Non-syndromic mental retardation segregating with an apparently balanced t(1;17) reciprocal translocation through three generations

Genome architecture catalyzes nonrecurrent chromosomal rearrangements

To investigate the potential involvement of genome architecture in nonrecurrent chromosome rearrangements, we analyzed the breakpoints of eight translocations and 18 unusual-sized deletions involving human proximal 17p. Surprisingly, we found that many deletion breakpoints occurred in low-copy repeats (LCRs); 13 were associated with novel large LCR17p structures, and 2 mapped within an LCR sequence (middle SMS-REP) within the Smith-Magenis syndrome (SMS) common deletion. Three translocation breakpoints involving 17p11 were found to be located within the centromeric alpha-satellite sequence D17Z1, three within a pericentromeric segment, and one at the distal SMS-REP. Remarkably, our analysis reveals that LCRs constitute >23% of the analyzed genome sequence in proximal 17p--an experimental observation two- to fourfold higher than predictions based on virtual analysis of the genome. Our data demonstrate that higher-order genomic architecture involving LCRs plays a significant role not only in recurrent chromosome rearrangements but also in translocations and unusual-sized deletions involving 17p.

Characterization of partial trisomy 9p due to insertional translocation by chromosomal (micro)FISH

We describe a family with an insertion 12;9 translocation occurring in a balanced form in a mother and two sons, but in an unbalanced form in the proband, resulting in trisomy of chromosome region 9p22-->9p24. The proband manifests typical features of trisomy 9p; the clinical signs were mental and growth retardation, microcephaly, epicanthus, low-set ears, micrognathia, clinodactyly and hypoplastic phalanges of the fifth fingers, hypoplasia or absence of toenails, and extremely small genitals. The GTG-banded findings were confirmed using (micro)FISH. Intriguingly, the mother and the two carrier sons exhibited major learning difficulties that were not present in the non-carrier sister of the mother: this may be due to a gene disruption or induction of abnormal expression. Dysmorphic features were not present in the three carriers. We compare our clinical and cytogenetic findings with other cases of partial trisomy 9p reported in the literature.