HTT EXON1

The huntingtin (HTT) protein plays critical roles in neuronal health and function throughout the adult brain, with its loss contributing to decreased neuronal excitability and degeneration [5]. Huntington's disease arises when CAG trinucleotide repeats in exon 1 of the HTT gene expand beyond 36 repeats, encoding abnormally long polyglutamine (polyQ) tracts that cause the protein to misfold and form toxic aggregates [1][2]. This structural prediction examined the Q23_exon1 variant containing 23 glutamine repeats, which falls within the normal range and does not cause disease, providing important baseline information about the native protein structure. The AlphaFold2 prediction achieved an average confidence score (pLDDT) of 79.6, indicating generally reliable structural predictions for most regions of this exon 1 segment. This confidence level suggests the model can capture key structural features of the normal huntingtin exon 1, though some regions may show lower confidence typical of intrinsically disordered segments. The polyglutamine tract itself is expected to adopt flexible conformations in the normal-length variant, contrasting with pathological expansions where extended polyQ tracts drive misfolding and aggregation [1][2]. Recent research has revealed that the length of CAG repeats in HTT exon 1 profoundly affects cellular function, with extensive transcriptomic changes observed in both cellular and animal models as repeat length increases [1]. In pathological contexts with expanded repeats, the mutant HTT protein forms inclusion bodies whose role remains debated—recent evidence suggests these aggregates may actually be protective rather than toxic, with ATF3-dependent inclusion body formation conferring cellular protection in human iPSC-derived neurons [2]. The mutant HTT transcript also contributes to disease through RNA-mediated toxicity by aberrantly recruiting RNA-binding proteins like MID1 [3]. Clinical implications extend beyond the well-established CAG repeat length, as loss of interrupting codons in the CAG and CCG repeats (CAG-CCG LOI variants) can lead to underestimation of repeat length by current diagnostic assays, affecting disease prediction and clinical trial interpretation [6]. The normal Q23 variant analyzed here provides structural reference data for understanding how pathological expansions disrupt protein structure and function. Striatal spiny projection neurons show particular vulnerability to mutant huntingtin toxicity, experiencing mitochondrial dysfunction, disrupted intracellular transport, and eventual degeneration [4][5], with DNA methylation changes in the striatum associated with disease progression [7].