HEXA+Gene+and+Hexosaminidase+A+Protein+--+Phylogenetic+Analysis

Although Tay-Sachs was researched extensively just a few decades ago, no treatment has been found. The research did lead to many scientific break throughs in molecular biology and genetic testing, but due to the rarity of the disease and the difficulty in treating protein deficient genetic diseases, it seems to most, especially those diagnosed with TSD and their parents, that we are no closer to a cure. But the fact is research is still continuing, and it is leading somewhere, even if just to understanding the mechanisms of the disease and its evolutionary roots. Most people subscribe to the Theory of Evolution postulated by Charles Darwin, believing that organisms - species - evolve. But what many do not realize is that not only do species evolve, but so do genes, and, in turn, proteins. Most people are aware of the Human Genome Project, in which an entire human genome was mapped, but significance of such a breathtaking endeavor is often overlooked. As more an more genomes are mapped, we are now able to compare these genomes across species to understand the mechanisms of evolution, using these comparisons to write a sort of biological history, or genealogy, of the world.

It is often forgotten that Darwin came up with his Theory of Evolution before knowing what genes, or even DNA, were, and that the biological sciences were largely empirical, observational sciences. Naturalists often grouped species into categories, taxons, phylums, classes, etc., based simply on traits that they could see. But genetic analysis has greatly broadened our ability to see relationships between species, allowing us to understand more fully how they have evolved, and, of course, how closely they are related. Now, we can compare individual genes and protein sequences to see not only how organisms have evolved, but how traits evolve as well.

The GM2 gangliosidoses arise from one of three genetic mutations, effecting the production of one of the three primary components of GM2 ganglioside hydrolysis: the alpha subunit polypeptide of hexosaminidase A (TSD), the beta subunit polypeptide of hexosaminidase A (Sandhoff disease) and the GM2 activating cofactor (AB variant). Because humans (Homo sapiens) are closely related to many other organisms, it is likely that we can find highly conserved orthologs of the genes that code for these proteins. Orthologs are similar (or homologous) genes found in different species that usually infer that they are products of linear descent, which means that they are similar traits in different species that usually indicate that they arose from the same common ancestor. Although we usually use these orthologs to find relationships between species, it may be possible to explore the inverse, that if species are similar, perhaps they will have a particular gene, or protein, of interest. With Tay-Sachs disease, the gene of interest is the HEXA gene, and the protein of interest is the alpha subunit of hexosaminidase A.

Using BLASTn, the sequence of the HEXA gene was found and identified.

61 tcgcaggacg ggcgacggcc ctctggccct ggcctcagaa cttccaaacc tccgaccagc 121 gctacgtcct ttacccgaac aactttcaat tccagtacga tgtcagctcg gccgcgcagc 181 ccggctgctc agtcctcgac gaggccttcc agcgctatcg tgacctgctt ttcggttccg 241 ggtcttggcc ccgtccttac ctcacaggga aacggcatac actggagaag aatgtgttgg 301 ttgtctctgt agtcacacct ggatgtaacc agcttcctac tttggagtca gtggagaatt 361 ataccctgac cataaatgat gaccagtgtt tactcctctc tgagactgtc tggggagctc 421 tccgaggtct ggagactttt agccagcttg tttggaaatc tgctgagggc acattcttta 481 tcaacaagac tgagattgag gactttcccc gctttcctca ccggggcttg ctgttggata 541 catctcgcca ttacctgcca ctctctagca tcctggacac tctggatgtc atggcgtaca 601 ataaattgaa cgtgttccac tggcatctgg tagatgatcc ttccttccca tatgagagct 661 tcacttttcc agagctcatg agaaaggggt cctacaaccc tgtcacccac atctacacag 721 cacaggatgt gaaggaggtc attgaatacg cacggctccg gggtatccgt gtgcttgcag 781 agtttgacac tcctggccac actttgtcct ggggaccagg tatccctgga ttactgactc 841 cttgctactc tgggtctgag ccctctggca cctttggacc agtgaatccc agtctcaata 901 atacctatga gttcatgagc acattcttct tagaagtcag ctctgtcttc ccagattttt 961 atcttcatct tggaggagat gaggttgatt tcacctgctg gaagcccaac ccagagatcc 1021 aggactttat gaggaagaaa ggcttcggtg aggacttcaa gcagctggag tccttctaca 1081 tccagacgct gctggacatc gtctcttctt atggcaaggg ctatgtggtg tggcaggagg 1141 tgtttgataa taaagtaaag attcagccag acacaatcat acaggtgtgg cgagaggata 1201 ttccagtgaa ctatatgaag gagctggaac tggtcaccaa ggccggcttc cgggcccttc 1261 tctctgcccc ctggtacctg aaccgtatat cctacggccc tgactggaag gatttctacg 1321 tagtggaacc cctggcattt gaaggtaccc ctgagcagaa ggctctggtg attggtggag 1381 aggcttgtat gtggggagaa tatgtggaca acacaaacct ggtccccagg ctctggccca 1441 gagcaggggc tgttgccgaa aggctgtgga gcaacaagtt gacatctgac ctgacatttg 1501 cctatgaacg tttgtcacac ttccgctgtg agttgctgag gcgaggtgtc caggcccaac 1561 ccctcaatgt aggcttctgt gagcaggagt ttgaacagac ctgagcccca ggcaccgagg 1621 agggtgctgg ctgtaggtga atggtagtgg agccaggctt ccactgcatc ctggccaggg 1681 gacggagccc cttgccttcg tgccccttgc ctgcgtgccc ctgtgcttgg agagaaaggg 1741 gccggtgctg gcgctcgcat tcaataaaga gtaatgtggc atttttctat aaaaaaaaaa 1801 aaaaaaaaaa a ||
 * **__ HEXA nucleotide sequence __** ||
 * 1 gagaccagcg ggccatgaca agctccaggc tttggttttc gctgctgctg gcggcagcgt

Table displays the base pair sequence of the HEXA gene. Each letter represents a base pair, adenine (a), thymine (t), guanine (g), and cytosine (c). Results from NCBI BLASTn.

By searching through NCBI protein bank, I was able to acquire multiple protein sequences for the alpha subunit of hexosaminidase A. Taking these sequences and using Clustal alignment in Molecular Evolutionary Genetics Analysis (MEGA 6), I was able to align the amino acid sequences and compare them to see how closely related they were. Below is a phylogenetic tree showing the bootstrap results of the alignment, and the evolutionary relationship between the proteins of each species examined.





This analysis was done with several species that produce a similar protein to the human hexosaminidase A alpha subunit. Some species were selected based on their evolutionary relationship to humans (such as the common chimp and orangutan) and some were selected as possible model organisms for research (zebra fish, above, and mouse, below). The species of most interest, however, was the Jacob sheep, as in 2010, autopsies revealed that four sheep that died from a progressive neurodegenerative disease within their first 6-8 months exhibited hexosaminidase A deficiency (Torres, P.A., et. al, 2010).



Both phylogenetic trees show similar results, with the Human hexosaminidase A alpha subunit being most closely related to that of the common chimp (Pan troglodytes). The two species form a clave, indicating a close relationship that is closer to each other than the next "relative," in this case, the orangutan. The two Macaca species (the Rhesus monkey and the crab-eating macaque) form their own clave, more closely related to the orangutan than the human/chimp clave. The Jacob sheep and bull species form another clave, with their hexosaminidase proteins being highly conserved (as indicated by their 100 bootstrap value). In both trees, the model organisms (Zebra fish and Mouse) form outgroups, groups that are most distinct from the others.

Based on this information, if protein isolation and therapy was to be pursued, the most likely match would be found in the common chimp. This is not to say that these findings suggest that chimpanzee hexosaminidase A would be a viable treatment for Tay-Sachs, especially since it was in found in 1998, through adenoviral gene therapy in knock-out mice, that over expression of both alpha and beta subunits through viral vectors was needed to obtain the most successful results (Guidotti, J.E., et. al, 1999).

Back to Tay-Sachs (Home)

HEXA Gene

HEXA Protein Interactions

Hexosaminidase A

Tay-Sachs Overview and History

GM2 Gangliosides and Sphingolipid Pathway

Materials and Methods

References