Non-syndromic Intellectual Disability: An Experimental In-Depth Exploration of Inheritance Pattern, Phenotypic Presentation, and Genomic Composition

Background Intellectual disability (ID), also termed mental retardation (MR), is a neurodevelopmental disorder characterized by an intelligence quotient (IQ) of 70 or below and a deficit in at least two behaviors associated with adaptive functioning. The condition is further classified into syndromic intellectual disability (S-ID) and non-syndromic intellectual disability (NS-ID). This study highlights the genes associated with NS-ID. Objectives A genetic study was performed on two Pakistani families to know the inheritance patterns, clinical phenotypes, and molecular genetics of affected individuals with NS-ID. Methodology Samples were collected from two families: families A and B. All affected individuals in both families were diagnosed by a neurologist. Written informed consent was taken from the affected individuals and guardians before collecting the data and sample. Family A belongs to the Swabi District of Pakistan having four affected individuals, out of whom three were male and one was female. Family B also belongs to the Swabi District of Pakistan having two affected individuals, out of whom one was male and one was female. A total of 10 candidate genes were selected and were further screened by microarray analysis. Results In family A, this analysis identified a region of 9.6 Mb on chromosome 17q11.2-q12 between the single nucleotide polymorphisms (SNPs) rs953527 and rs2680398. The region was genotyped using microsatellite markers to confirm the haplotypes in all family members. Based on the phenotype-genotype relationship, 10 possible candidate genes were selected out of more than 140 genes in this critical region of 9.6 Mb. In family B, homozygosity mapping through microarray identified four homozygous areas of affected individuals: two (27,324,822-59,122,062 and 96,423,252123,656,241) on chromosome 8, one (14,785,224-19,722,760) on chromosome 9, and one (126173647-126215644) on chromosome 11. Conclusion An autosomal recessive pattern was found in the pedigrees of both families A and B. Phenotypically affected individuals showed IQ levels below 70. Three genes, CDK5R1, OMG, and EV12A, were found on chromosome 17q11.2-q12 region of affected individuals in family A with high expression in the frontal cortex of the brain, hippocampus, and spinal cord, respectively. Other regions on chromosomes 8, 9, and 11 as evident from the affected individuals in family B can also contribute to the non-syndromic autosomal recessive intellectual disability (NS-ARID). Further research is needed to find the association of these genes with intelligence and other neuropsychiatric conditions.