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HUNTINGTIN-ASSOCIATED PROTEIN 1; HAP1

HUNTINGTIN-ASSOCIATED PROTEIN 1; HAP1

Alternative titles; symbolsHAP2NEUROAN 1HGNC Approved Gene Symbol: HAP1Cytogenetic location: 17q21.2 Genomic coordinates (GRCh38): 17:41,717,738-41,734,645 (...

Alternative titles; symbols

  • HAP2
  • NEUROAN 1

HGNC Approved Gene Symbol: HAP1

Cytogenetic location: 17q21.2 Genomic coordinates (GRCh38): 17:41,717,738-41,734,645 (from NCBI)

▼ Cloning and Expression
Several neurodegenerative disorders are known to involve trinucleotide expansions of the CAG codon for glutamine, including Huntington disease (HD; 143100), spinocerebellar ataxia type 1 (164400), Machado-Joseph disease (109150), and spinobulbar muscular atrophy (313700.0014). Li et al. (1995) identified a cDNA for a rat brain protein that binds to the mutant HD protein, huntingtin (HTT; 613004), by using the latter as bait in a 2-hybrid yeast screening assay. The protein, which they designated HAP1 (huntingtin-associated protein-1), was found to bind to huntingtin in proportion to the number of glutamines present in the glutamine repeat region. Two HAP1 cDNAs were found that differ at the C terminus, probably as a result of alternative splicing. The predicted 599- and 629-amino acid proteins are highly hydrophilic, rich in charged amino acids, and have no homology to known proteins. Two human PCR products, termed HAP1 and HLP (HAP-like protein), were obtained. The human HAP1 cDNA encoded a protein 96% identical to the rat HAP1 protein. Northern blots showed a 4.0-kb HAP1 transcript in rat brains and RT-PCR demonstrated expression in human brain, especially in the caudate and cortex, regions affected in Huntington disease. Coimmunoprecipitation experiments with rat brain tissue and HAP1-transfected cells confirmed that HAP1 binds to huntingtin in vivo, though they had not yet clearly shown the same in human brain tissue. Li et al. (1995) speculated that the ability of HAP1 to bind to glutamine repeats in huntingtin is influenced by adjacent amino acids, since in their yeast 2-hybrid assays there was no binding of HAP1 to atrophin-1, even though their atrophin-1 construct contained essentially the same number of glutamine repeats (21) as did their huntingtin construct (23).

Li et al. (1998) stated that genomic Southern blot analysis suggested the existence of separate genes for the previously identified human HAP1 and HLP1; however, Northern blot analysis detected the expression of human HLP1 only. Whether the HAP1 cDNA was expressed at a very low level or was the result of PCR artifact remained to be investigated. Li et al. (1998) obtained HLP1 cDNAs and renamed them as human HAP based on their homology to rat HAP1, their expression on Northern blots, and their binding to huntingtin. Human HAP encodes a 619-amino acid polypeptide that is 62% identical to rat HAP1 over its entire sequence and 82% identical in the putative huntingtin-binding region. Only 1 human isoform was detected. Northern blot analysis showed that HAP is expressed as a 4.1-kb transcript in human brain, with other minor transcripts detected in some brain regions. Western blot analysis detected HAP expression as a 75-kD protein whose expression was significantly decreased in parallel with decreased expression of huntingtin. In vitro studies showed that HAP interacts with the N-terminal region of huntingtin and that its interaction is greater when huntingtin carries an expanded CAG repeat. In transfected HEK293 cells, huntingtin and HAP colocalized in perinuclear inclusion bodies.

Nasir et al. (2000) determined that the human HAP1 protein, which is 62% identical to the mouse sequence, includes a leucine zipper motif and a putative tyrosine kinase phosphorylation site.

▼ Gene Function
Engelender et al. (1997) showed that HAP1 not only binds to huntingtin in a glutamine-repeat length-dependent manner, but also reacts with cytoskeleton proteins, namely the p150(Glued) subunit of dynactin (DCTN1; 601143) and the pericentriolar protein PCM1 (600299). Various observations suggested that HAP1 may function as an adaptor protein using coiled coils to mediate interactions among cytoskeletal, vascular, and motor proteins. Thus, HAP1 and huntingtin may play a role in vesicle trafficking within the cell, and disruption of this function could contribute to the neuronal dysfunction and death seen in HD.

Tang et al. (2003) used protein-binding experiments to identify a protein complex containing huntingtin (Htt), HAP1A, and the type 1 inositol 1,4,5-triphosphate (IP3) receptor (ITPR1; 147265) in neurons from rat brain. Functional experiments showed that HAP1A promoted Htt association with the C terminus of ITPR1. Both wildtype and Htt with expanded polyglutamine repeats bound to ITPR1, but only the expanded Htt caused increased sensitization of the ITPR1 receptor to activation by IP3. Expression of the expanded Htt protein in medium spiny striatal neurons, those affected in HD, resulted in an increase in intracellular calcium levels which may be toxic to neurons.

Gauthier et al. (2004) showed that huntingtin specifically enhances vesicular transport of brain-derived neurotrophic factor (BDNF; 113505) along microtubules. They determined that huntingtin-mediated transport involves HAP1 and the p150(Glued) subunit of dynactin, an essential component of molecular motors. BDNF transport was attenuated both in the disease context and by reducing the levels of wildtype huntingtin. The alteration of the huntingtin/HAP1/p150(Glued) complex correlated with reduced association of motor proteins with microtubules. The polyglutamine-huntingtin-induced transport deficit resulted in the loss of neurotrophic support and neuronal toxicity. Gauthier et al. (2004) concluded that a key role of huntingtin is to promote BDNF transport and suggested that loss of this function might contribute to pathogenesis.

By yeast 2-hybrid analysis of a rat hippocampus library, Kittler et al. (2004) found that rat Hap1 bound the intracellular domain of the GABA-A receptor beta subunits (e.g. GABRB1; 137190), but not the intracellular domain of GABA-A receptor alpha-1 (GABRA1; 137160), gamma-2 (GABRG2; 137164), or delta (GABRD; 137163) subunits or the intracellular C-terminal tail of the GABA-B receptor R1 subunit (GABBR1; 603540). Hap1 increased the number of GABA-A receptors at the cell surface of rat cortical neurons by facilitating internalized receptor recycling and reducing lysosomal degradation. Since GABA-A receptors are the major sites of fast synaptic inhibition in the brain, Kittler et al. (2004) concluded that the role of HAP1 in GABA-A receptor recycling contributes to the maintenance of inhibitory synapses.

Using immunoprecipitation analysis, Sheng et al. (2008) found that Ahi1 (608894) bound tightly to Hap1 in mouse brain lysates. Depletion of either protein reduced the amount of the other, and conversely, overexpression of either protein increased the endogenous level of the other, suggesting Ahi1 and Hap1 stabilize each other. Reduction of either Hap1 or Ahi1 also reduced the level of Trkb (NTRK2; 600456) and Trkb signaling, as indicated by reduced phosphorylation of Erk (see 601795) and Akt (see 164730). Sheng et al. (2008) concluded that HAP1 and AHI1 maintain the level and signaling of TRKB in neurons.

Using yeast 2-hybrid and immunoprecipitation analyses, Shimojo (2008) showed that human RILP (PRICKLE1; 608500) and huntingtin interacted directly with dynactin-1 to form a triplex. REST bound to the triplex through direct interaction with RILP, forming a quaternary complex involved in nuclear translocation of REST in non-neuronal cells. In neuronal cells, the complex also contained HAP1, which affected interaction of disease-causing mutant huntingtin, but not wildtype huntingtin, with dynactin-1 and RILP. Overexpression and knockout analyses demonstrated that the presence of HAP1 in the complex prevented nuclear translocation of REST and thereby regulated REST activity.

▼ Gene Structure
By genomic sequence analysis of a PAC library, Nasir et al. (2000) determined that the HAP1 gene spans approximately 12.1 kb and contains 11 exons.

▼ Mapping
Nasir et al. (1998) mapped the mouse homolog, Hap1, to chromosome 11 (band D), which shares extensive homology of synteny with human chromosome 17, including the region 17q21-q22, where frontotemporal dementia (600274) and other progressive degenerative disorders resulting from mutations in the MAPT gene (157140) map. Using FISH, Nasir et al. (2000) confirmed the localization of the human HAP1 gene to 17q21.2-q21.3.

▼ Molecular Genetics
By analysis of CEPH samples, Nasir et al. (2000) identified a polymorphism within intron 6 of the HAP1 gene with a heterozygosity of 70% and 10 different allele lengths in 20 alleles tested.

Among 889 patients with Huntington disease (HD; 143100), Metzger et al. (2008) found a significant association between age at onset and a thr441-to-met (T441M) substitution in the HAP1 gene (rs4523977). In HD patients with less than 60 CAG repeats, those who were homozygous for the met/met allele developed symptoms about 8 years later than HD patients with the thr/met or thr/thr genotypes (p = 0.015). In vitro studies showed that met441 bound mutated HTT more tightly than thr441, stabilized HTT aggregates, reduced the number of soluble HTT degraded products, and protected neurons against HTT-mediated toxicity. Metzger et al. (2008) concluded that the T441M SNP can modify the age at onset in adult patients with HD. They estimated that the T441M SNP may represent 2.5% of the variance in age at onset that cannot be accounted for by expanded CAG repeats in the HTT gene.

▼ Animal Model
Bertaux et al. (1998) cloned mouse Hap1 cDNA and demonstrated that expression is not enriched in areas specifically affected in Huntington disease. Bertaux et al. (1998) used the yeast 2-hybrid system to demonstrate that amino acids 171-230 of the huntingtin-associated protein are necessary for hap1-huntingtin binding and that Hap1 does not interact with the transgene exon 1 protein in a transgenic model of HD.

Chan et al. (2002) generated mice with homozygous disruption at the Hap1 locus. Loss of Hap1 expression did not alter the gross brain expression levels of its interacting partners, huntingtin and p150(Glued). Newborn Hap1 -/- animals were observed at the expected Mendelian frequency, suggesting a nonessential role of Hap1 during embryogenesis. Postnatally, Hap1 -/- pups showed decreased feeding behavior that ultimately led to malnutrition, dehydration, and premature death by postnatal day 9. Since Hap1 is particularly enriched in the hypothalamus, the authors suggested that Hap1 may play an essential role in regulating postnatal feeding.

Dragatsis et al. (2004) showed that Hap1-null mutants displayed suckling defects and starved within the first days after birth. Upon reduction of the litter size, some mutants survived into adulthood and displayed growth retardation with no apparent brain or behavioral abnormalities, suggesting that Hap1 function is essential only for early postnatal feeding behavior. Early lethality was rescued when Hap1 expression was restored in neuronal cells before birth. No synergism was observed between Hap1 and huntingtin mutation during mouse development. Dragatsis et al. (2004) concluded that Hap1 may have a fundamental role in regulating postnatal feeding in the first 2 weeks after birth, but a nonessential role in the adult mouse.

Zucker et al. (2005) compared mRNA levels of selected genes involved in NMDA receptor and Ca(2+) signaling pathways in medium spiny projection neurons (MSN) and neuronal NOS (NOS1; 163731)-positive interneurons (nNOS-IN) from 12-week-old R6/2 mice, a transgenic mouse model of HD, and wildtype littermates. There were different transcriptional alterations in R6/2 neurons for both MSN and nNOS-IN, indicating that global transcriptional dysregulation alone may not account for selective vulnerability. In the nNOS-IN population, several mRNAs were enriched, including Grin2d (602717), Psd95 (DLG4; 602887), and Hap1, as well as Nos1 mRNA itself. Zucker et al. (2005) suggested that these proteins may be involved in protective pathways that contribute to the resistance of this interneuron population to neurodegeneration in HD.

Using immunocytochemistry and Western blots, Sheng et al. (2006) demonstrated that Hap1 expression in rodent hypothalamus was upregulated by fasting and downregulated by intracerebroventricular administration of insulin due to increased degradation through ubiquitination. Decreasing the expression of mouse hypothalamic Hap1 by siRNA reduced the level and activity of hypothalamic GABA(A) receptors (see 137160) and caused a decrease in food intake and body weight. Sheng et al. (2006) noted that these findings link hypothalamic HAP1 to GABA in the stimulation of feeding and suggested that this mechanism is involved in the feeding-inhibitory actions of insulin in the brain.

Tags: 17q21.2