Nicolás Rosas¹, Ivanna Ihnatovych¹, Kinga Szigeti¹

Human‑specific genes contribute to the unique structural and functional features of the human brain. CHRFAM7A is one such gene, created by the fusion of the α7 nicotinic acetylcholine receptor (α7nAChR) and ULK4, a member of the ULK kinase family. CHRFAM7A has been associated with neuropsychiatric diseases.

Three CHRFAM7A alleles exist in the human population: the ancestral allele (0‑copy), allele harboring the direct‑orientation fusion gene (CHRFAM7A), and the allele with inverted fusion gene carrying a 2‑bp deletion in exon 6 (CHRFAM7AΔ2bp). Because animal models lack CHRFAM7A and its variants, studying its role in brain development and disease has been challenging. We developed a human isogenic iPSC model to elucidate the function of CHRFAM7A. We have previously shown that the direct allele modulates α7nAChR activity.

Here, we use iPSC‑derived medial ganglionic eminence (MGE) neuronal progenitors and primary human monocytes to investigate how the inverted CHRFAM7AΔ2bp allele influence neuronal development and decipher the underlying genetic mechanism. Through quantitative live-cell imaging, we assessed cytoskeletal debundling, growth cone dynamics, cell motility, and neurite arborization to characterize cellular processes affected by this human-specific variant.

We found that the inverted CHRFAM7A allele modulates ULK4 expression through a distinct genetic epistasis, driven by the emergence of an alternatively spliced ULK4 exon 12-18sh variant. The splice variant shifts ULK4 isoform balance toward the long isoform, leading to a microtubule cytoskeleton gain‑of‑function that enhances neuronal complexity and cell motility, ultimately strengthening functional connectivity in the human brain.

¹Department of Neurology, State University of New York at Buffalo, 875 Ellicott St., Buffalo, NY, 14203, USA

This work is supported in part by the Community Foundation for Greater Buffalo (Kinga Szigeti).