Molecular characterization of deletion breakpoints in adults with 22q11 deletion syndrome

Abstract

22q11 Deletion syndrome (22q11DS) is a common microdeletion syndrome with variable expression, including congenital and later onset conditions such as schizophrenia. Most studies indicate that expression does not appear to be related to length of the deletion but there is limited information on the endpoints of even the common deletion breakpoint regions in adults. We used a real-time quantitative PCR (qPCR) approach to fine map 22q11.2 deletions in 44 adults with 22q11DS, 22 with schizophrenia (SZ; 12 M, 10 F; mean age 35.7 SD 8.0 years) and 22 with no history of psychosis (NP; 8 M, 14 F; mean age 27.1 SD 8.6 years). QPCR data were consistent with clinical FISH results using the TUPLE1 or N25 probes. Two subjects (one SZ, one NP) negative for clinical FISH had atypical 22q11.2 deletions confirmed by FISH using the RP11-138C22 probe. Most (n = 34; 18 SZ, 16 NP) subjects shared a common 3 Mb hemizygous 22q11.2 deletion. However, eight subjects showed breakpoint variability: a more telomeric proximal breakpoint (n = 2), or more centromeric (n = 3) or more telomeric distal breakpoint (n = 3). One NP subject had a proximal nested 1.4 Mb deletion. COMT and TBX1 were deleted in all 44 subjects, and PRODH in 40 subjects (19 SZ, 21 NP). The results delineate proximal and distal breakpoint variants in 22q11DS. Neither deletion extent nor PRODH haploinsufficiency appeared to explain the clinical expression of schizophrenia in the present study. Further studies are needed to elucidate the molecular basis of schizophrenia and clinical heterogeneity in 22q11DS.

Fig. 1

22q11.2 deletion size and breakpoints in 44 adults with 22q11DS. Solid red arrowheads indicate the 21 qPCR screening markers used. Additional 15 qPCR markers were used to further delineate breakpoint regions in selected subjects (ID = 1, 23, 24). FISH probes used to validate deletion status are indicated by stars as follows: purple N25, blue Tuple1, and red RP11-138C22. Green boxes represent the four low copy repeat regions located in 22q11.2 (http://www.projects.tcag.ca/humandup/). Solid blue horizontal lines indicate the extent of 22q11.2 deletions tested by qPCR. Dotted blue lines represent the unknown interval between markers where the actual breakpoints reside. Broken red vertical lines show the positions of the seven breakpoint regions

Fig. 2

FISH analysis using the RP11-138C22 probe to confirm an atypical 22q11.2 deletion. Fluoresence in situ hybridization (FISH) validation of the RP11-138C22 probe to detect atypical 22q11DS deletions. A control individual (a) showing two copies of chromosome 22q11 using the RP11-138C22 probe. Individuals 1 (b) and 23 (c) are negative by FISH analysis for the TUPLE1 probe but are positive for the deletion using the RP11-138C22 probe due to the extent of their deletion, which was confirmed by qPCR analysis (Fig. 1)