Identification of Germline Mosaicism in a Family with 16p11.2 Microdeletion Syndrome

Abstract

Microdeletions and microduplications of chromosome 16p11.2 have been associated with language impairment disorders, intellectual disability, developmental delay and psychiatric disorders. The majority of 16p11.2 microdeletions occurs de novo, whereas most 16p11.2 microduplications are seen in familial cases. A 1.5 month old male has been diagnosed with severe microcephaly and developmental delay. Whole exome sequencing (WES) by Illumina Nextseq500 has revealed his acquisition of a 0.7 Mb deletion at chromosome 16 (16p11.2), involving 42 genes of which 32 are protein-coding genes. Family segregation analysis was performed by SYBR Green quantitative real-time polymerase chain reaction (qPCR) on an ABI 7300 system. In order to measure the fold change in copy number loss the 2^−ΔΔCt* formula was used which normalizes the average DNA content of the ‘tested’ triplicate Cycle threshold (CT) against the average CT values of the internal ABL gene. The results revealed that both of his parents are wild-type (negative) for the microdeletion. Unexpectedly, his affected maternal-aunt has been demonstrated to be a heterozygous carrier (affected) of the same 16p11.2 microdeletion just like the proband. Hence, the proposed hypothesis presumed that this autosomal dominant deletion has been inherited through germ-line mosaicism, which was anticipated to be accompanied by somatic mosaicism within his mother. Therefore, a buccal mucosal cells swab and hair samples were obtained from the mother. DNA was extracted by AutoMate Express DNA extraction system so as to perform a qPCR analysis of the buccal mucosa sample which showed a clear-cut sign of mosaicism in the mother. In conclusion, genetic counseling and pre-implantation genetic diagnosis (PGD) is recommended so as to rule out any risk of recurrence due to the presence of germline mosaicism. *ΔΔCt = (average Ct value for the tested aberration − average Ct value for ABL control) for the tested individual − (average Ct value for the tested aberration − average Ct value for ABL control) for a normal individual