Handedness, the preference for using one hand over the other, is a trait observed globally, with approximately 90% of the population being right-handed and 10% left-handed. This distribution remains consistent across different regions and has persisted through human history. Handedness not only reflects individual variation but also serves as a window into the broader understanding of brain asymmetry and human neurobiology.
Historical and Global Perspectives on Handedness
The prevalence of right-handedness has been a constant feature throughout human history, with only minor regional and temporal fluctuations. This pattern suggests a deep-rooted biological underpinning rather than cultural or social influences. Studies tracing back through various historical periods and across different cultures have consistently found a similar ratio of right-handed to left-handed individuals.
Neurological Basis of Handedness
Handedness is intrinsically linked to brain asymmetry. In right-handed individuals, the left hemisphere usually exhibits dominance in controlling preferred hand activities, and the reverse is true for left-handers. This lateralization of brain function is evident from as early as ten weeks of gestational age, indicating a preprogrammed, genetic component in determining handedness.
Genetic and Developmental Insights
The genetic basis for handedness has been partially unraveled through twin and family studies, which estimate that left-handedness has a heritability of about 25%. However, the specific genetic variants responsible for handedness have been elusive, with early genome-wide association studies (GWAS) in smaller cohorts failing to identify significant loci. It was only in larger studies, particularly those utilizing the UK Biobank dataset, that significant genetic loci associated with left-handedness were identified, highlighting genes involved in microtubule formation and cellular structure.
Microtubules and Handedness
The association of genes related to microtubules, such as TUBB, MAP2, and MAPT, with left-handedness points towards the role of these cellular components in brain development and the establishment of left-right axis. Microtubules, integral to cell structure and function, may influence the development of brain asymmetry and, consequently, handedness. This connection underscores the complex interplay between genetics, cellular biology, and neurodevelopmental processes in determining handedness.
Expanding the Genetic Framework
Further large-scale GWAS and meta-analyses have expanded the genetic landscape associated with handedness, revealing numerous loci linked to microtubule-associated proteins and axon development. This extensive genetic network suggests a multifaceted genetic architecture underlying handedness, with potential implications for understanding neurodevelopmental pathways and brain asymmetry.
Rare Genetic Variants and Handedness
While common genetic variants have been the focus of most studies, there is growing interest in the role of rare, protein-altering variants in determining handedness. Preliminary investigations into families with a high prevalence of left-handedness have not yet yielded conclusive results, highlighting the need for larger, more comprehensive studies to explore the impact of rare genetic variations.
Handedness and Neurodevelopmental Disorders
Research has consistently shown an increased prevalence of left-handedness in individuals with neurodevelopmental disorders such as autism and schizophrenia. These findings suggest a potential link between the genetic factors influencing handedness and those contributing to these disorders. The genetic overlap between handedness and neurodevelopmental conditions underscores the shared biological pathways that may underlie these traits.
Genetic Correlations with Neurodegenerative Diseases
There is also evidence of a genetic correlation between left-handedness and certain neurodegenerative diseases, including Parkinson’s disease. This relationship, partly attributed to genetic loci shared between these conditions, provides insight into the broader neurological implications of the genetics of handedness.
Future Directions in Handedness Research
The exploration of rare coding variants in large datasets like the UK Biobank offers a promising avenue for advancing our understanding of the genetic factors contributing to handedness. By examining the associations between handedness and genes implicated in various neurological and neurodevelopmental disorders, researchers can gain deeper insights into the complex genetic networks that shape this fundamental human trait.
DISCUSSION : Insight into the Genetics of Handedness: Unraveling the Role of Rare Variants
Utilizing the expansive dataset from the UK Biobank, our research delves into the genetic nuances of left-handedness, highlighting the subtle but significant role of rare coding variants. Although the heritability attributed to these variants is modest, under 1%, their impact on individuals carrying specific mutations in genes like TUBB4B, DSCAM, and FOXP1 is profound, hinting at a deeper genetic interplay in determining handedness and brain asymmetry.
Microtubules at the Forefront
The recurring theme in the genetic underpinnings of handedness is the role of microtubules. The discovery that TUBB4B, among other tubulin genes, is significantly associated with left-handedness reinforces the hypothesis of microtubule involvement in brain asymmetry. Unlike common variants affecting gene expression, the rare coding variants in TUBB4B suggest that even minor alterations in the protein sequence can influence handedness, perhaps by modulating microtubule dynamics and stability.
Beyond Neurological Disorders: The Broader Spectrum of TUBB4B Variants
The association of TUBB4B variants with left-handedness expands the phenotypic spectrum of this gene beyond known sensorineural and ciliopathic disorders. This connection prompts a reevaluation of how microtubule dynamics, influenced by genetic variations, play a role in broader neurodevelopmental processes, potentially affecting the brain’s left-right axis formation without leading to overt clinical disorders.
Microtubules, Cilia, and Developmental Asymmetry
Microtubules are instrumental in ciliary function and cellular asymmetry, which are vital for the early developmental processes that establish body and organ laterality. However, the link between microtubules and brain asymmetry seems distinct from their role in visceral organ placement, suggesting that brain and body asymmetries might arise through divergent developmental mechanisms.
Exploring the Complexity of Brain Asymmetry
While the focus has been on tubulins and microtubule-associated proteins, genes like DSCAM and FOXP1, implicated in neurodevelopmental disorders, also emerge as potential contributors to handedness. These findings suggest that the mechanisms influencing brain asymmetry and handedness could overlap with pathways involved in complex neurodevelopmental conditions.
Future Directions in Handedness Research
The modest heritability of left-handedness attributed to rare coding variants beckons larger-scale studies to uncover additional genetic factors involved. This pursuit is not just about understanding handedness but also about decoding the intricate genetic and developmental blueprint that governs brain asymmetry and functional lateralization.
Conclusion
Our investigation into the genetic architecture of handedness, particularly through the lens of rare coding variants, illuminates the intricate biological pathways that sculpt brain asymmetry and handedness. It underscores the delicate balance between genetic predisposition and developmental biology in shaping one of the most fundamental aspects of human neurodiversity.
REFERENCE LINK : https://www.nature.com/articles/s41467-024-46277-w#Sec7
