Chronic intestinal pseudo-obstruction syndrome and gastrointestinal malrotation in an infantwith schaaf-yang syndrome - Expanding the phenotypic spectrum

MAGEL2 (patho-)physiology and Schaaf-Yang syndrome

Schaaf-Yang syndrome (SYS) is a complex neurodevelopmental disorder characterized by autism spectrum disorder, joint contractures, and profound hypothalamic dysfunction. SYS is caused by variants in MAGEL2, a gene within the Prader-Willi syndrome (PWS) locus on chromosome 15. In this review, we consolidate decades of research on MAGEL2 to elucidate its physiological functions. Moreover, we synthesize current knowledge on SYS, suggesting that while MAGEL2 loss-of-function seems to underlie several SYS and PWS phenotypes, additional pathomechanisms probably contribute to the distinct and severe phenotype observed in SYS. In addition, we highlight recent therapeutic advances and identify promising avenues for future investigation.

Advancing in Schaaf-Yang syndrome pathophysiology: from bedside to subcellular analyses of truncated MAGEL2

BACKGROUND

Schaaf-Yang syndrome (SYS) is caused by truncating mutations in MAGEL2, mapping to the Prader-Willi region (15q11-q13), with an observed phenotype partially overlapping that of Prader-Willi syndrome. MAGEL2 plays a role in retrograde transport and protein recycling regulation. Our aim is to contribute to the characterisation of SYS pathophysiology at clinical, genetic and molecular levels.

METHODS

We performed an extensive phenotypic and mutational revision of previously reported patients with SYS. We analysed the secretion levels of amyloid-β 1-40 peptide (Aβ_1-40) and performed targeted metabolomic and transcriptomic profiles in fibroblasts of patients with SYS (n=7) compared with controls (n=11). We also transfected cell lines with vectors encoding wild-type (WT) or mutated MAGEL2 to assess stability and subcellular localisation of the truncated protein.

RESULTS

Functional studies show significantly decreased levels of secreted Aβ_1-40 and intracellular glutamine in SYS fibroblasts compared with WT. We also identified 132 differentially expressed genes, including non-coding RNAs (ncRNAs) such as HOTAIR, and many of them related to developmental processes and mitotic mechanisms. The truncated form of MAGEL2 displayed a stability similar to the WT but it was significantly switched to the nucleus, compared with a mainly cytoplasmic distribution of the WT MAGEL2. Based on the updated knowledge, we offer guidelines for the clinical management of patients with SYS.

CONCLUSION

A truncated MAGEL2 protein is stable and localises mainly in the nucleus, where it might exert a pathogenic neomorphic effect. Aβ_1-40 secretion levels and HOTAIR mRNA levels might be promising biomarkers for SYS. Our findings may improve SYS understanding and clinical management.

The N-terminal domain of the Schaaf-Yang syndrome protein MAGEL2 likely has a role in RNA metabolism

MAGEL2 encodes the L2 member of the melanoma-associated antigen gene (MAGE) protein family, truncating mutations of which can cause Schaaf-Yang syndrome, an autism spectrum disorder. MAGEL2 is also inactivated in Prader-Willi syndrome, which overlaps clinically and mechanistically with Schaaf-Yang syndrome. Studies to date have only investigated the C-terminal portion of the MAGEL2 protein, containing the MAGE homology domain that interacts with RING-E3 ubiquitin ligases and deubiquitinases to form protein complexes that modify protein ubiquitination. In contrast, the N-terminal portion of the MAGEL2 protein has never been studied. Here, we find that MAGEL2 has a low-complexity intrinsically disordered N-terminus rich in Pro-X_n-Gly motifs that is predicted to mediate liquid-liquid phase separation to form biomolecular condensates. We used proximity-dependent biotin identification (BioID) and liquid chromatography-tandem mass spectrometry to identify MAGEL2-proximal proteins, then clustered these proteins into functional networks. We determined that coding mutations analogous to disruptive mutations in other MAGE proteins alter these networks in biologically relevant ways. Proteins identified as proximal to the N-terminal portion of MAGEL2 are primarily involved in mRNA metabolic processes and include three mRNA N 6-methyladenosine (m⁶A)-binding YTHDF proteins and two RNA interference-mediating TNRC6 proteins. We found that YTHDF2 coimmunoprecipitates with MAGEL2, and coexpression of MAGEL2 reduces the nuclear accumulation of YTHDF2 after heat shock. We suggest that the N-terminal region of MAGEL2 may have a role in RNA metabolism and in particular the regulation of mRNAs modified by m⁶A methylation. These results provide mechanistic insight into pathogenic MAGEL2 mutations associated with Schaaf-Yang syndrome and related disorders.

Diagnosis of Schaaf-Yang syndrome in Korean children with developmental delay and hypotonia

Schaaf-Yang syndrome (SYS) is a recently identified disorder caused by a loss-of-function mutation in a maternally imprinted gene, MAGEL2, at 15q11.2q13. Due to its extreme rarity and wide range of clinical severity, clinical suspicion is difficult for a physician. In the current study, its frequency among the Korean pediatric patients with developmental delay (DD) or intellectual disability (ID) was assessed. As the first report of Korean patients with SYS, our study aims to increase the awareness of this condition among the physicians taking care of the pediatric patients with DD/ID and hypotonia.The patients diagnosed with SYS by whole-exome sequencing (WES) among the 460 Korean pediatric patients with DD/ID were included, and their clinical and molecular features were reviewed.Four patients (0.9%) were diagnosed with SYS. Profound DD (4 patients), multiple anomalies including joint contractures and facial dysmorphism (4 patients), generalized hypotonia (3 patients), and severe respiratory difficulty requiring mechanical ventilation (3 patients) were noted in most cases, similar to those in previous reports. Sleep apnea (2 patients), autistic features (2 patients), a high grade of gastroesophageal reflux (1 patient), and seizures (1 patient) were found as well. A total of 3 different truncating MAGEL2 mutations were identified. A previously-reported mutation, to be the most common one, c.1996dupC, was found in 2 patients. The other 2 mutations, c.2217delC and c.3449_3450delTT were novel mutations. As MAGEL2 is maternally imprinted, 2 patients had inherited the MAGEL2 mutation from their respective healthy fathers.SYS is an extremely rare cause of DD/ID. However, hypotonia, joint contractures, profound DD/ID and facial dysmorphism are the suggestive clinical features for SYS. As a maternally imprinted disorder, it should be reminded that SYS may be inherited in form of a mutation from a healthy father.

MAGEL2-related disorders: A study and case series

Pathogenic MAGEL2 variants result in the phenotypes of Chitayat-Hall syndrome (CHS), Schaaf-Yang syndrome (SYS) and Prader-Willi syndrome (PWS). We present five patients with mutations in MAGEL2, including the first patient reported with a missense variant, adding to the limited literature. Further, we performed a systematic review of the CHS and SYS literature, assess the overlap between CHS, SYS and PWS, and analyze genotype-phenotype correlations among them. We conclude that there is neither a clinical nor etiological difference between CHS and SYS, and propose that the two syndromes simply be referred to as MAGEL2-related disorders.

A Recurrent Variant in MAGEL2 in Five Siblings with Severe Respiratory Disturbance after Birth

Schaaf-Yang syndrome (SHFYNG) is caused by truncating mutations in the paternal allele of the MAGEL2 gene located in the Prader-Willi syndrome region. We report 5 newborns affected with SHFYNG in one family. Trio exome analysis revealed a heterozygous c.1996dupC frameshift mutation in MAGEL2 inherited from the unaffected father. The phenotypes showed strong resemblance, especially for severe respiratory disturbance requiring mechanical ventilation at birth. After discharge from the hospital, 4 of the patients died of respiratory insufficiency within 1 or 2 weeks after birth, and 1 child died after 110 days of aggravated apnea. Apnea or respiratory failure was the main cause of early death in this family. Respiratory distress is a common manifestation of SHFYNG, especially in patients with c.1996dupC mutations. Hypotonia is a main cause of respiratory disturbance, and we propose another possible cause affecting the respiratory center of the brain.

Schaaf-Yang syndrome overview: Report of 78 individuals

Schaaf-Yang Syndrome (SYS) is a genetic disorder caused by truncating pathogenic variants in the paternal allele of the maternally imprinted, paternally expressed gene MAGEL2, located in the Prader-Willi critical region 15q11-15q13. SYS is a neurodevelopmental disorder that has clinical overlap with Prader-Willi Syndrome in the initial stages of life but becomes increasingly distinct throughout childhood and adolescence. Here, we describe the phenotype of an international cohort of 78 patients with nonsense or frameshift mutations in MAGEL2. This cohort includes 43 individuals that have been reported previously, as well as 35 newly identified individuals with confirmed pathogenic genetic variants. We emphasize that intellectual disability/developmental delay, autism spectrum disorder, neonatal hypotonia, infantile feeding problems, and distal joint contractures are the most consistently shared features of patients with SYS. Our results also indicate that there is a marked prevalence of infantile respiratory distress, gastroesophageal reflux, chronic constipation, skeletal abnormalities, sleep apnea, and temperature instability. While there are many shared features, patients with SYS are characterized by a wide phenotypic spectrum, including a variable degree of intellectual disability, language development, and motor milestones. Our results indicate that the variation in phenotypic severity may depend on the specific location of the truncating mutation, suggestive of a genotype-phenotype association. This evidence may be useful in both prenatal and pediatric genetic counseling.

The role of obesity in the fatal outcome of Schaaf-Yang syndrome: Early onset morbid obesity in a patient with a MAGEL2 mutation

Schaaf-Yang syndrome (SYS) was recently identified as a genetic condition resembling Prader-Willi syndrome. It is caused by mutations on the paternal allele of the MAGEL2 gene, a gene that has been mapped in the Prader-Willi critical region. Here, we present an infant with SYS who sadly died because of the combination of hypotonia, sleep apnea, and obesity. A heterozygous premature stop mutation in MAGEL2 was identified in the patient. The main factors reported in the mortality of SYS are lethal arthrogryposis multiplex congenita, fetal akinesia, and pulmonary problems. Our clinical report indicates that obesity and its complications are an important additional factor in the mortality associated with SYS. Therefore, we advise to strictly monitor weight and intensively treat overweight and obesity in SYS.