Complex rearrangements of chromosome 15 in two patients with mild/atypical Prader Willi syndrome.

Summary: Complex rearrangements of chromosome 15 in two patients with MILD/ATYPICAL Prader Willi syndrome: Multiple mechanisms are responsible for the development of Prader Willi syndrome (PWS), the most common genetic cause of obesity in childhood. Molecular findings are usually deletions and uniparental disomy (UPD) of the 15q11-13 region. Rarely, structural rearrangements of the pericentromeric region of chromosome 15 are also detected. Two cases with mild PWS phenotype and complex maternal UPD identified by microsatellite analysis are described: the first patient had uniparental iso and heterodisomy and the second displayed biallelic inheritance and uniparental isodisomy. Key-words: Complex UPD - Prader Willi Syndrome INTRODUCTION Prader-Will syndrome (PWS) is a complex neurogenetic disorder characterized by obesity, neonatal hypotonia, mental retardation, short stature, small hands and feet and hypogonadism. Several genetic mechanisms are implicated in the development of PWS. Deletions in the critical region of the paternal 15q11-13 are responsible for 70% of cases, while maternal uniparental disomy (UPD) accounts for the remaining ones (5). Structural rearrangements involving the critical region of 15q are rarely reported (2). We present the cytogenetic and molecular findings of two patients with PWS due to complex rearrangements of chromosome 15. CLINICAL REPORTS The 1st patient was a 7 month old boy born with normal delivery after an uneventful pregnancy. Birth weight was 2,900 g (25th PC); length 51 cm (50th PC) and head circumference 35 cm (50th PC). The parents were phenotypically normal and consanguinity was not reported. Father was 40 years old and mother 37 years olds. Soon after birth severe hypotonia was noticed without feeding difficulties. Mild dysmorphic features, such as almond shaped eyes, strabismus, high forehead, fish-like mouth and small tapering fingers were present. At the age of 11 months reevaluation revealed psychomotor development of an 8 month old child. The 2nd patient was a boy, the second child of phenotypically normal, non-consanguineous parents, aged 50 (father) and 43 years (mother) respectively. His birth weight was 2,450 g (25th PC), length 53 cm (75th PC) and head circumference 35 cm (50th PC). Soon after birth he was hospitalized because of hypocalcaemia and hypoglycemia. Hypotonia, micrognathia, downslanting palpebral fissures and cryptorchidism were evident. Screening tests for metabolic diseases and myopathies were negative. During early childhood the parents did not mention hyperphagia or obesity. Clinical examination at the age of 16 years showed slight dysmorphic features, weight 70 kg (95th PC), height 170 cm (90th PC), moderate intelligence (IQ=65) and learning difficulties at school. CYTOGENETIC AND MOLECULAR STUDIES Conventional cytogenetic analysis from peripheral blood samples was performed on trypsin-Giemsa banded chromosomes, using standard techniques. Fluorescent in situ hybridization (FISH) with the commercially available locus specific DNA probes SNRPN/PML (Q-BIOgene, France) was applied following the manufacturer's protocol. The methylation status of the SNRPN gene in the critical PWS region was determined by specific PCR (MSPCR), using DNA treated with sodium bisulfate (CpG Genome(TM) Modification Kit, Q-BIOgene) (3). Molecular studies were performed by examining chromosome 15 microsatellite repeats by PCR analysis as previously described (4). Eight polymorphic markers within the PWS/AS critical region were used to identify deletion or UPD in the specific loci and two polymorphic markers near the telomere of chromosome 15 were analyzed for evaluation of UPD. Results were visualized on the Visible Genetics Open Gene(TM) System. Paternity was proved by analysis of markers on chromosomes 5 and 16. RESULTS Conventional cytogenetic analysis of both patients showed a normal male 46, XY karyotype and FISH results revealed normal hybridization patterns for the SNRPN locus. …