Developmental delay referrals and the roles of Fragile X testing and molecular karyotyping: A New Zealand perspective

Abstract

Global developmental delay (GDD) affects ~1-3% of children, many of whom will also have intellectual disability (ID). Fragile X is the major genetic cause of GDD with mental retardation (MR) in males, accounting for ~20% of all X-linked MR. As Fragile X has serious genetic implications, the overwhelming majority of developmental delay (DD) cases referred to our laboratory are concerned with the exclusion of a diagnosis of Fragile X, along with simultaneous karyotype analysis to confirm chromosome aberrations. Critically, molecular laboratories have generally experienced a falling positive detection frequency of Fragile X. In this context, the recent implementation of array-based technology has significantly increased the likelihood of detecting chromosome aberrations that underpin DD. In the current study, we report a Fragile X mutation detection frequency for DD referrals that is comparable with the falling UK detection frequencies. In addition, we find that there is a 9-fold greater likelihood of detecting clinically significant chromosomal aberrations than of detecting a full Fragile X mental retardation 1 (FMR1) gene CGG repeat expansion in cases referred on the basis of DD. We propose a more efficent sequential testing algorithm that involves an initial molecular karyotype, cascading to FMR1 gene analysis in the event of a negative result.