Human fibroblasts (HFs) were obtained from the two 9p24.1 DUP/TRP carriers; unexpectedly, G-band chromosomal karyotyping identified mosaicism for an abnormal mar element (DL3363, 56.2%; DL55459, 25.0%) (Figures 1D and 1E; Table S1). Mar elements are structurally abnormal (and typically small) chromosomes that cannot be resolved by conventional banding cytogenetics alone; their clinical significance varies depending on the specific genetic material (ISCN, 2013). Because karyotypic mosaicism can underestimate the prevalence of the mar element (false negatives occur due to the small size of the mar element), these results were confirmed by FISH in interphase cells for a probe to glycine decarboxylase (GLDC), a gene included in the 9p24.1 TRP region. FISH analysis indicated that in both carriers the mar element contained sequences from the 9p24.1 region, and also confirmed that HF lines from both carriers contain a similar level of mar mosaicism (55% ± 0.5%) (Figures 1F and 1G; Table S1).
