Prader-Willi syndrome (PWS) is clinically characterized by severe hypotonia and feeding difficulties in early infancy, followed by excessive eating and development of morbid obesity in later infancy or early childhood. Cognitive impairment is seen in almost all individuals but varies in severity. A behavioral phenotype with temper tantrums, stubbornness, manipulative behavior and obsessive compulsive disorder is common. Hypogonadism is present in both males and females, and is seen as genital hypoplasia, incomplete pubertal development; and most individuals are infertile. Short stature, and small hands and feet are common features. Characteristic facial features, strabismus and scoliosis are often present. Clinical diagnostic criteria for PWS have been developed (Holm et al 1993; Gunay-Aygun et al 2001); however, confirmation of the clinical diagnosis with molecular genetic testing is required.
The 15q11-q13 chromosomal region contains imprinted genes, some of which are exclusively expressed from either of the parental alleles (Figure 1). Paternally expressed genes are: MKRN3, MAGEL2, NDN, PWRN1, C15orf2, SNURF–SNRPN and several snoRNA genes (SNORD64, SNORD107, SNORD108, SNORD109A, SNORD109B, SNORD115 and SNORD116). SNORD115 and SNORD116 are present in 47 and 24 gene copies, respectively, while the other snoRNA genes are single copy genes. Deficiency of SNORD116 causes the key characteristics of the PWS phenotype (Sahoo T et al. 2008; de Smith et al. 2009). PWS is caused by lack of expression of the paternally contributed 15q11-q13 genes, while lack of expression of maternally contributed 15q11-q13 genes causes Angelman syndrome (AS OMIM 105830).
PWS can be due to a paternally derived de novo deletion of 15q11-q13, maternal uniparental disomy (UPD) of chromosome 15 or an imprinting defect. Common breakpoints for the deletions exist, where the deletion typically extends from distal breakpoint BP3 to one of the two proximal breakpoints BP1 or BP2, but unique deletions have also been observed (Kim et al. 2012). Deletions can be associated with chromosomal rearrangements, but this is very rare. Both maternal heterodisomy and isodisomy of chromosome 15 have been observed. In rare cases a parental translocation or marker chromosome can predispose to maternal UPD. Imprinting defects can either be due to primary epimutations without DNA sequence alterations or it can be due to small deletions in the imprinting centre (IC) critical region (PWS-SRO) (Buiting et al. 1995). It is important to establish the underlying genetic mechanism as the recurrence risks depend on this. Prenatal diagnosis is possible for pregnancies with increased risk if the genetic mechanism is known. A summary of genetic mechanisms with frequencies and the recurrence risks are given in table 1.
Guidelines for molecular analysis of Prader-Willi syndrome are available (Ramsden et al. 2010). DNA methylation analysis will diagnose PWS in >99% of cases. Most diagnostic laboratories in Europe now use MS-MLPA as method of choice. MS-MLPA will detect both methylation abnormalities and a deletion if present. For individuals with methylation abnormalities but without a deletion, microsatellite analysis using polymorphic markers will distinguish between UPD and another defect. If microsatellite analysis shows biparental inheritance of chromosome 15, analysis of the IC region should be performed to distinguish between an isolated epimutation and a deletion in the IC.
Some laboratories perform MS-PCR for methylation analysis. If abnormal methylation is detected FISH analysis or MLPA analysis is required to test for presence of a deletion. If no deletion is present the testing strategy is similar as that described for MS-MLPA.
In cases of de novo deletions the father should be further investigated to rule out paternal chromosomal rearrangements.
PWS should be considered in a newborn with hypotonia and poor suck. Later in childhood, other symptoms such as developmental delay and excessive eating with central obesity (and with a history of hypotonia and poor suck) should lead to suspicion of PWS. From adolescence and in adulthood additional symptoms such as mild intellectual disability, hypothalamic hypogonadism and typical behavioral phenotype should lead to testing for PWS. An early diagnosis of PWS is important for proper management of the disease, which includes (but are not restricted to) strict supervision of food intake and in some cases growth hormone replacement.
Table 1. Frequencies of (epi)genetic mechanisms and recurrence risks.
Figure 1: Genetic map of 15q11.2-q13 region.
Genes in blue are expressed only from the paternal allele, genes in red only from the maternal allele and genes in green show biallelic expression. SNORD116 and SNORD115 are present in more copies than indicated on the figure. The imprinting center (IC) is shown below as a horizontal line located between C15orf2 and SNURF-SNRPN, PWS-SRO in blue and AS-SRO in red. BP1 to BP5 indicated positions of breakpoints. The bottom solid black lines indicate extension of class I and class II deletions, respectively. Only rarely, do the deletions extend to BP4 or BP5. Not all genes with biallelic expression are shown on the map. The figure is not drawn to scale.
Figure 2: Testing strategy of PWS. MS-MLPA or MS_PCR is the two preferred strategies used. Blue colored circles show either confirmation or discarding of PWS diagnosis. Pink circles show determination of the genetic mechanism. *Testing with FISH or MLPA is only necessary if initial analysis method was MS-PCR.
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