## is a human gene whose product aHI1:** Abelson helper integration site 1 is a WD40 repeat-containing protein that localizes to the basal bodies of primary cilia. It plays critical roles in ciliary signaling, neuronal migration, and synaptic function. AHI1 interacts with the Jouberin protein and is involved in the sonic hedgehog pathway. In brain, it is highly expressed in regions affected in [Parkinson disease](/diseases/parkinsons-disease). This page covers the gene's normal function, disease associations, expression patterns, and key research findings relevant to neurodegeneration.
## is a human gene whose product aHI1:** Abelson helper integration site 1 is a WD40 repeat-containing protein that localizes to the basal bodies of primary cilia. It plays critical roles in ciliary signaling, neuronal migration, and synaptic function. AHI1 interacts with the Jouberin protein and is involved in the sonic hedgehog pathway. In brain, it is highly expressed in regions affected in [Parkinson disease](/diseases/parkinsons-disease). This page covers the gene's normal function, disease associations, expression patterns, and key research findings relevant to neurodegeneration.
Gene Summary
CLN2 encodes TPP1 (tripeptidyl peptidase 1), a lysosomal serine protease. It catalyzes the cleavage of N-terminal tripeptides from substrates. Deficiency causes classic late infantile Batten disease, characterized by seizures, vision loss, and neurodegeneration. [@volk1999]
Gene Information
Function
Normal Function
AHI1: Abelson helper integration site 1 is a WD40 repeat-containing protein that localizes to the basal bodies of primary cilia. It plays critical roles in ciliary signaling, neuronal migration, and synaptic function. AHI1 interacts with the Jouberin protein and is involved in the sonic hedgehog pathway. In brain, it is highly expressed in regions affected in Parkinson disease. [@ebrahimifakhari2022]
GAD1: Glutamate decarboxylase 1 produces the 67-kDa isoform (GAD67), which is responsible for most GABA synthesis in the brain. GAD67 is expressed in GABAergic [neurons](/entities/neurons) throughout the CNS and is essential for inhibitory neurotransmission. It is regulated at transcriptional and post-translational levels.
GAD2: Glutamate decarboxylase 2 produces the 65-kDa isoform (GAD65), which is concentrated at synaptic terminals. GAD65 is particularly important for activity-dependent GABA synthesis and is a major autoantigen in type 1 diabetes and some neurological disorders.
CLN2: Ceroid lipofuscinosis 2 encodes TPP1, a lysosomal aspartyl protease that degrades proteins in the late endosome/lysosome. TPP1 cleaves N-terminal tripeptides from various substrates and is essential for neuronal health. The enzyme is secreted and can be delivered to deficient cells.
TECPR2: Tectonin beta-propeller repeat containing 2 is an [autophagy](/entities/autophagy) receptor that facilitates selective autophagy of damaged ER and protein aggregates. It interacts with Atg proteins and helps maintain ER homeostasis. Loss of function leads to impaired autophagy and ER stress.
Disease Associations
AHI1: [Joubert syndrome](/diseases/joubert-syndrome) - Biallelic mutations cause Joubert syndrome characterized by cerebellar vermis hypoplasia and developmental delay. [AHI1 variants](/diseases/parkinsons-disease) are also associated with [Parkinson disease](/diseases/parkinsons-disease) risk. [1](https://pubmed.ncbi.nlm.nih.gov/17160772/)
GAD1: [Epilepsy](/diseases/epilepsy) - GAD1 deficiency leads to reduced GABAergic inhibition and seizure susceptibility. [Schizophrenia](/diseases/schizophrenia) - GAD1 expression is reduced in schizophrenia brains. [2](https://pubmed.ncbi.nlm.nih.gov/14534350/)
GAD2: [Epilepsy](/diseases/epilepsy) - Autoantibodies against GAD2 are found in some epilepsy patients. [Type 1 diabetes](/diseases/diabetes) - GAD2 is a major autoantigen. [3](https://pubmed.ncbi.nlm.nih.gov/8366929/)
CLN2: [Late infantile neuronal ceroid lipofuscinosis](/diseases/batten-disease) - Also known as classic Batten disease, CLN2 deficiency causes progressive neurodegeneration with seizures, vision loss, and cognitive decline. Enzyme replacement therapy (cerliponase alfa) is available. [4](https://pubmed.ncbi.nlm.nih.gov/10435979/)
TECPR2: [Hereditary spastic paraplegia](/diseases/hereditary-spastic-paraplegia) - Autosomal recessive mutations cause complex HSP type 49 with neuropathy, dysautonomia, and often autism. [5](https://pubmed.ncbi.nlm.nih.gov/21937952/)
Expression Pattern
AHI1: Expressed in fetal and adult brain, with highest expression in cerebellum, basal ganglia, and [hippocampus](/brain-regions/hippocampus). Also expressed in kidney and testis. [6](https://pubmed.ncbi.nlm.nih.gov/11025711/)
GAD1: Broad expression in GABAergic neurons throughout the brain, including [cortex](/brain-regions/cortex), hippocampus, basal ganglia, and cerebellum. More abundant than GAD2 in most brain regions. [7](https://pubmed.ncbi.nlm.nih.gov/2547584/)
GAD2: Expressed in GABAergic neurons with synaptic localization. Higher expression in hippocampus and cerebellum compared to GAD1 in some regions. [8](https://pubmed.ncbi.nlm.nih.gov/1339266/)
CLN2: Expressed in all tissues with highest expression in brain, particularly in neurons. Lysosomal expression in most cell types. [9](https://pubmed.ncbi.nlm.nih.gov/9546342/)
TECPR2: Highest expression in brain, especially in motor neurons and Purkinje cells. Also expressed in peripheral nerves. [10](https://pubmed.ncbi.nlm.nih.gov/21937952/)
Therapeutic Implications
AHI1: Gene therapy approaches to restore AHI1 function are being explored for Joubert syndrome. Ciliary signaling modulators may provide benefit.
GAD1: Enhancing GAD1 expression or activity may help treat epilepsy and neuropsychiatric disorders. GABAergic agents targeting GAD1 pathways are in development.
GAD2: Immunomodulatory approaches to neutralize anti-GAD2 antibodies may benefit autoimmune epilepsy. GAD2 vaccines are being studied for diabetes.
CLN2: Cerliponase alfa (Brineura) is an FDA-approved enzyme replacement therapy for CLN2 disease. Early treatment can slow disease progression. Gene therapy approaches are in clinical trials. [11](https://pubmed.ncbi.nlm.nih.gov/26334758/)
TECPR2: Enhancing autophagy through [mTOR](/mechanisms/mtor-signaling-pathway) modulation or direct autophagy induction may compensate for TECPR2 deficiency. Gene therapy is under investigation. [12](https://pubmed.ncbi.nlm.nih.gov/35260468/)