Genetic subtypes and phenotypic characteristics of 110 patients with Prader-Willi syndrome

Prader-Willi syndrome (PWS; OMIM#176,270) is a complex genetic disorder caused by lacking paternal expression of imprinted genes on chromosome 15q11-q13. The estimated prevalence of PWS is 1/10,000–30,000 live births [1]. PWS has several genetic subtypes: paternal deletions (65–75%), maternal uniparental disomy (UPD) (20–30%), imprinting center defects (1–3%) and other rare cases of paternal chromosome 15 translocation (0.1%) [2]. Five common breakpoint regions (BP1-BP5) have been identified along chromosome 15q11-q13 [3]. The paternal deletions of PWS are usually recurrent involving BP1-BP3 (type I) and BP2-BP3 (type II), and sometimes atypical with smaller or larger deletion sizes [4‐6]. Maternal UPD happens due to meiosis errors during female gametogenesis: isodisomy is caused by meiosis II error and later monosomy rescue; heterodisomy is caused by meiosis I error without crossover events; and mixed UPD is related to meiosis I error with crossover events [7, 8].

PWS is a multisystemic condition with phenotype changing dramatically with age. Infantile hypotonia is characteristic, demonstrating weak cry, poor suck, and failure to thrive. Male cryptorchidism is frequent owing to hypogonadism [9]. In early childhood, they gradually exhibit excessive appetite and obesity if unrestricted from food [10]. Developmental delay and/or intellectual disability (DD/ID) are presented as delayed motor/language skills and learning problems [1]. Behavior and psychological problems are common including stubbornness, compulsions, temper tantrums, skin picking, anxiety, and autistic traits [11]. Short stature and small hands/feet are caused by growth hormone deficiency [12]. Hypothalamic dysfunction underlies many features including hyperphagia, temperature instability, pituitary hormone deficiency, high pain threshold, and sleep apnea [13]. Dysmorphic faces and hypopigmentation are also common in PWS patients [2]. To date, phenotypic disparities have been reported between the UPD and deletion types of PWS, showing inconsistent findings. The differences within the UPD and deletion subtypes remain less known.

In this study we presented the overall data of 110 cases with PWS diagnosed from 8,572 pediatric patients in Northeast China. The phenotypic characters were evaluated and compared among different genetic subtypes to explore possible associations. The effect of growth hormone (GH) treatment on 21 cases was also evaluated.

Imprinted genes are regulated by methylation for differential expression according to the parental origin. MS-MLPA has great advantage in the diagnosis of imprinted disorders by detecting both copy number and methylation ratio [6, 21]. In the present study, a high diagnostic rate of 7.70% by ME028 was obtained from 662 individuals with suspected PWS features including neonatal hypotonia with feeding problems, male cryptorchidism, short stature with small hands/feet, obesity with hyperphagia and extreme food seeking. These PWS features deserve high attention of pediatricians, and MS-MLPA was recommended as first-tier molecular technique for rapid diagnosis of PWS. The proportion of maternal UPD was 37.25% in our study, which was higher than the conventional 20–30% [2], and similar to 36.27% in a recent large multisite cohort study [22]. This might be due to higher maternal age nowadays or lower diagnostic rate of UPD previously. In the 7,910 individuals with unexplained DD/ID, MLPA P245 screening was cost-effective to identify multiple microdeletion syndromes at one time. The diagnostic yield for 15q11.2 deletions by P245 was 1.25% in our study, which was lower than 2.07% in 10,026 Chinese pediatric patients with developmental disorders by chromosomal microarray [23], but much higher than 0.14% in 29,085 Western children with unexplained developmental delay by array comparative genomic hybridization [24]. The differences might come from the race, inclusion criteria, or detection methods. The diagnostic rate of paternal deletion PWS was 0.75% by P245-ME028 assays, with the maternal UPD PWS missed out in this group.

The probes of ME028 encompassing BP1 to distal BP3 facilitated the subclassification of paternal deletion PWS. Those atypical deletions exceeding distal BP3 were delineated by CNV-sequencing. The 91 PWS cases with paternal deletions were subdivided into 31.87% of type I, 61.54% of type II and 6.59% of atypical deletions. Limited researches demonstrated the subtypes of paternal deletion PWS. A multisite cohort study including 303 deletion PWS patients (primarily Caucasians) showed 38.9% of type I, 54.5% of type II and 6.6% of atypical deletions [22]. Our data are comparatively lower in type I and higher in type II. More researches in large scale are needed to refine the ratios. PWS with maternal UPD could be subgrouped based on the meiosis errors and crossover events [7, 8]. In the present study by microsatellite genotyping, we found 57.89% of heterodisomy and 42.11% of isodisomy at 15q11-q13 region, corresponding to meiosis I and II errors respectively. This added to the currently less known data about the subtype ratio of PWS with maternal UPD. The mixed form of UPD induced by crossover events at distal 15q was not evaluated.

The clinical characters were evaluated and compared among the PWS genetic subtypes. Overall, the universal phenotype included neonatal hypotonia, feeding problem, motor delay, learning disabilities, and male cryptorchidism. Genital hypoplasia in females was easily overlooked and not assessed. Studies on various populations have compared phenotype between the UPD and deletion types of PWS, showing disparities including maternal age, hypopigmentation, birth length, IQ score, self-injury, psychotic illness, pain threshold and sleeping disorders [25‐29]. Our UPD patients had higher maternal age, higher frequencies of anxiety and autistic traits, and lower frequencies of hypopigmentation and skin picking than the deletion patients. Other previously indicated differences were not found including higher diagnosis age in UPD, or more frequent feeding problems, temperature instabilities, sleeping disorders, dysmorphic faces in the deletion group. Our type I patients showed earlier diagnosis age and higher frequency of speech delay, indicating more severe symptoms in response to larger deletion regions than type II. We did not found behavioral differences between type I and type II groups, whereas a previous study on 12 type I and 14 type II PWS individuals showed worse adaptive and more common obsessive–compulsive behaviors in the type I group [29]. Phenotypic variations between heterodisomy and isodisomy in the UPD group are rarely reported. Some individual cases have been reported about the concomitant autosomal recessive disorders of ichthyosis and Tay-Sachs disease caused by pathogenic variants in the homozygosity region [30, 31]. We did not find additional phenotype in the UPD group. A relatively higher frequency of anxiety was observed in the maternal chr 15 isodisomy than heterodisomy group. This has not been reported before, and needs confirmation in more PWS individuals with maternal UPD.

Patients with atypical deletions are informative for understanding the function of distinct deletion regions. Many hallmark traits of PWS were positive in patient 1 retaining the MKRN3, MAGEL2 and NDN genes compared with type II patients, indicating the SNRPN-SNORD regions were sufficient for PWS pathogenesis. Patient 2 differed to patient 3 in the OCA2 gene which correlated to the hypopigmentation phenotype. The 15q11.2 BP1–BP2 microdeletion of the NIPA1, NIPA2, CYFIP1, and TUBGCP5 genes causes Burnside-Butler syndrome with abnormalities in brain morphology, behavior, and cognition [32]. Patient 2 and patient 3 with partial deletion of BP1–BP2 (NIPA1 retained and TUBGCP5 deleted) were indistinguishable to the majority of PWS patients. Previous reports of expanded deletion in PWS demonstrated cardiovascular, renal or other congenital anomalies [33]. The three patients with expanded deletions to distal BP4 and BP5 displayed severe phenotype including tube feeding and absence of speech, without structural anomalies.

The benefits of GH treatment have been established on body composition, psychomotor development, cognition, adaptive functioning, and linear growth, without adverse effects on glucose parameters, lipid profile and blood pressure [34]. In the present cohort, 19.09% of PWS patients received GH treatment for more than one year, and with complete laboratory data. We did not fragment these patients into subtypes. Significant improvements were found in body height and serum IGF-I. Central hypothyroidism has been found in very young patients with PWS, and improves with age [35]. Our data of TSH, fT4, and fT3 were within reference ranges before and after GH treatment, with no obvious changes. The glucose and lipid metabolic parameters were also within reference ranges. These possibly due to the relatively early nutritional phases of the patients.

This study provided detailed genetic and phenotypic characters of 110 PWS patients diagnosed from 8,572 Chinese pediatric individuals. We added more data about the phenotypic associations with various genetic subtypes, which help to promote awareness of this complex neurodevelopmental disorder.