ROUTINE SCREENING FOR MICRODELETIONS BY FISH IN 77 PATIENTS SUSPECTEDOF HAVING PRADER-WILLI OR ANGELMAN SYNDROMES USING YAC CLONE 273A2 (D15S10)

About 70% of patients with Prader-Willi syndrome (PWS) and Angelman sy ndrome (AS) have a common interstitial de novo microdeletion encompass ing paternal (PWS) or maternal (AS) loci D15S9 to D15S12. Most of the non-deletion PWS patients and a small number of non-deletion AS patien ts have a maternal or paternal uniparental disomy (UPD)15, respectivel y. Other chromosome 15 rearrangements and a few smaller atypical delet ions? some of the latter being associated with an abnormal methylation pattern, are rarely found. Molecular and fluorescence in situ hybridi zation (FISH) analysis have both been used to diagnose PWS and AS. Her e, we have evaluated, in a typical routine cytogenetic laboratory sett ing, the efficiency of a diagnostic strategy that starts with a FISH d eletion assay using Alu-PCR (polymerase chain reaction)-amplified D1SS 10-positive yeast artificial chromosome (YAC) 273A2. We performed FISH in 77 patients suspected of having PWS (n = 66) or AS (n = 11) and co mpared the results with those from classical cytogenetics and wherever possible with those from DNA analysis. A FISH deletion was found in 1 6/66 patients from the PWS group and in 3/11 patients from the AS grou p. One example of a centromere 15 co-hybridization performed in order to exclude cryptic translocations or inversions is given. Of the PWS p atients, 14 fulfilled Holm's criteria, but two did not. DNA analysis c onfirmed the commmon deletion in all patients screened by the D15S63 m ethylation test and in restriction fragment length polymorphism dosage blots. In 3/58 non-deletion patients, other chromosomal aberrations w ere found. Of the non-deleted group, 27 subjects (24 PWS, 3 AS) were t ested molecularly, and three patients with an uniparental methylation pattern were found in the PWS group. The other 24/27 subjects had neit her a FISH deletion nor uniparental methylation, but two had other cyt ogenetic 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 mutati on-type analysis of patients suspected of having PWS or AS.