Diagnostic yield of whole-exome sequencing in non-syndromic intellectual disability

Aetiological diagnosis in non-syndromic intellectual disability (NSID) still poses a diagnostic challenge to clinicians.

Screening is currently achieved by chromosomal microarrays followed by whole-exome sequencing (WES). In search for the aetiological yield of WES in patients with NSID, 59 unrelated patients were studied.

Among the 59 patients, 44 (74.6%) were from consanguineous unions. Epilepsy was present in 11 (37.9%), behavioural problems in 12 (41.4%) and autistic features in 14 (48.3%). WES analysis resulted in molecular diagnosis in 29 patients (49.2%). Some of the genes were specific for nervous system functioning, like HERC1, TBC1D7, LINS, HECW2, DEAF1, HNMT, DLG3, NRXN1 and HUWE1. Others were ubiquitously expressed genes involved in fundamental cellular processes, like IARS, UBE3A, COQ4, TAF1, SETBP1, ARV1, ZC4H2, KAT6A, ASXL3, THOC6, HNRNPH2, TUBA8 and KIF1A. Twenty-two (75.8%) were consanguineously married; however, only 12 (41.4%) of the detected genes caused autosomal recessive phenotypes.

This cohort suggests that recessive genes probably represent an actually smaller subgroup of NSID, even among families with consanguinity. Although in societies with high consanguinity rates, considering the recessive inheritance first seems to be an advantageous strategy, de novo mutations in autosomal dominantly expressed genes represent the major aetiological group in patients with NSID, even among those patients from consanguineous families.