Routine screening for microdeletions by FISH in 77 patients suspected of having Prader-Willi or Angelman syndromes using YAC clone 273A2 (D15S10).

About 70% of patients with Prader-Willi syndrome (PWS) and Angelman syndrome (AS) have a common interstitial de novo microdeletion encompassing paternal (PWS) or maternal (AS) loci D15S9 to D15S12. Most of the non-deletion PWS patients and a small number of non-deletion AS patients have a maternal or paternal uniparental disomy (UPD) 15, respectively. Other chromosome 15 rearrangements and a few smaller atypical deletions, some of the latter being associated with an abnormal methylation pattern, are rarely found. Molecular and fluorescence in situ hybridization (FISH) analysis have both been used to diagnose PWS and AS. Here, we have evaluated, in a typical routine cytogenetic laboratory setting, the efficiency of a diagnostic strategy that starts with a FISH deletion assay using Alu-PCR (polymerase chain reaction)-amplified D15S10-positive yeast artificial chromosome (YAC) 273A2. We performed FISH in 77 patients suspected of having PWS (n = 66) or AS (n = 11) and compared the results with those from classical cytogenetics and wherever possible with those from DNA analysis. A FISH deletion was found in 16/66 patients from the PWS group and in 3/11 patients from the AS group. One example of a centromere 15 co-hybridization performed in order to exclude cryptic translocations or inversions is given. Of the PWS patients, 14 fulfilled Holm's criteria, but two did not. DNA analysis confirmed the common deletion in all patients screened by the D15S63 methylation test and in restriction fragment length polymorphism dosage blots. In 3/58 non-deletion patients, other chromosomal aberrations were found. Of the non-deleted group, 27 subjects (24 PWS, 3 AS) were tested molecularly, and three patients with an uniparental methylation pattern were found in the PWS group. The other 24/27 subjects had neither a FISH deletion nor uniparental methylation, but two had other cytogenetic aberrations. Given that cytogenetic analysis is indispensable in most patients, we find that the FISH deletion assay with YAC 273A2 is an efficient first step for stepwise diagnostic testing and mutation-type analysis of patients suspected of having PWS or AS.