Chromosome 6 between‐arm intrachromosomal insertion with intrasegmental double inversion: A four‐break model

Intrachromosomal insertions are very rare; about 40 cases have been published [Farrell and Chow, 1992; Madan and Menko, 1992; Ardalan et al., 2005]. Due to the rarity, the nature of this type of chromosomal rearrangement is not well studied and how it was formed remains unknown. The chromosomal segment can be intercalated into the other arm (between-arm or pericentric) or the same arm (within-arm or paracentric). Insertions sometimes could not be distinguished from inversions by G-banding [Madan and Nieuwint, 2002]. In fact, two cases of between-arm intrachromosomal insertion were misinterpreted as pericentric inversion [Starke et al., 2001; Ardalan et al., 2005]. At least four cases of within-arm intrachromosomal insertion [Madan and Menko, 1992] were misdiagnosed as, and four cases [Madan and Nieuwint, 2002] potentially were paracentric inversion. The risk of having an abnormal child for carriers of within-arm intrachromosomal insertion is about 15%, and can be as high as 30–50% [Madan and Menko, 1992]. In contrast, most paracentric inversions are harmless [Madan, 1995], except a recent case report for cri-du-chat syndrome [South et al., 2006]. Therefore, it is important to distinguish between these two types of rearrangements. Chromosome 6 between-arm intrachromosomal insertion is extremely rare. To the best of our knowledge, only one carrier mother with such rearrangement, which gave rise to an interstitial deletion of 6q13-q15 in her son, was ever reported [Rose et al., 1992]. In this study, we studied a carrier with a chromosome 6 between-arm intrachromosomal insertion. She had two phenotypically normal children, one spontaneous abortion at 6 weeks (specimen was unavailable for analysis), and two abnormal children with chromosome 6q13-q14.2 deletion. The clinical presentation of the two affected children (Patients 5 and 6) and the pedigree of this family has been recently reported [Wang et al., 2009]. The peripheral blood was analyzed by standard G-banding techniques. Dual-color fluorescence in situ hybridization analysis with labeled bacterial artificial chromosome probes (BAC-FISH) was implemented to further delineate the between-arm intrachromosomal insertion. The BAC clones were selected from the March 2006 University of California at Santa Cruz (UCSC) freeze of the human genome [Kent et al., 2002], and the probes were obtained from The Centre for Applied Genomics (TCAG). The BAC-FISH analysis was performed with 1ml of each BAC probe and 1mg of Cot-1 DNA. The slide with added probe mixture was co-denatured at 72 C for 5 min, and hybridized overnight at 37 C. The slide was washed next day in 0.4 SSC/0.3% NP-40 at 73 C for 2 min, and in 2 SSC/0.1% NP-40 at room temperature for 1 min, counterstained with DAPI, and visualized under fluorescent microscope. Genome-wide human SNP array was applied to this carrier to exclude microdeletion or microduplication flanking the breakpoints. Genomic DNA was extracted from peripheral blood lymphocytes using Gentra Puregene kit (Qiagen, Mississauga, ON). The array analysis using Affymetrix genome-wide human SNP array 6.0 (Affymetrix, Santa Clara, CA) was performed by The Centre for Applied Genomics (TCAG) (www.tcag.ca).