigments.
What is the type of mutation?
Albinism is a POINT MUTATION or FRAMESHIFT MUTATION depending on the type of Albinism. Most commonly, albinism is caused by a frameshift mutation.
Frameshift mutations: occurs when one base is added or removed
There are two types of frameshift mutations. An insertion is a mutation in which one or a few nitrogenous bases are added to a DNA sequence. This results in a sequence that is a few bases longer than it should be. In albinism, this is caused by a single based insertion in the tyrosinase base in the exon 2 that shifts the reading frame and creates a "premature termination signal" (Harrison, 2013). Therefore tyrosine remains inactive, jeopardizing the function of melanin. A deletion is a mutation in which one or a few bases are removed, resulting in a shorter-than-normal sequence. In albinism, this includes the deletion of tyrosine, therefore obstructing its conversion to melanin.
Insertions and deletions are called frameshift mutations because they do not just affect one codon, a 3-base sequence that codes for one amino acid, like in base substitutions (Attwood, 2014). Instead, frameshift mutations affect all the codons that occur after the point mutation. This changes how a DNA sequence is read or the reading frame.
What is the function of the protein the gene codes for?
The gene tyrosinase (TYR), which is located in chromosome 11, codes for the production of melanin. Melanin pigment is responsible for giving colour to the eyes, skin and hair. If a mutation occurs to this gene, then melanin will not be properly synthesized and the eyes, skin and hair will be lacking pigment.
What effect does the mutation have on the structure of the protein the gene codes for?
Many different mutations can cause the gene tyrosinase (TYR) to not function the way it should. A mutation can disrupt the conversion of the amino acid tyrosine, which then codes for the pigment melanin. Without this amino acid the skin, eyes and hair will not have proper pigmentation. Mutations to the tyrosinase gene cause oculocutaneous albinism Type 1 (OCA1), which is an autosomal recessive disorder, illustrated by reduced melanin pigment. Autosomal recessive disorders are passed down through families. Two copies of the abnormal gene must be present in order for the disease to develop. OCA1 is further divided into two categories, OCA1A and OCA1B. The difference between these two is that OCA1A tyrosinase activity and melanin synthesis are undetectable. In OCA1B tyrosinase activity and melanin synthesis are still present but reduced compared to normal levels.
What is the mechanism of the disorder (what effects does the alteration in function of the protein have on the workings of the cell and the body)?
Albinism consists of a group of inherited abnormalities of melanin synthesis and is typically characterized by a congenital reduction or absence of melanin pigment. The phenotypic heterogeneity of albinism is due to different gene mutations affecting the enzymes and proteins in the melanin pathway, resulting in varying degrees of decreased production.
Melanin is a photoprotective pigment in the skin that absorbs UV light from the sun that is formed in the melanosome organelle of the melanocyte. Melanocytes are found in the basal layer of the epidermis,hair follicles, and pigmented tissues of the eye. People with albinism have mutations in a gene called TYR which is responsible for production of the enzyme tyrosinase, used by cells to convert the amino acid tyrosine into pigment.
Cross section of skin, showing melanocyte and keratinocytes in the basal layer of the epidermis
Effects on Skin:
1) Pale appearance
2) Increased sensibility towards sun
3) Increased risk of skin cancer
Effects on the Optic System:
Since the development of the optic system is highly dependent on the presence of melanin the following effects may occur.
1) Pale irises, red pupils—caused by light reflecting off of the blood vessels in the retina
2) Blindness—without melanin the neuronal tracts leading from the eye to the visual cortex of the brain develop irrationally
3) Diminished depth perception
4) Nystagmus, or fluttering of the eyes—the overstimulation of retinal photoreceptors sends confusing messages to the brain
Diagram of the wild type and mutated type protein and the pathway
Characteristics of wild-type and uw-mutant primary murine melanocytes in culture.
Subcellular distribution of melanogenic proteins in wild-type and mutant primary melanocytes. Wild-type and mutant melanocytes were stained with antibodies and fluorescent probes
What are the symptoms of the disorder?
Albinism is associated with vision problems like:
Albinism is a lifelong condition that does not worsen with time. Although there is no way to treat poor or absent pigment production or correct abnormal vision development, proper eye evaluation and management can be useful. Eye misalignment can be treated with glasses or surgery. Glasses can be prescribed to improve vision and reduce light sensitivity. Magnifying glasses for reading and low vision aids for distance vision can also be helpful. Genetic counseling of affected individuals and their families is recommended. Counselors can provide a detailed explanation of the disorder including the chances of future children being affected.
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