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Chromosome 15q11-q13 is a region that harbors several genes regulated by genomic imprinting, a phenomenon in which genes are expressed preferentially from one parental allele. As a result, genes subject to regulation by genomic imprinting are functionally haploid, having only a single functional copy [1]. Prader-Willi syndrome (PWS) and Angelman syndromes (AS) are 2 clinically distinct disorders associated with multiple anomalies and intellectual disability (ID), typically resulting from deletion of the paternal for PWS or maternal for AS copies of the chromosome region 15q11-q13. Each one occurs with a frequency of approximately 1/15,000 to 1/30,000 live births [2]. The deletion of 15q11.2-q13 locus is probably one of the most common pathogenic deletions observed in humans. There are 4 known genetic mechanisms of PWS and AS: de novo maternal or paternal deletions involving chromosome region 15q11.2-q13, uniparental disomy (UPD) of 15q11.2-q13, mutations in the imprinting center (IC) and causing specifically AS UBE3A gene point mutations. PWS and AS should be differentiated from disorders caused by other microdeletions, duplications and point mutations in genes associated with ID. UPD confirmation thus requires genotyping the patient and both parents, commonly performed by STR analysis which involves PCR amplification of polymorphic short tandem repeat loci [3]. Materials and Methods.Group of 60 patients with PWS phenotype (18 females and 42 males) and 32 patients with AS phenotype (21 and 11 respectively) has been collected. To identify methylation status of the SNRPN gene all collected samples were analyzed by methylation-specific PCR (MS-PCR). For UPD15 detection we performed a linkage analysis of the nuclear families using STR markers D15S817, D15S1513, D15S822, D15S1002 genotyping. The STR markers were selected according to their high heterozygosity and location in the typical deletion region adjacent of PWS and AS loci. Results. In the analyzed group of patients with PWS phenotype the absence of unmethylated MS-PCR product was detected in 25 patients (42%). In AS group the absence of methylated allele was detected 9 patients (28%). DNA samples of both parents were available for 10patients (probands) with PWS phenotype. Five of the patients had both methylated and unmethylated MS-PCR product. And other 5 probands showed an absence of unmethylatedallele, for those families the linkage analysis of chosen STR-markerswas performed. In 3 patients the paternal region deletion in the 15q11-q13 chromosome was identified. Two patients had maternal UPD15 (table 1).

DIFFERNTIAL DIAGNOSIS OF DELETION AND UNIPARENTAL DISOMY IN 15Q11.2-Q13 LOCUS