Molecular analysis of mosaicism for two different de novo acrocentric rearrangements demonstrates diversity in Robertsonian translocation formation

Abstract

Robertsonian translocations (ROBs) involving chromosome 21 occur in about 5% of individuals with Down syndrome. ROBs are the most common chromosomal rearrangements in humans and are formed through whole arm exchanges of any two acrocentric chromosomes. The de novo formation of ROBs occurs at exceptionally high rates. The present case concerns a child with mosaic Down syndrome who has two cell lines that contain two different de novo ROBs: 45,XX,rob(14;21)(q10;q10) and 46,XX,rea(21;21)(q10;q10),+21. To elucidate the mechanisms by which the rearrangements formed, somatic cell hybrids were constructed to allow the parental origins of the chromosomes involved in the ROBs to be distinguished. The analysis of the hybrids showed that the rob(14q21q) must have formed postzygotically because it contained a maternal chromosome 14 and a paternal chromosome 21. Furthermore, hybrid analysis of the rea(21q21q) demonstrated two copies of the same chromosome from the mother and thus, by definition, was an isochromosome [i(21q)]. All free-lying chromosomes 21 isolated in hybrids were of maternal origin. These chromosomes may have originated from either of the patient's cell lines. We present four hypotheses for the formation of the two cell lines of this child. This case is part of an ongoing project to determine the mechanism(s) of de novo ROB formation and the results differ from the other de novo rob(14q21q) studied in our laboratory (n = 7) in that all previously studied translocations were maternally derived, leading to the conclusion that most de novo rob(14q21q) occur in oogenesis. The current case illustrates that other mechanisms may contribute to ROB formation.

Robertsonian Translocations: An Overview of 872 Robertsonian Translocations Identified in a Diagnostic Laboratory in China

“…The formation of these mosaic ROBs occurred mainly postzygotically, somehow different from non-mosaic ROBs which usually occurred during meiosis [ 2 , 3 , 13 ]. It was also noticed that different subtypes of mosaic ROBs occurred through different multiple-step mechanisms [ 30 , 31 , 34 ]. Mosaicism with more than one ROBs are extremely rare [ 24 ].…”

Comparison of human prostate specific glandular kallikrein 2 and prostate specific antigen gene expression in prostate with gene amplification and overexpression of prostate specific glandular kallikrein 2 in tumor tissue

“…More than 95% of rea(21q21q) arise de novo [Shaffer et al, 1993]. About 2.7% of cases of Down syndrome are due to mosaicism [Giraud and Mattei, 1975], but mosaicism involving a ROB is rare [Berend et al, 1998].…”

Fragility in the 14q21q translocation region

“…The breakage/reunion model provides a plausible model which explains all observed types of pre-and postzygotically derived Robertsonian translocations: monocentric and dicentric with/without NOR and β-satellite sequences (Berend et al, 1998;Catalan et al, 2000). It does not require (in translocations other than those involving chromosome 14 and either chromosomes 13 or 21) the occurrence of an unusual U-type recombination and is more consistent with the general absence of β-satellite signal within the translocation region of human Robertsonian chromosomes (Cheung et al, 1990;Earle et al, 1992;Gravholt et al, 1992;Wolff and Schwartz, 1992).…”