Prader-Willi syndrome (PWS; OMIM 176270) was first reported by Prader et al (1) in 1956. As a complicated neurodevelopmental genetic disorder, PWS classically presents with severe hypotonia and feeding difficulties in the neonatal period, sometimes with concurrent anorexia and a sucking deficit (2). Challenged by the difficulties in early diagnosis, care and treatment, the majority of patients gain excessive weight at three or four years of age, and begin to exhibit signs of developmental delay and hypogonadism, eventually leading to severe early obesity, intellectual disability, cognitive impairment, genital hypoplasia and infertility (3,4). PWS is characterized by a sporadic occurrence, which does not discriminate among gender or race, with an estimated prevalence of 1 in 15,000–30,000 live births (5). An interstitial deletion of paternal origin at 15q11.2-q13 may result in the absence of paternal expression of imprinted genes located in this chromosomal region. An imprinting defect (ID) or maternal uniparental disomy (UPD) of chromosome 15 may also be an explanation for the pathogenic mechanism of this disease (6). Due to the severe complications and high mortality of PWS, a multidisciplinary strategy is required to aid early diagnosis and optimize clinical management in order to improve quality of life and prognosis. Pediatricians worldwide are focusing effort on prenatal diagnosis. However, some clinical aspects of PWS in utero, including abnormal extremity positions, fetal hypomobility and polyhydramnios, can only guide PWS diagnosis owing to the lack of high specificity and sensitivity of these features. To date, there have been few reports examining the prenatal diagnosis of PWS using ultrasound or as an accidental finding by cytogenetic molecular techniques (7–9). In this present report, the case of an infant with PWS resulting from complete maternal heterodisomy (hetero-UPD) of chromosome 15 is presented. This case was not identified by fetal ultrasound examination, chromosome karyotype analysis or chromosome microarray (CMA) conducted during the pregnancy, but was identified retrospectively by postnatal diagnosis of the syndrome according to fetal ultrasound findings, postnatal clinical features and molecular genetic investigations.

As a sporadic genetic disorder with remarkable developmental consequences, PWS is usually triggered by a paternal deletion or maternal UPD of the chromosome region 15q11-q13. The current genetic epidemiology is based on western populations, according to which 15q11.2-q13 paternal deletion is responsible for 70–75%, maternal UPD for 25–28%, and ID for 2–5% of the cases of PWS (16–18). However, studies of Asian populations reveal a smaller proportion of PWS arising from maternal UPD than has previously been recognized (5,19–21). Furthermore, research on Western populations also revealed significant discrepancies in both the genotypes and phenotypes in cases of PWS arising from maternal UPD compared to those with chromosomal region deletions, exhibiting fewer typical facial phenotypes, a shorter course of gavage feeding, later onset of hyperphagia, significantly higher birth weights and a lesser degree of hypotonia (22). These milder clinical manifestations among maternal UPD patients are usually related to missed diagnosis, and thus should arouse vigilance among pediatricians.

Widely accepted mechanisms for the development of maternal UPD 15 are as follows: i) Trisomy rescue, triggered by the loss of one copy of chromosome 15 in a trisomic fetus; ii) gamete complementation, formed in a disomic egg fertilized by a nullisomic sperm; and iii) post-zygotic duplication (23,24). Trisomy rescue, the most commonly accepted mechanism, may give rise to a number of outcomes in somatic cells mitosis, leading to hetero-UPD (rescue), a normal cell (rescue) or trisomy (no or incomplete rescue). Incomplete rescue of trisomy 15 may lead to the development of mosaic mutations and chromosome rearrangements, which are associated with cases of PWS with maternal UPD. In this case, complete heterozygosity of the DNA markers implies non-disjunction in the maternal meiosis II, which is also consistent with studies demonstrating that the incidence of chromosome 15 non-disjunction may exponentially increase with maternal age, and that the children of older mothers have an increased rate of maternal UPD for chromosome 15 compared to mothers who are under 35 years of age at delivery (23,25).

Due to the severe complications associated with PWS, it is important to clarify its features so that early diagnoses can be made. At present, PWS is difficult to diagnose prenatally due to a lack of precise and well-characterized fetal phenotypes, which, otherwise, would have provided a basis for further molecular genetic examinations. To date, only a few reports of PWS at the prenatal stage have been published. Most of these reported nonspecific prenatal signs indicative of PWS, and were accidentally identified as trisomy 15 in a routine prenatal examination such as chorionic villus sampling or amniotic fluid culture (26–28), or as a retrospective discovery in postnatal diagnosis according to fetal ultrasound findings and/or postnatal clinical features (7–9,29–33).

Specific prenatal signs are required to allow further molecular genetic examination for PWS. Studies have identified a number of clinical features of the fetus as indicators for PWS, including the abnormal position of feet and toes (9), polyhydramnios (9,31), cerebral anomalies (33), decreased fetal activity (9,34) and hypoplasia of external genitalia (33,35), with the hope that these features can facilitate an early prenatal diagnosis of PWS. However, none of these are representative. In 2008, Bigi et al (9) issued a report in which a possible fetal phenotype was delineated for the first time, including abnormal extremity positions accompanied by reduced fetal movement and polyhydramnios, which are also regarded as informative in the diagnosis of PWS. Further studies followed these, but there remains an absence of detailed prenatal characteristics of fetuses with PWS. Therefore, prenatally-diagnosed cases of PWS described in the literature have been reviewed in the present report and the major characteristics detailed in Table SI (9,26,28–41). A previous report on a total of 28 prenatal cases with PWS suggests that the phenotypes of the fetuses are variable despite the similarities shared by most of the cases. Among the phenotypic features commonly exhibited in prenatal individuals harboring PWS, the top 10 are breech position (6/7, 85.7%), polyhydramnios (13/26, 50%), intrauterine growth restriction (IUGR) (10/26, 38.5%), decreased fetal movement (8/26, 30.8%), facial dysmorphism (5/17, 29.4%), extended legs/feet with flexed toes (5/17, 29.4%), low abdominal circumference (4/16, 25%), low femoral length (4/16, 25%), clenched hands (3/16, 18.8%), hypogonadism (3/16, 18.8%) and low occipital frontal circumference (2/16, 12.5%). Some phenotypic features presented in the present case are consistent with previous reports, including polyhydramnios (9,30,33), breech position (9), clenched hands (33), peculiar position of the extremities (9,31,33) and diminished fetal movement (9). Some of the features, including IUGR and facial dysmorphism, have not been noted or summarized in previous studies to the best of our knowledge. This phenotypic inconsistency was considered to be associated with multiple factors, including the differences in the deletion sizes and affected genes, and even the clinical experience of the obstetricians and ultrasonologists. IUGR and facial dysmorphism, which occur frequently in cases of PWS, should be included as indicators for the prenatal diagnosis of PWS.

It should be noted that all the indicators have a lack of specificity to allow a confirmed prenatal PWS diagnosis. While a prenatal diagnosis solely relying on ultrasound findings seems implausible, nevertheless, when fetal ultrasound examinations detect these indicators, genetic analysis should be considered to diagnose PWS.

A diagnosis of PWS is established in a proband following a DNA methylation analysis demonstrating abnormal parent-specific imprinting within the Prader-Willi critical region on chromosome 15 in which the region demonstrates maternal-only imprinting. Three molecular mechanisms that give rise to PWS include paternal deletion, maternal UPD15 and ID. Optimizing a diagnostic strategy requires clinicians to fully understand the options available for testing for PWS, including the conditions to use these methods, their technical superiorities and limitations, and the cost of the testing (42). For example, DNA methylation analysis is the only technique able to diagnose PWS caused by the three aforementioned genetic mechanisms and to differentiate PWS from AS in cases of deletion (43). Therefore, DNA methylation analysis is regarded as the primary choice for differentiating PWS from other confounders which cannot be identified using laboratory information, before or after birth. Upon the diagnosis of PWS, further testing is required to explore the underlying mechanism, with the purpose of discriminating the individuals with a high recurrence risk from the larger population with a very low recurrence risk (deletions and maternal UPD). The genetic subtypes and testing methods used in the diagnosis of Prader-Willi syndrome are summarized in Table SII. As a universally applied tool for CNV detection, CMA has substituted karyotype analysis and some other specific tests as one of the first choices for patients with developmental/intellectual disorders, autism, congenital abnormalities and other dysmorphic features (44–47). The Affymetrix CytoScan HD array has great value in identifying human chromosomal aberrations due to its broad genomic coverage, and is therefore deemed to be a reliable method capable of specifically detecting 25- to 50-kb copy number changes across the genome with allelic SNP call corroboration. However, although CMA using the CytoScan HD has proved to be successful in identifying maternal UPD patients, including all iso-UPD cases and most of the iso/hetero-UPD cases, it is likely to fail in the identification of complete hetero-UPD. In the case reported in this present study, the diagnosis was missed in the fetal ultrasound examination, chromosome karyotype analysis and CMA conducted during the pregnancy, and identified retrospectively by postnatal examinations according to fetal ultrasound findings, postnatal clinical features, parent-child trio CMA-SNP array and STR based linkage analysis (48).

As far as the present study is concerned, a failure to diagnosis PWS occurred in prenatal ultrasound and routine prenatal CMA examinations due to the technical limitations of these approaches for detecting complete hetero-UPD. It was the postnatal clinical features that allowed a diagnosis of PWS, which was determined by trio CMA analysis and a multiplex-fluorescence-labeled STR assay developed on the basis of linkage analysis as a rapid and economic detection method for chromosomal region deletion or maternal UPD15. It is now important to determine which approach to the prenatal testing of PWS is sufficient enough to make confirmed diagnoses. Certain clinical features of the fetus, including anomalous extremity positions and subdued fetal movement, combined with polyhydramnios, could be considered indicators of the need for further testing to allow the early prenatal diagnosis of PWS owing to the high potential risk of PWS and the technical limitation of CMA. If clinical signs strongly suggest PWS (abnormal ultrasonic results and advanced maternal age), methylation and/or UPD analysis is highly recommendable.