Cytogenetic investigation
Cytogenetic analysis revealed a normal karyotype in 59 patients, a visible cytogenetic deletion in the region 15q11-13 in 3 cases and a Robertsonian translocation 45 XY, t (15; 15) in 1 case.

Molecular investigation and fluorescent in situ hybridisation analysis
Molecular investigation of patients using PCR analysis confirmed the diagnosis of PWS or AS with the detection of absent paternal or maternal alleles, respectively. Schematic presentations of molecular results are shown in Figures 1 and 2.  Diagnosis was confirmed in 39/114 of referrals for PWS (34%) and revealed 28 deletions (72%) and 11 cases with uniparental disomy (28%). Uniparental disomy was observed also in the patient with the 45XY, t (15; 15) karyotype. Most of the typical PWS patients (28/40) had a molecular alteration (70%), while molecular findings were also observed in 11% of atypical cases (7/62). 

Table 1. Laboratory findings in Prader Willi and Angelman syndrome patients.

Table 2. Clinical characteristics of 40 patients with a typical Prader Willi syndrome (PWS) phenotype and 32 patients with a typical Angelman syndrome (AS) phenotype.

Table 3. Correlation between phenotype and genotype in Prader Willi (PWS) and Angelman syndrome (AS) patients.

^a Includes deletions, UPD.

Diagnosis was confirmed in 18/57 of referrals for AS examination (32%) and all patients had deletions. Deletions were observed in 56% of the patients with typical AS presentation. No deletions were found in patients with atypical presentation.

PWS patients No 118 and No 119, identical twins with typical clinical presentation, had a large deletion encompassing the whole 15q11-13 region except the S543 locus. Patients No 77 and No 78 were also identical twins with typical AS and deletions limited to the S543 locus.
FISH analysis using the SNRPN probe revealed two hybridization signals in 33 cases, which were not informative by PCR analysis, including the case of 45XY, t (15; 15). In 1 patient in whom molecular investigation was not informative, only one signal was observed, indicating a deletion for the SNRPN gene.