## 4 Discussion
By intersecting the top features across these three models, we distilled a consensus list of six genes, thereby reducing algorithm-specific selection biases and focusing on features with broad predictive relevance. Next, we reinforced reproducibility by re-applying meta-analysis with a stringent fold-change cut-off of 1.0, yielding 13 DEGs. The intersection of these 13 genes with our six consensus ML-selected genes produced a core set of five candidates. This set was subjected to two external validation cohorts by building a nomogram and ROC curve analysis. An inverse-variance-weighted meta-analysis across both cohorts further confirmed the robustness of the findings, with the final two biomarkers, _GABRD_ and _PHACTR1_, achieving AUCs of 0.958 and 0.930, underscoring their high discriminative power and generalizability. Such performance in an external cohort underscores the robustness of our integrative approach and parallels outcomes seen in other ML biomarker studies with rigorous external testing. The GABAergic 8 subunit, _GABRD_, was consistently given priority by the ML models, which aligns with the recognized role of GABAergic dysfunction in the development of HD. A previous study identified that _GABRD_ is abundantly expressed in the cerebellum, cortex, and hippocampus, localized in the cytosol and membranes of the neurons [56]. [PERSON] et al. identified a differential expression of _GABRD_ is associated with impairment of memory and learning induced by a low dose of cadmium at different stages of the mouse offspring [57]. In different HD mouse models, differential regulation of _GABRD_ is associated with disease pathogenicity, such as, using YAC128 transgenic mice, [PERSON] et al. identified upregulation of _GABRD_ mRNA at the onset of the disease, particularly at the age of 6-12 months, indicating a dynamic response during the disease course. On the other hand, [PERSON] et al. identified that HD severity is associated with loss of _GABRD_ in R6/1 mice at 6 months, but the expression was remained unchanged at the presymptomatic stage (3 months), and the loss of _GABRD_ expression was associated with emerging motor and cognitive deficits [58]. In addition, [PERSON] et al. observed that striatal neurons from _GABRD\({}^{-/-}\)_ mice, deficient in 8-mediated tonic GABA currents, had decreased survival after an in vitro excitotoxic challenge with quinolinic acid, indicating a neuroprotective function against excitotoxic damage in the adult striatum [59]. More importantly, in experimental settings, _GABRD_ is identified as a blood biomarker for Alzheimer's disease (AD) detection by untargeted proteomics profiling, where _GABRD_-positive neuronal extracellular vesicles represent synaptic dysfunction and neuronal death [56]. On the other hand, _PHACTR1_, which is implicated in dendritic arborization in the gene enrichment analysis, was repeatedly selected, highlighting novel facets of HD biology that warrant further mechanistic study. _PHACTR1_ encodes protein phosphatase and actin regulator 1 that binds actin and protein phosphatase 1 (PP1) and plays a critical role in the actin cytoskeleton and dynamics. _PHACTR1_ has been identified in the synaptosome, co-localizing with the synaptic marker synaptophysin, detectable in dendrites at DIV21, and strongly expressed in differentiated neurons within the adult mouse cerebral cortex [60; 61]. Dendritic spine formation and synaptic plasticity are processes known to be impaired in HD, and they are supported by actin dynamics in neurons. [PERSON] et al. showed that BDNF treatment downregulates _PHACTR1_ expression in cortical neurons by MEK the ERK/MAPK cascade [62]. Although there is a lack of direct mechanistic studies on _PHACTR1_ in HD, a recent investigation revealed that _PHACTR1_ is among a limited number of genes that are more prevalent in the striatum and whose dysregulation correlates with the emergence of somatic CAG repeats and neurodegeneration in HD murine models [63].