Huntingtin

The 5'-end (five prime end) of the HTT gene has a sequence of three DNA bases, cytosine-adenine-guanine (CAG), coding for the amino acid glutamine, that is repeated multiple times. This region is called a trinucleotide repeat. The usual CAG repeat count is between seven and 35 repeats.

The HTT gene is located on the short arm (p) of chromosome 4 at position 16.3, from base pair 3,074,510 to base pair 3,243,960.{{Cite web | url=http://ghr.nlm.nih.gov/gene/HTT | title=HTT gene | access-date=2016-02-18 | archive-date=2016-02-02 | archive-url=https://web.archive.org/web/20160202113623/http://ghr.nlm.nih.gov/gene/HTT | url-status=live }}

The function of huntingtin (Htt) is not well understood but it is involved in axonal transport.{{cite journal | vauthors = Vitet H, Brandt V, Saudou F | title = Traffic signaling: new functions of huntingtin and axonal transport in neurological disease | journal = Current Opinion in Neurobiology | volume = 63 | pages = 122–130 | date = August 2020 | pmid = 32408142 | doi = 10.1016/j.conb.2020.04.001 | s2cid = 218596089 }} Huntingtin is essential for development, and its absence is lethal in mice. The protein has no sequence homology with other proteins and is highly expressed in neurons and testes in humans and rodents.{{cite journal | vauthors = Cattaneo E, Zuccato C, Tartari M | title = Normal huntingtin function: an alternative approach to Huntington's disease | journal = Nature Reviews. Neuroscience | volume = 6 | issue = 12 | pages = 919–30 | date = December 2005 | pmid = 16288298 | doi = 10.1038/nrn1806 | s2cid = 10119487 }} Huntingtin upregulates the expression of brain-derived neurotrophic factor (BDNF) at the transcription level, but the mechanism by which huntingtin regulates gene expression has not been determined.{{cite journal | vauthors = Zuccato C, Ciammola A, Rigamonti D, Leavitt BR, Goffredo D, Conti L, MacDonald ME, Friedlander RM, Silani V, Hayden MR, Timmusk T, Sipione S, Cattaneo E | display-authors = 6 | title = Loss of huntingtin-mediated BDNF gene transcription in Huntington's disease | journal = Science | volume = 293 | issue = 5529 | pages = 493–8 | date = July 2001 | pmid = 11408619 | doi = 10.1126/science.1059581 | s2cid = 20703272 }} From immunohistochemistry, electron microscopy, and subcellular fractionation studies of the molecule, it has been found that huntingtin is primarily associated with vesicles and microtubules.{{cite journal | vauthors = Hoffner G, Kahlem P, Djian P | title = Perinuclear localization of huntingtin as a consequence of its binding to microtubules through an interaction with beta-tubulin: relevance to Huntington's disease | journal = Journal of Cell Science | volume = 115 | issue = Pt 5 | pages = 941–8 | date = March 2002 | pmid = 11870213 | doi = 10.1242/jcs.115.5.941 }}{{cite journal | vauthors = DiFiglia M, Sapp E, Chase K, Schwarz C, Meloni A, Young C, Martin E, Vonsattel JP, Carraway R, Reeves SA | display-authors = 6 | title = Huntingtin is a cytoplasmic protein associated with vesicles in human and rat brain neurons | journal = Neuron | volume = 14 | issue = 5 | pages = 1075–81 | date = May 1995 | pmid = 7748555 | doi = 10.1016/0896-6273(95)90346-1 | s2cid = 18071283 | doi-access = free }} These appear to indicate a functional role in cytoskeletal anchoring or transport of mitochondria. The Htt protein is involved in vesicle trafficking as it interacts with HIP1, a clathrin-binding protein, to mediate endocytosis, the trafficking of materials into a cell.{{cite journal | vauthors = Velier J, Kim M, Schwarz C, Kim TW, Sapp E, Chase K, Aronin N, DiFiglia M | display-authors = 6 | title = Wild-type and mutant huntingtins function in vesicle trafficking in the secretory and endocytic pathways | journal = Experimental Neurology | volume = 152 | issue = 1 | pages = 34–40 | date = July 1998 | pmid = 9682010 | doi = 10.1006/exnr.1998.6832 | s2cid = 36726422 }}{{cite journal | vauthors = Waelter S, Scherzinger E, Hasenbank R, Nordhoff E, Lurz R, Goehler H, Gauss C, Sathasivam K, Bates GP, Lehrach H, Wanker EE | display-authors = 6 | title = The huntingtin interacting protein HIP1 is a clathrin and alpha-adaptin-binding protein involved in receptor-mediated endocytosis | journal = Human Molecular Genetics | volume = 10 | issue = 17 | pages = 1807–17 | date = August 2001 | pmid = 11532990 | doi = 10.1093/hmg/10.17.1807 | doi-access = free }} Huntingtin has also been shown to have a role in the establishment in epithelial polarity through its interaction with RAB11A.{{cite journal | vauthors = Elias S, McGuire JR, Yu H, Humbert S | title = Huntingtin Is Required for Epithelial Polarity through RAB11A-Mediated Apical Trafficking of PAR3-aPKC | journal = PLOS Biology | volume = 13 | issue = 5 | pages = e1002142 | date = May 2015 | pmid = 25942483 | pmc = 4420272 | doi = 10.1371/journal.pbio.1002142 | doi-access = free }}

Huntingtin has been found to interact directly with at least 19 other proteins, of which six are used for transcription, four for transport, three for cell signalling, and six others of unknown function (HIP5, HIP11, HIP13, HIP15, HIP16, and CGI-125).{{cite journal | vauthors = Harjes P, Wanker EE | title = The hunt for huntingtin function: interaction partners tell many different stories | journal = Trends in Biochemical Sciences | volume = 28 | issue = 8 | pages = 425–33 | date = Aug 2003 | pmid = 12932731 | doi = 10.1016/S0968-0004(03)00168-3 }} Over 100 interacting proteins have been found, such as huntingtin-associated protein 1 (HAP1) and huntingtin interacting protein 1 (HIP1), these were typically found using two-hybrid screening and confirmed using immunoprecipitation.{{cite journal|author23-link=Bernhard Landwehrmeyer | vauthors = Goehler H, Lalowski M, Stelzl U, Waelter S, Stroedicke M, Worm U, Droege A, Lindenberg KS, Knoblich M, Haenig C, Herbst M, Suopanki J, Scherzinger E, Abraham C, Bauer B, Hasenbank R, Fritzsche A, Ludewig AH, Büssow K, Buessow K, Coleman SH, Gutekunst CA, Landwehrmeyer BG, Lehrach H, Wanker EE | title = A protein interaction network links GIT1, an enhancer of huntingtin aggregation, to Huntington's disease | journal = Molecular Cell | volume = 15 | issue = 6 | pages = 853–65 | date = Sep 2004 | pmid = 15383276 | doi = 10.1016/j.molcel.2004.09.016 | doi-access = free }}{{cite journal | vauthors = Wanker EE, Rovira C, Scherzinger E, Hasenbank R, Wälter S, Tait D, Colicelli J, Lehrach H | title = HIP-I: a huntingtin interacting protein isolated by the yeast two-hybrid system | journal = Human Molecular Genetics | volume = 6 | issue = 3 | pages = 487–95 | date = Mar 1997 | pmid = 9147654 | doi = 10.1093/hmg/6.3.487 | doi-access = free }}

Huntingtin has also been shown to interact with:

{{div col|colwidth=20em}}

• HIP2,{{cite journal | vauthors = Kalchman MA, Graham RK, Xia G, Koide HB, Hodgson JG, Graham KC, Goldberg YP, Gietz RD, Pickart CM, Hayden MR | title = Huntingtin is ubiquitinated and interacts with a specific ubiquitin-conjugating enzyme | journal = The Journal of Biological Chemistry | volume = 271 | issue = 32 | pages = 19385–94 | date = Aug 1996 | pmid = 8702625 | doi = 10.1074/jbc.271.32.19385 | doi-access = free }}
• MAP3K10,{{cite journal | vauthors = Liu YF, Dorow D, Marshall J | title = Activation of MLK2-mediated signaling cascades by polyglutamine-expanded huntingtin | journal = The Journal of Biological Chemistry | volume = 275 | issue = 25 | pages = 19035–40 | date = Jun 2000 | pmid = 10801775 | doi = 10.1074/jbc.C000180200 | doi-access = free }}
• OPTN,{{cite journal | vauthors = Hattula K, Peränen J | title = FIP-2, a coiled-coil protein, links Huntingtin to Rab8 and modulates cellular morphogenesis | journal = Current Biology | volume = 10 | issue = 24 | pages = 1603–6 | year = 2000 | pmid = 11137014 | doi = 10.1016/S0960-9822(00)00864-2 | s2cid = 12836037 | doi-access = free | bibcode = 2000CBio...10.1603H }}
• PRPF40A,
• SETD2,{{cite journal | vauthors = Faber PW, Barnes GT, Srinidhi J, Chen J, Gusella JF, MacDonald ME | title = Huntingtin interacts with a family of WW domain proteins | journal = Human Molecular Genetics | volume = 7 | issue = 9 | pages = 1463–74 | date = Sep 1998 | pmid = 9700202 | doi = 10.1093/hmg/7.9.1463 | doi-access = free }}
• TRIP10,{{cite journal | vauthors = Holbert S, Dedeoglu A, Humbert S, Saudou F, Ferrante RJ, Néri C | title = Cdc42-interacting protein 4 binds to huntingtin: neuropathologic and biological evidence for a role in Huntington's disease | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 5 | pages = 2712–7 | date = Mar 2003 | pmid = 12604778 | pmc = 151406 | doi = 10.1073/pnas.0437967100 | bibcode = 2003PNAS..100.2712H | doi-access = free }}
• ZDHHC17.{{cite journal | vauthors = Singaraja RR, Hadano S, Metzler M, Givan S, Wellington CL, Warby S, Yanai A, Gutekunst CA, Leavitt BR, Yi H, Fichter K, Gan L, McCutcheon K, Chopra V, Michel J, Hersch SM, Ikeda JE, Hayden MR | title = HIP14, a novel ankyrin domain-containing protein, links huntingtin to intracellular trafficking and endocytosis | journal = Human Molecular Genetics | volume = 11 | issue = 23 | pages = 2815–28 | date = Nov 2002 | pmid = 12393793 | doi = 10.1093/hmg/11.23.2815 | doi-access = free }}

{{Div col end}}

Huntingtin is a scaffolding protein in the ATM oxidative DNA damage response complex. Mutant huntingtin (mHtt) plays a key role in mitochondrial dysfunction involving the inhibition of mitochondrial electron transport, inhibition of mitochondrial import processes, higher levels of reactive oxygen species and increased oxidative stress.{{cite journal |vauthors=Liu Z, Zhou T, Ziegler AC, Dimitrion P, Zuo L |title=Oxidative Stress in Neurodegenerative Diseases: From Molecular Mechanisms to Clinical Applications |journal=Oxid Med Cell Longev |volume=2017 |pages=2525967 |date=2017 |pmid=28785371 |pmc=5529664 |doi=10.1155/2017/2525967 |doi-access=free }}{{cite journal |last1=Maiuri |first1=Tamara |last2=Mocle |first2=Andrew J. |last3=Hung |first3=Claudia L. |last4=Xia |first4=Jianrun |last5=van Roon-Mom |first5=Willeke M. C. |last6=Truant |first6=Ray |title=Huntingtin is a scaffolding protein in the ATM oxidative DNA damage response complex |journal=Human Molecular Genetics |date=25 December 2016 |volume=26 |issue=2 |pages=395–406 |doi=10.1093/hmg/ddw395 |pmid=28017939 |doi-access=free }} The promotion of oxidative damage to DNA may contribute to Huntington's disease pathology.{{cite journal |vauthors=Ayala-Peña S |title=Role of oxidative DNA damage in mitochondrial dysfunction and Huntington's disease pathogenesis |journal=Free Radic. Biol. Med. |volume=62 |pages=102–10 |date=September 2013 |pmid=23602907 |pmc=3722255 |doi=10.1016/j.freeradbiomed.2013.04.017 }}

{{main|Huntington's disease}}

Huntington's disease (HD) is caused by a mutated form of the huntingtin gene, where excessive (more than 36) CAG repeats result in formation of an unstable protein.{{cite journal | vauthors = Walker FO | title = Huntington's disease | journal = Lancet | volume = 369 | issue = 9557 | pages = 218–28 | date = Jan 2007 | pmid = 17240289 | doi = 10.1016/S0140-6736(07)60111-1 | s2cid = 46151626 }} These expanded repeats lead to production of a huntingtin protein that contains an abnormally long polyglutamine tract at the N-terminus. This makes it part of a class of neurodegenerative disorders known as trinucleotide repeat disorders or polyglutamine disorders. The key sequence which is found in Huntington's disease is a trinucleotide repeat expansion of glutamine residues beginning at the 18th amino acid. In unaffected individuals, this contains between 9 and 35 glutamine residues with no adverse effects. However, 36 or more residues produce an erroneous mutant form of Htt, (mHtt). Reduced penetrance is found in counts 36–39.{{cite journal | vauthors = Chong SS, Almqvist E, Telenius H, LaTray L, Nichol K, Bourdelat-Parks B, Goldberg YP, Haddad BR, Richards F, Sillence D, Greenberg CR, Ives E, Van den Engh G, Hughes MR, Hayden MR | title = Contribution of DNA sequence and CAG size to mutation frequencies of intermediate alleles for Huntington disease: evidence from single sperm analyses | journal = Human Molecular Genetics | volume = 6 | issue = 2 | pages = 301–9 | date = Feb 1997 | pmid = 9063751 | doi = 10.1093/hmg/6.2.301 | doi-access = free }}

N-terminal fragments of mHtt have been discovered in Huntington's disease patients. These fragments can be generated by protease enzymes that cut this elongated protein into fragments. Moreover, recent research has identified aberrant splicing to affect the mutant gene products, yielding fragments that coincide with the first exon of the protein.{{cite journal | vauthors = Sathasivam K, Neueder A, Gipson TA, Landles C, Benjamin AC, Bondulich MK, Smith DL, Faull RL, Roos RA, Howland D, Detloff PJ, Housman DE, Bates GP | title = Aberrant splicing of HTT generates the pathogenic exon 1 protein in Huntington disease | journal = Proc Natl Acad Sci U S A | volume = 110 | issue = 6 | pages = 2366-70 | date = Feb 2013 | pmid = 23341618 | doi = 10.1073/pnas.1221891110 | doi-access = free | hdl = 1721.1/79814 | hdl-access = free }} These protein fragments are observed to form abnormal clumps, known as neuronal intranuclear inclusions (NIIs), inside nerve cells, and may attract other, normal proteins into the clumps. The characteristic presence of these clumps in patients was thought to contribute to the development of Huntington disease.{{cite journal | vauthors = Davies SW, Turmaine M, Cozens BA, DiFiglia M, Sharp AH, Ross CA, Scherzinger E, Wanker EE, Mangiarini L, Bates GP | title = Formation of neuronal intranuclear inclusions underlies the neurological dysfunction in mice transgenic for the HD mutation | journal = Cell | volume = 90 | issue = 3 | pages = 537–48 | date = Aug 1997 | pmid = 9267033 | doi = 10.1016/S0092-8674(00)80513-9 | s2cid = 549691 | doi-access = free }} However, later research raised questions about the role of the inclusions (clumps) by showing the presence of visible NIIs extended the life of neurons and acted to reduce intracellular mutant huntingtin in neighboring neurons.{{cite journal | vauthors = Arrasate M, Mitra S, Schweitzer ES, Segal MR, Finkbeiner S | title = Inclusion body formation reduces levels of mutant huntingtin and the risk of neuronal death | journal = Nature | volume = 431 | issue = 7010 | pages = 805–10 | date = Oct 2004 | pmid = 15483602 | doi = 10.1038/nature02998 | bibcode = 2004Natur.431..805A | doi-access = free }} One confounding factor is that different types of aggregates are now recognised to be formed by the mutant protein, including protein deposits that are too small to be recognised as visible deposits in the above-mentioned studies.{{cite journal | vauthors = Sahl SJ, Lau L, Vonk WI, Weiss LE, Frydman J, Moerner WE | title = Delayed Emergence of Subdiffraction-Sized Mutant Huntingtin Fibrils Following Inclusion Body Formation | journal = Q Rev Biophys | volume = 49 | pages = e2 | date = 2016 | pmid = 26350150 | doi = 10.1017/S0033583515000219 | doi-access = free | pmc = 4785097 }} The likelihood of neuronal death remains difficult to predict. Likely multiple factors are important, including: (1) the length of CAG repeats in the huntingtin gene and (2) the neuron's exposure to diffuse intracellular mutant huntingtin protein. NIIs (protein clumping) can be helpful as a coping mechanism—and not simply a pathogenic mechanism—to stem neuronal death by decreasing the amount of diffuse huntingtin.{{cite journal | vauthors = Orr HT | title = Neurodegenerative disease: neuron protection agency | journal = Nature | volume = 431 | issue = 7010 | pages = 747–8 | date = Oct 2004 | pmid = 15483586 | doi = 10.1038/431747a | bibcode = 2004Natur.431..747O | s2cid = 285829 }} This process is particularly likely to occur in the striatum (a part of the brain that coordinates movement) primarily, and the frontal cortex (a part of the brain that controls thinking and emotions). Further, it is possible the pathogenic mechanism lay more with the RNA transcripts and their potential CAG repeats to exhibit RNAi than with the actual huntingtin protein itself.{{cite journal | url=https://www.nature.com/articles/s41419-022-05494-1 | doi=10.1038/s41419-022-05494-1 | title=The length of uninterrupted CAG repeats in stem regions of repeat disease associated hairpins determines the amount of short CAG oligonucleotides that are toxic to cells through RNA interference | date=2022 | last1=Murmann | first1=Andrea E. | last2=Patel | first2=Monal | last3=Jeong | first3=Si-Yeon | last4=Bartom | first4=Elizabeth T. | last5=Jennifer Morton | first5=A. | last6=Peter | first6=Marcus E. | journal=Cell Death & Disease | volume=13 | issue=12 | pmid=36585400 | pmc=9803637 }}

People with 36 to 40 CAG repeats may or may not develop the signs and symptoms of Huntington disease, while people with more than 40 repeats will develop the disorder during a normal lifetime. When there are more than 60 CAG repeats, the person develops a severe form of HD known as juvenile HD. Therefore, the number of CAG (the sequence coding for the amino acid glutamine) repeats influences the age of onset of the disease. No case of HD has been diagnosed with a count less than 36.

As the altered gene is passed from one generation to the next, the size of the CAG repeat expansion can change; it often increases in size, especially when it is inherited from the father. People with 28 to 35 CAG repeats have not been reported to develop the disorder, but their children are at risk of having the disease if the repeat expansion increases.

In the pathogenesis of the disease, there is further somatic expansion of CAG repeats. It takes decades to reach 80 repeats, then years to reach 150 repeats. Beyond 150, cellular toxicity start to manifest. Over months, the neuron slowly loses its cell identity until cell death pathways are activated.{{cite journal |last1=Handsaker |first1=Robert E. |last2=Kashin |first2=Seva |last3=Reed |first3=Nora M. |last4=Tan |first4=Steven |last5=Lee |first5=Won-Seok |last6=McDonald |first6=Tara M. |last7=Morris |first7=Kiely |last8=Kamitaki |first8=Nolan |last9=Mullally |first9=Christopher D. |last10=Morakabati |first10=Neda R. |last11=Goldman |first11=Melissa |last12=Lind |first12=Gabriel |last13=Kohli |first13=Rhea |last14=Lawton |first14=Elisabeth |last15=Hogan |first15=Marina |last16=Ichihara |first16=Kiku |last17=Berretta |first17=Sabina |last18=McCarroll |first18=Steven A. |title=Long somatic DNA-repeat expansion drives neurodegeneration in Huntington's disease |journal=Cell |date=February 2025 |volume=188 |issue=3 |pages=623–639.e19 |doi=10.1016/j.cell.2024.11.038|doi-access=free |pmid=39824182 |pmc=11822645 |pmc-embargo-date=February 6, 2026 }}

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{{Reflist|35em}}

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• {{cite journal | vauthors = Bates G | title = Huntingtin aggregation and toxicity in Huntington's disease | journal = Lancet | volume = 361 | issue = 9369 | pages = 1642–4 | date = May 2003 | pmid = 12747895 | doi = 10.1016/S0140-6736(03)13304-1 | s2cid = 7587406 }}
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• {{cite journal | vauthors = Rangone H, Humbert S, Saudou F | title = Huntington's disease: how does huntingtin, an anti-apoptotic protein, become toxic? | journal = Pathologie-Biologie | volume = 52 | issue = 6 | pages = 338–42 | date = Jul 2004 | pmid = 15261377 | doi = 10.1016/j.patbio.2003.06.004 }}
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{{refend}}

• {{MeshName|Huntingtin+protein,+human}}
• [https://web.archive.org/web/20090327130709/http://www.stanford.edu/group/hopes/causes/huntprot/p1.html The Huntingtin Protein and Protein Aggregation] at [https://hopes.stanford.edu/ HOPES] {{Webarchive|url=https://web.archive.org/web/20210212014602/https://hopes.stanford.edu/ |date=2021-02-12 }} : Huntington's Outreach Project for Education at Stanford
• [https://www.hda.org.uk/ The HDA] Huntington's Disease Association UK
• {{OMIM|143100}}
• {{EntrezGene|3064}}
• [https://web.archive.org/web/19980211231514/http://bioinformatics.weizmann.ac.il/cards-bin/carddisp?HD GeneCard]
• [https://web.archive.org/web/20120205103304/http://www.ihop-net.org/UniPub/iHOP/bng/88980.html iHOP]

Category:Human proteins

Category:Huntington's disease

Category:Genes on human chromosome 4