Are de novo rea(21;21) chromosomes really de novo?

Abstract

Key Clinical MessageWe report a rare case of recurrent trisomy 21 caused by an isochromosome 21q and what is very likely to be maternal germ-line cell mosaicism. Over 90% of cases of rob(21;21) reported in the literature are due to an isochromosome 21q, with a risk of recurrence of more than 10%.

The Second Case of Non-Mosaic Trisomy of Chromosome 26 with Homologous Fusion 26q;26q in the Horse

“…Can it be that ECA26 is more prone for centric fusion than other equine small acrocentric chromosomes? Chromosome-specific effects have been observed in humans where a small percentage of cases of Down and Patau syndrome with trisomy HSA21 and HSA13, respectively, have the extra chromosome in the form of Robertsonian fusion or an isochromosome [ 15 , 16 , 17 , 35 , 36 ]. In Down syndrome, there are even rare mosaic cases where one cell line carries HSA21 isochromosome and another, a Robertsonian fusion [ 17 ].…”

“…In rare occasions, trisomies of acrocentric autosomes are combined with Robertsonian fusion or isochromosome formation [ 15 , 16 , 17 ], so that despite of aneuploidy, the diploid chromosome number remains normal. For example, about 5–6% of cases with Down syndrome carry unbalanced heterologous or homologous fusions involving HSA21 [ 15 , 17 ].…”

“…In rare occasions, trisomies of acrocentric autosomes are combined with Robertsonian fusion or isochromosome formation [ 15 , 16 , 17 ], so that despite of aneuploidy, the diploid chromosome number remains normal. For example, about 5–6% of cases with Down syndrome carry unbalanced heterologous or homologous fusions involving HSA21 [ 15 , 17 ]. The mechanism for heterologous fusions is Robertsonian translocation, of which the most common (82%) in Down syndrome patients is rob (14q;21q), with the remaining 8% represented by rob(13q;21q), rob(15q;21q) and rob(21q;22q) [ 17 ].…”

“…On the other hand, trisomy due to homologous fusion of (21q;21q) can result from different mechanisms—by isochromosome i(21q) formation or due to Robertsonian translocation rob(21q;21q). Since isochromosomes result from the duplication of a single chromosome arm [ 18 ], the duplicated parts are genetically identical and can be distinguished from homologous translocation by genotyping for allelic variation using chromosome specific polymorphic short tandem repeat (STR) markers [ 15 , 16 , 18 , 19 ].…”

De Novo Robertsonian Translocation t(21; 21) in a Child with Down Syndrome

“…Interestingly, Herve et al 7 have reported a case of recurrent trisomy 21 caused by an isochromosome 21q {i(21q)} very likely secondary to a maternal germline cell mosaicism. They hypothesize that vast majority of apparently de novo cases of rea(21;21) may be due to an i(21q) with maternal or paternal inheritance.…”