CHRFAM7A, a uniquely human fusion gene, was recently shown to lead to actin cytoskeleton gain of function leading to a reinforced neuronal matrix with precise connections and enlarged synaptic area due to a shift to lamellipodia dendritic spine. The actin phenotype affects all neuronal substructures, including the cell body, growth cone, and dendritic spine. The reinforced membrane in the presence of CHRFAM7A leads to post-developmental adaptation to changes in stiffness associated with biological and pathological processes (Szigeti et al., 2023). The mechanism suggests that CHRFAM7A infers a more efficient and resilient brain.

CHRFAM7A is a fusion gene between CHRNA7, the subunit of α7 nAChR, and ULK4 (Sinkus et al., 2015). Based on locus specific dual genotyping 75% of the human population is carrier of the direct, expressed allele (Szigeti et al., 2020). Gene expression localization in the human brain corresponds to areas where α7 nAChR is present (Hellström-Lindahl et al., 1998). α7 nAChR is abundant in the human association cortices and has been associated with cognition (Zoli et al., 2018; Wang et al., 2020; Borroni and Barrantes, 2021). CHRFAM7A has two alleles, the direct allele that has been shown to be translated, gets incorporated into the α7 nAChR as up to 3 subunits, and creates a CHRFAM7A/α7 nAChR heteropentamer that is hypomorphic (Benfante et al., 2011; de Lucas-Cerrillo et al., 2011; Wang et al., 2014; Ihnatovych et al., 2019, 2020; Leonard and Benfante, 2023). The inverted allele does not modify α7 nAChR function and its translation remains hypothetical. Data thus far indicate that the inverted allele is non-functional from the α7 nAChR perspective (Sinkus et al., 2015; Szigeti et al., 2020). The direct allele non-carriers are inverted, however the heterozygous individuals in the direct allele carrier group also harbor the inverted allele.

Significant improvements in magnetic resonance imaging (MRI) technology and post-processing techniques today enables reliable quantification of the brain volume and its change throughout the human lifespan. Large databases allow us to define brain volume trajectories that could serve as potent tool toward understanding the healthy aging process and the effect of neurological/psychiatric diseases (Hedman et al., 2012; Bethlehem et al., 2022). Only recently, brain charts that define normative sex-stratified and age-related changes increase the sensitivity in detecting genetic and environmental influences on the brain structure and allow quantification of effect sizes when compared to pathological state (Bethlehem et al., 2022). Moreover, MRI techniques such as functional MRI (fMRI) allow assessment of the functional connectivity, a measure describing the concurrent activity of two brain regions within a network during rest or while performing certain task. These networks constantly adapt throughout the aging process and some modulations such as increased neural efficacy may confer additional resilience to pathology (Dennis and Thompson, 2014; Arenaza-Urquijo and Vemuri, 2018).

One of the most commonly discussed general modifiers of life-long cognitive performance is the cognitive and/or brain reserve (Stern, 2009). The reserve was originally introduced in the field of Alzheimer’s Disease (AD) where patients with greater pre-morbid brain volume and higher educational attainment tent to withstand greater amount of pathology and ultimately have better long-term outcomes (Stern, 2012). The neurocognitive and structural relationship can differ based on multiple modifiable (education, lifestyle and leisure activity) and non-modifiable (genetic background, sex) factors (Levine et al., 2021). The impact of the genetic background on the structural and functional brain function has been previously investigated for well-known dementia risk factors such as APOE e4 and BDNF Val66Met polymorphisms (Wisdom et al., 2011; Brown et al., 2014). Investigating other genetic candidates that could potentially influence the neurocognitive-structural MRI correlation would further increase our ability to explain the individual variability throughout the lifespan.

We hypothesize that the translated CHRFAM7A allele leads to a neuronal gain of function in the human brain through activation of the actin cytoskeleton. We performed a neurocognitive-structural and resting state functional MRI correlation pilot study on 46 normal human subjects to explore the effect of CHRFAM7A on human brain function.

The study participants had a mean age of 51.8 (SD = 14.9), and 69% were females and 89% white. 71.7% of participants were CHRFAM7A direct allele carriers consistent with previously reported frequencies in Caucasians (Benfante et al., 2011; Sinkus et al., 2015; Szigeti et al., 2020; Leonard and Benfante, 2023). Demographic variables (age and years of education) were comparable between direct allele carriers and non-carriers (Table 1). Motor function measured by 25FTW and 9PHT was similar between CHFRAM7A direct allele carriers and non-carriers and depression scores (BDI-FS) were low for both groups (Table 1). Cognitive measures of processing speed (SDMT), verbal and visual learning and memory were consistently higher in the CHRFAM7A direct allele carrier-group, reaching statistical significance in visual immediate recall (27.5 vs. 21.1, p = 0.003, partial η = 0.202, FDR corrected p-value = 0.018) (Table 1, Figure 1). Structural MRI suggested larger whole brain volume in the non-carriers (1549.4 mL vs. 1503.1 mL, uncorrected p = 0.046, partial η = 0.094) in the context of similar lateral ventricular volume (38.9 mL vs. 37.0 mL, p = 0.708, partial η = 0.03) and T2-lesion volume (0.6 mL vs. 0.6 mL, p = 0.876, partial η = 0.001) (Table 2, Figure 1). The smaller WBV in CHRFAM7A direct allele carriers was a result of smaller white matter (742.5 mL vs. 764.4, uncorrected p = 0.077, partial η = 0.074), gray matter (760.6 mL vs. 785.0 mL, uncorrected p = 0.082 partial η = 0.072) and deep gray matter volumes (59.2 mL vs. 61.9 mL, p = 0.050 partial η = 0.09). The structural MRI analysis is likely underpowered in this exploratory study (Table 2).

Lastly, 37 (80%) out of the 46 participants had an available fMRI sequence as part of their protocol and their functional connectivity was analyzed. In particular, 28 direct allele careers and 9 direct allele non-carriers were included in this comparison (Table 3). In this exploratory fMRI analysis, the CHRFAM7A direct allele carriers had lower connectivity within the V2 network resting activity when compared to direct allele non-carriers (FDR corrected p-value = 0.027).

CHRFAM7A is one of the human restricted genes created by fusion between CHRNA7 and part of ULK4. Mechanistic insights from isogenic induced pluripotent stem cell (iPSC) models identified a direct allele mediated actin cytoskeleton gain of function. Consistent with emerging literature that functional effects of the human specific genes are most frequently observed in the brain, immune system and metabolism we demonstrated gain of function in the neuronal lineage and previously have shown immune gain of function in microglia (Ihnatovych et al., 2020; Szigeti et al., 2023). Human specific genes are thought to drive traits that make us human and typically are present in all humans while mutations or absence lead to developmental consequences or disease. CHRFAM7A thus far the only human specific gene that is not developmentally essential as 0.7% of the population without CHRFAM7A alleles are indistinguishable from carriers, suggesting that it may provide different context for disease when present. Furthermore, the locus is biallelic, harboring direct and inverted CHRFAM7A. suggesting that the contextual consideration divides the human population 1–3, or 25% non-carriers and 75% carriers of the direct, translated allele. Early data suggest that allele frequencies differ in racial and ethnic groups perhaps adding an additional layer of diversity for disease context.

Recently we reported that CHRFAM7A translated from the direct allele leads to an actin cytoskeleton gain of function in the neuronal lineage in the form of switching cell membrane structure from filopodia to lamellipodia, and as a result leads to reinforced synaptic structure, adaptation to tissue stiffness and axon guidance. These structural changes suggest a more efficient synapse and we proceeded to test the hypothesis in a neurocognitive-structural and resting state functional MRI correlation study. The study population is very similar to clinical trial populations in neuropsychiatric diseases consisting of mostly Caucasian participants and 2/3 being women. Motor and cognitive measures demonstrated a split as motor function was not affected by CHRFAM7A direct allele carrier status while the cognitive measures for processing speed, learning and memory consistently favored carriers, reaching statistical significance on immediate visual learning. The greater differences within the BVMT-R immediate versus delayed tasks suggest that the CHRFAM7A direct allele may influence the process of active learning and working memory versus improving the process of consolidation and/or decreasing the memory decay over time. These findings correlate and colocalize with the high expression of the α7 nACHR in the hippocampus and the medial prefrontal cortex which are essential regions for memory processing (Pastor and Medina, 2023). Understanding whether verbal and visual memory are affected differentially requires larger well-powered studies.

BVMT-R requires visual spatial perception in that examinees are awarded points for the accuracy of reproducing what they see, and the location of stimuli. BVMT-R is associated with right hemisphere visual networks in MS (Fuchs et al., 2019). Visual spatial function in middle age has emerged as the main predictor of cognitive decline later in life, and this correlation has been hypothesized as the first measurable decline (Possin, 2010; Salimi et al., 2018; Kolli et al., 2021; Berente et al., 2022). Informed by the underlying cell biology of CHRFAM7A, the association between visual spatial function and cognitive decline could be driven by a more resilient neuronal structure measured by the visual spatial relative strength, latently detecting CHRFAM7A direct allele carriers.

Structural MRI validated the prediction for a more efficient neuronal structure from the iPSC model. Larger brain size has been associated with higher cognitive function traditionally, thus the inverse relationship between t whole brain volume and cognitive function in CHRFAM7A direct allele carriers accentuates the enhanced cognitive findings. The resting state functional MRI findings further support the more efficient neural structure hypothesis in the sense that direct allele carriers require less connectivity in the unstimulated resting state.

Further studies are needed to validate these findings and to expand how CHRFAM7A direct allele affects higher cognitive function through α7 nAChR associated domains such as learning and memory and how it contributes to cognitive reserve. The role of the α7 nAChRs in memory comes mostly from rodent and other animal model experiments and experimental restrictions in humans preclude direct confirmation of the molecular mechanism. Of note, α7 nAChRs agonists, such as encenicline has been studied in clinical trials in both Alzheimer’s disease and schizophrenia and while animal models showed consistent efficacy all human clinical trials failed identifying a translational gap (NCT01969136 and NCT01969123) (Prickaerts et al., 2012). While CHRFAM7A modifies the α7 nAChR into a hypomorphic ionotropic receptor, at the same time shifts Ca²⁺ dynamics in the neuron that activates the actin cytoskeleton (Szigeti et al., 2023). Actin cytoskeleton is a fundamental mechanism in memory formation through stabilizing and maturing synapses resulting in long term potentiation that could account for the observed memory gain of function (Lamprecht, 2021). Synaptic actin dynamics has been shown to contribute to behavioral visual acuity measured by the visual water maze task in rats (Bi et al., 2021). Thus, similar to other human restricted genes (ARHGAP11B, SRGAP2) an apparent loss of function mutation leads to a novel gain of function and the observed outcome is the sum of these effects (Heide et al., 2020).

There are several limitations to our study as it is exploratory in nature. While the sample size and the genotype distribution for carriers and non-carriers were meaningful and consistent with previously reported allele frequencies in Caucasians, we need larger and more diverse studies to confirm these findings. This cross-sectional study does not address whether the efficiency translates into cognitive reserve. Larger longitudinal datasets are needed to answer this very important question. Moreover, we only utilized a small set of tests that only investigated three cognitive domains of cognitive processing speed, verbal and visuospatial learning and memory. Future studies can include greater number of tests that will include measures of executive function, logical memory, visual integration and language. While the inverted allele does not seem to affect α7 nAChR function, it has been implicated in psychiatric disease. Understanding the function of the inverted allele will refine the interpretation of these findings. Due to the small sample size, we restricted the analysis on pre-determined and established networks and did not perform an independent difference in functional connectivity between the direct allele carriers vs. non-carriers. Future more comprehensive fMRI analysis would be able to determine differences that may be more specific to the CHRFAM7A.

These preliminary data indicate that human brain may have unique characteristics due to an actin cytoskeleton gain of function in 75% of the population and raises the possibility that this structural change may contribute to resilience and cognitive reserve. Cognitive reserve has been implicated in brain aging and susceptibility to disease thus considering CHRFAM7A genotype may facilitate risk stratification and identify new targetable mechanisms for neurodegeneration, neuroinflammation and stroke.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by University at Buffalo, State University of New York. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

DJ: Writing – original draft, Writing – review & editing, Data curation, Formal analysis, Investigation, Methodology, Project administration, Visualization. RD: Data curation, Investigation, Methodology, Project administration, Writing – original draft, Writing – review & editing. MRe: Data curation, Investigation, Methodology, Project administration, Writing – review & editing. AB: Data curation, Investigation, Methodology, Project administration, Writing – review & editing. NB: Data curation, Investigation, Methodology, Project administration, Writing – review & editing. MRa: Data curation, Investigation, Methodology, Project administration, Writing – review & editing. MD: Data curation, Investigation, Methodology, Project administration, Writing – review & editing. RB: Data curation, Investigation, Methodology, Project administration, Writing – review & editing. RZ: Data curation, Funding acquisition, Investigation, Methodology, Project administration, Writing – review & editing. KS: Conceptualization, Data curation, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review & editing.