About 10% of the mutations that are known to cause human monogenic disease are deletions of more than 20 bp (gross deletions). To date, 2127 disease causing gross deletions have been listed in the Human Gene Mutation Database (www.hgmd.org).¹ For only few of these mutations, breakpoints have been determined at the sequence level because this information is often of little relevance in clinical diagnostics. Those gross deletions that have been analysed in greater detail most often show DNA sequences with direct or inverted similarity at 5′ and 3′ breakpoints. The presence of repeated sequences has suggested models that help to explain the formation of these mutations.²

Mutations in the RB1 gene can cause retinoblastoma, a childhood tumour of the eye. We have searched for gross deletions in samples of constitutional and tumour DNA from patients with this tumour. Most of these samples had previously been screened for RB1 gene point mutations but with negative results. Using quantitative multiplex polymerase chain reaction (PCR)³ we identified a spectrum of gross deletions that was heterogeneous with respect to extent and location. In three patients, we found deletions in a region that contains exon 24 of the RB1 gene. Further analysis showed that the 5′ and 3′ deletion breakpoints of these three mutations are located close to each other in an L1HS and MER21B element, respectively.⁴ These two DNA elements belong to different classes of interspersed repetitive DNA. The regions surrounding the 5′ and 3′ breakpoints do not show any sequence similarity. However, they are localised at the borders of strong scaffold/matrix attachment elements that mark the position of recombinogenic DNA structures.

METHODS

Samples

DNA from peripheral blood and from fresh frozen retinoblastoma samples was extracted as described previously.⁵ We investigated constitutional DNA from 60 patients with bilateral or familial …