Future Directions for Studying the Potential of Mammalian Dscam in Autism Spectrum Disorder and Alzheimer's Disease: Insights from Dose Sensitivity.

Down syndrome cell adhesion molecule (Dscam), a macromolecular member of the immunoglobulin (Ig) superfamily, is widely distributed in the nervous system. Over the past few decades, significant progress has been made in studies of the Dscam gene and its protein products across multiple species, advancing our understanding of its alternative splicing mechanisms, isoform-specific homophilic binding properties, and crucial neurological functions during neural development. In Drosophila, the Dscam gene undergoes extensive alternative splicing, generating thousands of isoforms that differ in their extracellular and/or transmembrane domains. These isoforms confer unique cellular identities and mediate cell-cell recognition and downstream signaling cascades primarily via homophilic interactions. In contrast, mammalian Dscam lacks the extreme alternative splicing and vast isoform diversity found in Drosophila, yet it retains crucial neurological functions. Studies indicate that the expression levels serve as an important regulator of Dscam-dependent neural processes, underlying its dosage-sensitive phenotypes. Abnormal Dscam expression has been implicated in the pathology of several neurological diseases. For example, heterozygous loss of Dscam function is convincingly associated with autism spectrum disorder (ASD), while trisomy of the Dscam gene is linked to Down syndrome (DS). Recent studies also suggest a connection between Dscam overexpression and Alzheimer's disease (AD), implicating Dscam in previously unrecognized neurodegenerative mechanisms. However, efforts to clarify Dscam's role in the neuropathology of diseases are severely hampered by the etiological and phenotypic heterogeneity of these diseases, necessitating novel approaches. This review integrates cross-species evidence on Dscam's dose sensitivity to elucidate the molecular mechanisms behind its dosage-dependent phenotypes in mammals, thereby advancing the understanding of how dysregulated Dscam expression contributes to phenotypic heterogeneity in ASD and disease onset in AD. Insights into Dscam's dose sensitivity highlight that alterations in dosage likely perturb genetic regulatory networks, leading to nonlinear phenotypic consequences through multi-level molecular interactions. Therefore, we propose a biphasic framework to address current mechanistic challenges in future research: (1) systematic identification of key regulatory nodes within genetic networks using emerging big-data methodologies, followed by (2) mechanistic validation through targeted experimental studies of prioritized molecular pathways. These efforts may establish Dscam as a promising therapeutic target for modulating pathological cascades in both ASD and AD.