Table 2. Association of catechol-O-methyltransferase with neurological disorders.

COMT: catechol-O-methyltransferase; GMV: gray matter volume; PD: Parkinson's disease; Val: Valine; Met: Methionine; CSF: cerebrospinal fluid; AD: Alzheimer's disease; AA: adenine-adenine; AG: adenine-guanine; SNP: single nucleotide polymorphism

Author	Year of publication	Type of study	No. of subjects	Purpose of study	Results/conclusion
Sampedro F et al. [12]	2020	Observational	120	To determine the GMV in COMT Val/Val Parkinson’s disease (PD) patients with preserved cognition and its association with cognitive decline.	PD patients with a reduced GMV with a Val/Val genotype are predisposed to cognitive decline.
Fang YJ et al. [27]	2019	Observational	54	To assess the executive functions in PD patients with different COMT gene variants.	PD patients with Val/Val genotype showed poor performance in the set-shifting task. COMT gene polymorphism influences working memory in PD.
Babić Leko M et al. [28]	2020	Observational	233	To assess the association of COMT Val(158)Met genotype with the CSF biomarker level in Alzheimer’s disease (AD) patients.	The levels of t-tau and p-tau181 were found to be increased in AD patients with AA in comparison to the AG COMT Val158Met genotype.
Porcelli S et al. [29]	2019	Observational	457	To explore the effects of gene variants on psychiatric comorbidities in AD.	The rs4680 variant of the COMT genotype was associated with AD, and the rs174696 variant of COMT was associated with depression in Greek AD patients.
Driscoll I et al. [30]	2019	Observational	2857	To study the association of SNPs in a few genes, including COMT, with the pathogenesis of AD and risk of dementia in postmenopausal women.	COMT gene was associated with probable dementia.