40-kDa huntingtin-associated protein (HAP40)

The 40-kDa huntingtin-associated protein (HAP40, encoded by F8A1) is a conserved human protein entirely located within intron 22 of the factor VIII gene, but transcribed in the opposite direction[2][4]. The gene region containing F8A1 is duplicated in two other locations on the X chromosome (as F8A2 and F8A3)[4]. HAP40 acts as a RAB5A effector involved in early endosome trafficking, regulating the recruitment of huntingtin protein onto endosomes and modulating interactions with the cytoskeleton (increased actin interactions, reduced endosome motility)[2][3]. Its expression is abundant in many tissues, and its evolutionary conservation suggests a biologically important role, although its precise physiological or pathological functions are still under investigation[2][4]. Alterations in HAP40 protein levels are observed in Huntington’s Disease, suggesting a potential link between its function and neurodegenerative processes, but it is not a direct target for current therapeutic drugs[3]. The proximity of F8A1 to factor VIII is relevant in genetic studies, as recombination or inversion events involving F8A1/F8A2/F8A3 may disrupt factor VIII and cause hemophilia A (but F8A1 itself is not a coagulation factor nor involved in blood clotting)[6].\n\nSummary of Most Important Points:\n- HAP40 (F8A1) is a vesicular trafficking effector, not a receptor, enzyme, transporter, or direct therapeutic target[2][3][4].\n- It is closely associated biologically with the huntingtin protein, and changes in its levels are relevant in Huntington's Disease[3].\n- It is *not* involved in coagulation itself; rather, its genomic location in the factor VIII gene is key for understanding inversion events causing hemophilia[2][6].\n- No known drugs, biomarkers, safety concerns, or therapeutic targeting mechanisms involve HAP40 to date.

Other names

Coagulation factor VIII associated 1F8A1HAP40DXS522EFactor VIII intron 22 proteinCpG island proteinFactor VIII-associated (intronic transcript) 1Factor VIII associated proteinHuntingtin-associated protein 40