Multi-tissue cytogenetic analysis for the diagnosis of mosaic Down syndrome: A case report

Abstract

Less than one percent of individuals with Down syndrome exhibit mosaicism, a biological phenomenon that describes an individual who has two or more genetically distinct cell lines. The percentage of mosaicism in different tissues can impact the presence of clinical findings and hinder cytogenetic diagnosis. We report a case of mosaicism for trisomy 21 diagnosed after multi-tissue cytogenetic analysis of peripheral blood and buccal mucosa, associated with significant intellectual disability, dysmorphic facial features, congenital heart defects, macropenis, and imperforate anus.

Keywords: Down syndrome; buccal smear; chromosome disorders; cytogenetics; macropenis; mosaicism; trisomy 21.

FIGURE 1

Fluorescence in situ hybridization with a panel of probes specific for the detection of chromosome trisomies for 13 and 21 performed on buccal swab slides. The LSI 13/21 probe hybridizes to the D21S259, D21S34, and D21S342 regions within 21q22.13‐q22.2 and to the RB‐1 region at 13q14. Three red signals were observed on 78 of 501 interphase nuclei, consistent with mosaicism for trisomy 21. These are interphase nuclei signals where the DNA is extended. The RB‐1 probe covers 440 kb, and the chromosome 21 probes cover 200 kb. Additional signals correspond to the spreading of the probes signal across a larger nuclear region