Huntingtin and Huntington's Disease


This website was produced as an assignment for Genetics 677 at UW-Madison Spring 2009


Gene Ontology


I used two different databases to search for gene ontology terms for human HTT.  Other databases I searched did not have a human HTT gene annotated. 

AmiGO

The AmiGO database listed 11 ontology terms under human HTT, four each for biological processes and molecular function and three for cellular component.  The site provides a simple method to create trees showing linkage between the different terms.  The results for each compenent are shown below.  Additionally, I created a tree showing linkage between all 11 terms, shown directly below. 

Biological process

The biological process terms indicate the role the gene plays in the larger biological framework of the cell.  Of the terms listed on AmiGO, apoptosis and pathogenesis at first appeared the most significant in describing the role of HTT in Huntington's disease.  It has been demonstrated that the mutant form of huntingtin eventually leads to apoptosis in neurons (1).  The role in organ morphogenesis was seen mice knockouts for huntingtin (2). 

GO Terms
Organ morphogenesis    GO:0009887
Induction of apoptosis     GO:0006917
Pathogenesis              GO:0009405

Cellular component

The cellular component terms describe the locations in the cell in which the gene is actively functioning.  I was not surprised to see both cytoplasm and the nucleus listed, as huntingtin is described as active in both in primary papers I previously came across.  References for both are listed on AmiGO.  The soluble fraction term essentially indicates the protein is found in the water soluble components of the cell, which includes the cytoplasm and nucleus.  The golgi apparatus listing led me to a research paper describing the interaction of huntingtin with optineurin, which maintains the shape of the golgi apparatus (3). 

GO Terms
Cytoplasm                 GO:0005737
Golgi apparatus       GO:0005794
Nucleus                     GO:0005634
Soluble Fraction        GO:0005625

Molecular function

Molecular function terms describe the precise role the protein plays in the cell on the molecular level.  The GO terms for molecular function included three similar terms in microtuble binding, protein binding, and transporter activity, all of which point to huntingtin actively taking part in transport in the cell.   The reference listed under transporter activity describe the role of huntingin as anchoring microtubules or transporting mitochondria, vesicles, or other organelles (4).  The transcriptional corepressor reference describes an interaction of mutant huntingtin with p53, repressing p53-driven promoters p21 and MDR-1 (5).  Furthermore, mutant huntingtin binds CREB binding protein, which overall results in reduced control of transcription (5). 

GO Terms
Microtubule binding     GO:0008017
Protein binding GO:0005515
Transcription corepressor GO:0003714
Transporter activity   GO:0005215

GOALL

The GOALL database listed 55 GO terms under the gene name HTT for humans.  Of the biological processes, some corresponded with functions I would consider highly connected to the nature of Huntington's disease, including brain development and locomotory behavior.  Others were surprising, such as iron ion homeostasis.   The cellular component and molecular function terms followed the same pattern, with more terms listed for each compared to AmiGo.

Subcellular Localization

Based upon my results for gene ontology, I used a sequence search in the pTARGET database to determine subcellular locations of the protein based upon its structure.  pTARGET demonstrated with relatively high confidence that huntingtin locates to the cytoplasm based upon its structure, with a confidence value of 87.6%.  I found this interesting, as the protein sequence I used was the full huntngtin (3,144 aa).  The localization to the nucleus listed in my GO terms is proposed to occur only when the protein is cleaved.

Analysis

The AmiGO program provided what I considered the most useful information regarding the gene ontology of huntingtin.  I thought the tree creation aspect of the database was a useful way to manipulate the ontology terms to visualize how the protein's function is mapped out.  Also, the direct link to the primary papers accounting for the papers provided useful information and a good way to build up background knowledge on each specific aspect of the protein.  Overall, the AmiGO program outlined the importance of huntingtin in transport activities, with the possiblity of function in both the cytoplasm and the nucleus.  This database would be useful to gather intial information about a particular gene or protein to gain an understanding of its function.  GO ALL, in contrast, provided more terms, though the large number made the characterization of the actual function of the protein more difficult.  GO ALL would probably be more useful to search for more unique functions of a protein. 


References:
(1)  Saudou et. al 1998.  Huntingtin acts in the nucleus to induce apoptosis but death does not correlate with the formation of intranuclear inclusionsCell 95(1):55-66.doi:10.1016/S0092-8674(00)81782-1  
(2)  Nasir et al. 1995.  Targeted disruption of the Huntington's disease gene results in embryonic lethality and behavioral and morphological changes in heterozygotes.  Cell 81(5):811-23. doi:10.1016/0092-8674(95)90542-1  
(3)  Sahlender et al. 2005.  Optineurin links myosin VI to the Golgi complex and is involved in Golgi organization and exocytosis.  J Cell Bio 169(2)285-95.  doi:10.1083/jcb.200501162
(4)  Gutekunst et al. 1995.  Identification and localization of huntingtin in brain and human lymphoblastoid cell lines with anti-fusion protein antibodies.  PNAS 92(19) .  Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/7568002 
(5) 
Steffan et al. 2000.  The Huntington's disease protein interacts with p53 and CREB-binding protein and represses transcription.  PNAS 97(12):6763-8.  Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/10823891

Created by Eric Nickels enickels@wisc.edu       5/9/2009      Genetics 677 Webpage