Sunday, 22 April 2012

Genetic Tools for Disease Discovery

Genetic Tools for Disease Discovery - Maddison Norton (s42580061)

Recently, a clinical study was conducted by a group of medical professionals in an attempt to identify an unknown disease. Two children had presented with this unknown disease and had exhibited signs and symptoms such as skin lesions the day after birth, chronic diarrhoea from the first week of life, postulated skin, very short hair, disorganised eye brows and eyelashes, fatigue, thickened fingernails and reoccurring gastrointestinal infections (Blaydon et al 2011). A picture of the affected son can be seen below,



The parents of the affected children were first cousins and while they did not exhibit any of the signs or symptoms associated with this mystery disease, the research indicated that they were carriers. Unfortunately, the affected daughter died of malnutrition caused by the chronic diarrhoea at the age of 12 and as such could not be included in the research. The family pedigree can be seen on the following page,


A number of tests were conducted using skin and blood samples from the surviving son, unaffected mother and control subjects.  In particular, the blood sample of the affected son was genotyped and a 4 base pair deletion was found in exon 5 on chromosome 2. Specifically, CAGA had been deleted from the ADAM17 gene.  The location of the ADAM17 gene is signified by a red line in the following diagram,



The third child, who did not display any of the same symptoms as his siblings, was then also genotyped. He did not have this deletion.  Both the mother and father had one copy of the mutated gene and one unaffected copy of the gene.

This particular gene, ADAM17, encodes for a protein which effectively functions to ‘cut open’ vesicles containing a number of different substances but specifically TNF- α (Tumor Necrosis Factor-alpha).  TNF- α is a cell-signalling protein which controls the immune cells (Davidson College 200).
The mutation observed on the ADAM17 gene was a 4 base pair deletion. As codons are read in groups of 3, this deletion caused a reading frame shift. This frame shift is particularly devastating as “all the nucleotides that are downstream of the deletion or insertion are improperly grouped” (Reece et al 2010).  This means that the wrong amino acids are sequenced. An example of the effect that a frame shift can be seen below,



The frame shift in the aforementioned ADAM17 mutation causes a premature stop codon (UAG) which results in a severely shortened protein (Hein, Pattison & Arena 2012. While the resulting protein is still made, it suffers from major functional deficiencies.
Immunohistochemical characterisation was then conducted on the affected boy to test the level of antigens in his body. The results indicated that the affected boy had very few B cells but an abundance of killer T cells in his skin follicles.  These results supported the idea that the ADAM17 gene mutation was causing an absence of TNF- α.  The elevated level of killer T cells in the affect boy’s skin follicles can be seen below in picture A, compared to an unaffected individual in picture B.



These results indicated that the disease the son and daughter suffered from was caused by the deletion mutation in the ADAM17 gene.  Follow up research was conducted into mice with the ADAM17 mutation and while the mice exhibited similar symptoms, all of the mice with the mutation died within weeks of birth due to multiple organ system failure. This was particularly important as it suggested that humans are capable of adapting pathways to compensate for the absence of the ADAM17 protein.  This mutation also lends itself to further research in regards to inhibiting the ADAM17 gene in individuals who suffer from chronic inflammatory disease.

The original article can be accessed at: http://www.nejm.org/doi/full/10.1056/NEJMoa1100721


References:
Blaydon, D, Biancheri, P, Di, W, Plagnol, V, Cabral, R, Brooke, M, van Heel, D, Ruschendorf, F, Toynbee, M, Walne, A, O’Toole, E, Martin, J, Lindley, K,  Vulliamy, T, Abrams, D, MacDonald, T, Harper J & Kelsell, D 2011, ‘Inflammatory Skin and Bowel Disease Linked to ADAM17 Deletion’, The New England Journal of Medicine, vol. 365, no. 16, pp. 1502-1508.

Davidson College 2000, TNF-alpha, viewed 16 March 2012, <http://www.bio.davidson.edu/courses/immunology/Students/spring2000/wolf/tnfalpha.html >.

Hein, M, Pattison, S & Arena, S 2012, ‘Genetics’, in Wiley J (ed.), Introduction to General, Organic, and Biochemistry, 10th edn, John Wiley & Sons, Inc, New York, pp. 82-84.

Reece, J, Meyers, N, Urry, L, Cain, M, Wasserman, S, Minorsky, P, Jackson, R & Cooke, B 2010, ‘Genetics’, in Wilbur B (ed.), Campbell Biology, 9th edn, Pearson Benjamin Cummings, San Francisco, pp. 352.

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