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PAK3
p21 (RAC1) activated kinase 3
PAK3 is located on the long (q) arm of chromosome X, at band Xq23. Arm ratio per GRCh38 - banding schematic.
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Overview
PAK3 (p21 RAC1 activated kinase 3) encodes a member of the p21-activated kinase family, which functions in intracellular signalling pathways essential for brain development. The protein acts downstream of small GTPases, including RAC1 and CDC42, to regulate cytoskeletal dynamics and neuronal architecture. Pathogenic changes in PAK3 have been associated with X-linked intellectual disability, presenting with cognitive impairment and developmental delay. Because the gene resides on the X chromosome, males who inherit a pathogenic variant typically exhibit more pronounced features than carrier females, who may show variable expression due to X-inactivation patterns.
What the gene does
The PAK3 protein functions as a serine/threonine kinase activated by binding to GTP-loaded RAC1 or CDC42. Upon activation, the kinase phosphorylates downstream substrates involved in cytoskeletal remodelling, particularly the actin cytoskeleton that shapes neuronal dendrites and axons. This signalling cascade is crucial during synapse formation and maturation, influencing how neurons establish connections in the developing brain. PAK3 activity also modulates gene transcription and cell survival pathways, contributing to the maintenance of neuronal populations. Experimental evidence suggests the protein plays a role in synaptic plasticity, the process by which neuronal connections strengthen or weaken in response to activity. Disruption of PAK3 function therefore impairs the structural and functional development of neural circuits, which may manifest as learning difficulties and cognitive deficits in affected individuals.
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Chromosome location
PAK3 is located on the X chromosome at band Xq23. The gene spans multiple exons encoding the full-length kinase, although the precise exon count and transcript architecture have not been comprehensively detailed in all public databases. Its X-chromosomal position explains the inheritance pattern observed in families with PAK3-related conditions, where males are typically more severely affected than females.
Protein structure
Domain architecture has not been experimentally characterised in detail for this protein.
Key variants
Pathogenic variants in PAK3 include missense changes that disrupt kinase activity or regulatory interactions with GTPases. Because the gene is X-linked, hemizygous males carrying a single pathogenic allele exhibit the associated phenotype, whereas heterozygous females may display a milder or variable presentation depending on X-inactivation. The spectrum of reported variants includes substitutions in functionally critical regions of the kinase domain, which compromise the protein's ability to phosphorylate target substrates effectively.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.1066G>T | p.Glu356Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Intellectual disability, X-linked 30 |
c.1306C>T | p.Arg436Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Inborn genetic diseases |
c.298C>T | p.Arg100Ter | Pathogenic/Likely pathogenic | ★★☆☆ | PAK3-related disorder |
c.1094C>A | p.Ala365Glu | Pathogenic | ★☆☆☆ | not provided |
c.199C>T | p.Arg67Cys | Pathogenic | ★☆☆☆ | not provided |
c.490_491insGGAG | p.Pro164fs | Pathogenic | ★☆☆☆ | not provided |
c.535G>T | p.Glu179Ter | Pathogenic | ★☆☆☆ | Inborn genetic diseases |
c.729dup | p.Ser244fs | Pathogenic | ★☆☆☆ | not provided |
c.976G>C | p.Val326Leu | Pathogenic | ★☆☆☆ | Microcephaly |
c.1337G>C | p.Trp446Ser | Pathogenic | - | Intellectual disability, X-linked 30 |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Variants in PAK3 are associated with X-linked intellectual disability, characterised by cognitive impairment ranging from mild to moderate severity. Affected individuals may also exhibit developmental delay, speech and language difficulties, and behavioural features. The condition follows an X-linked pattern, with males predominantly affected and carrier females occasionally displaying milder signs. Clinical presentation can vary between families, reflecting differences in the specific variant and individual genetic background.
No disease links recorded for this gene in our reference set.
UK clinical status
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
How is PAK3-related intellectual disability inherited?
PAK3-related intellectual disability follows an X-linked pattern, meaning the gene is located on the X chromosome. Males, who have one X chromosome, are typically more severely affected if they inherit a pathogenic variant, while females, with two X chromosomes, may be carriers with milder or no symptoms.
What does the PAK3 protein do in the brain?
The PAK3 protein regulates the actin cytoskeleton in neurons, influencing the shape and connectivity of brain cells. It plays a key role in synapse formation and plasticity, processes essential for learning and memory.
Can PAK3 variants be detected through genetic testing?
Yes, PAK3 variants can be identified through clinical genetic testing, including gene panel sequencing or exome sequencing. Families with a history of X-linked intellectual disability may be offered testing to confirm a diagnosis and inform reproductive planning.