Coupling clinical exome sequencing with functional characterization studies to diagnose a patient with familial Mediterranean fever and MED13L haploinsufficiency syndromes

MED13L-related intellectual disability due to paternal germinal mosaicism

The MED13L-related intellectual disability or MRFACD syndrome (Mental retardation and distinctive facial features with or without cardiac defects; MIM # 616789) is one of the most common forms of syndromic intellectual disability with about a hundred cases reported so far. Affected individuals share overlapping features comprising intellectual disability, hypotonia, motor delay, remarkable speech delay, and a recognizable facial gestalt. De novo disruption of the MED13L gene by deletions, duplications, or sequence variants has been identified as deleterious. Siblings affected by intragenic deletion transmitted from a mosaic parent have been reported once in the literature. We now present the first case of paternal germinal mosaicism for a missense MED13L variant causing MRFACD syndrome in one of the father's children and being the likely cause of intellectual disability and facial dysmorphism in the other. As part of the Mediator complex, the MED proteins have an essential role in regulating transcription. Thirty-two subunits of the Mediator complex genes have been linked to congenital malformations that are now acknowledged as transcriptomopathies. The MRFACD syndrome has been suggested to represent a recognizable phenotype.

Performance of meta-predictors for the classification of MED13L missense variations, implication of raw parameters

MED13L syndrome is a rare congenital disorder comprising moderate intellectual disability, hypotonia and facial dysmorphism. Whole exome or genome sequencing in patients with non-specific neurodevelopmental disorders leads to identification of an increasing number of MED13L missense variations of unknown signification. The aim of our study was to identify relevant annotation parameters enhancing discrimination between candidate pathogenic or neutral missense variations, and to assess the performance of seven meta-predictor algorithms: BayesDel, CADD, DANN, FATHMM-XF, M-CAP, MISTIC and REVEL for the classification of MED13L missense variants. Significant differences were identified for five parameters: global conservation through verPhyloP and verPhCons scores; physico-chemical difference between amino acids estimated by Grantham scores; conservation of residues between MED13L and MED13 protein; proximity to phosphorylation sites for pathogenic variations. Among the seven selected in-silico tools, BayesDel, REVEL, and MISTIC provided the most interesting performances to discriminate pathogenic from neutral missense variations. Individual gene parameter studies with MED13L have provided expertise on elements of annotation improving meta-predictor choices. The in-silico approach allows us to make valuable hypotheses to predict the involvement of these amino acids in MED13L pathogenic missense variations.

Report of a de novo c.2605C > T (p.Pro869Ser) change in the MED13L gene and review of the literature for MED13L-related intellectual disability

Background

MED13L-related intellectual disability is a new syndrome that is characterized by intellectual disability (ID), motor developmental delay, speech impairment, hypotonia and facial dysmorphism. Both the MED13L haploinsufficiency mutation and missense mutation were reported to be causative. It has also been reported that patients carrying missense mutations have more frequent epilepsy and show a more severe phenotype.

Case presentation

We report a child with ID, speech impairment, severe motor developmental delay, facial deformity, hypotonia, muscular atrophy, scoliosis, odontoprisis, abnormal electroencephalogram (EEG), and congenital ureteropelvic junction obstruction (UPJO) combined with high ureter attachment. We used whole-exome sequencing (WES) to detect the genetic aberration of the child and found a de novo mutation, c.2605C > T (p.Pro869Ser), in the MED13L gene. Neither of her parents carried the mutation. Additionally, we review the literature and summarize the phenotypes and features of reported missense mutations. After reviewing the literature, approximately 17 missense mutations in 20 patients have been reported thus far. For 18 patients (including our case) whose clinical manifestations were provided, 100% of the patients had ID or developmental delay (DD). A total of 88.9, 83.3 and 66.7% of the patients had speech impairment, delayed milestones and hypotonia, respectively. A total of 83.3% of the patients exhibited craniofacial deformity or other dysmorphic features. Behavioral difficulties and autistic features were observed in 55.6% of the patients. Cardiac anomalies were seen in only 27.8% of the patients. Of these patients, 44.4% had epileptic seizures. Of the 17 mutations, 2 were located in the N-terminal domain, 8 were located in the C-terminal domain, and 1 was located in an α-helical sequence stretch. One of them was located in the MID domain of the MedPIWI module.

Conclusions

We report a new patient with a reported missense mutation, c.2605C > T (p.Pro869Ser), who exhibited some infrequent manifestations except common phenotypes, which may broaden the known clinical spectrum. Additionally, by reviewing the literature, we also found that patients with missense mutations have a higher incidence of seizures, MRI abnormalities, autistic features and cardiac anomalies. They also have more severe ID and hypotonia. Our case further demonstrates that Pro869Ser is a hotspot mutation of the MED13L gene.

Is MED13L-related intellectual disability a recognizable syndrome?

INTRODUCTION

MED13L-related intellectual disability is characterized by moderate intellectual disability (ID), speech impairment, and dysmorphic facial features. We present 8 patients with MED13L-related intellectual disability and review the literature for phenotypical and genetic aspects of previously described patients.

MATERIALS AND METHODS

In the search for genetic aberrations in individuals with ID, two of the patients were identified by chromosomal microarray analysis, and five by exome sequencing. One of the individuals, suspected of MED13L-related intellectual disability, based on clinical features, was identified by Sanger sequencing.

RESULTS

All 8 individuals had de novo MED13L aberrations, including two intragenic microdeletions, two frameshift, three nonsense variants, and one missense variant. Phenotypically, they all had intellectual disability, speech and motor delay, and features of the mouth (open mouth appearance, macroglossia, and/or macrostomia). Two individuals were diagnosed with autism, and one had autistic features. One had complex congenital heart defect, and one had persistent foramen ovale. The literature was reviewed with respect to clinical and dysmorphic features, and genetic aberrations.

CONCLUSIONS

Even if most clinical features of MED13L-related intellectual disability are rather non-specific, the syndrome may be suspected in some individuals based on the association of developmental delay, speech impairment, bulbous nasal tip, and macroglossia, macrostomia, or open mouth appearance.

MED13L-related intellectual disability: involvement of missense variants and delineation of the phenotype

Molecular anomalies in MED13L, leading to haploinsufficiency, have been reported in patients with moderate to severe intellectual disability (ID) and distinct facial features, with or without congenital heart defects. Phenotype of the patients was referred to "MED13L haploinsufficiency syndrome." Missense variants in MED13L were already previously described to cause the MED13L-related syndrome, but only in a limited number of patients. Here we report 36 patients with MED13L molecular anomaly, recruited through an international collaboration between centers of expertise for developmental anomalies. All patients presented with intellectual disability and severe language impairment. Hypotonia, ataxia, and recognizable facial gestalt were frequent findings, but not congenital heart defects. We identified seven de novo missense variations, in addition to protein-truncating variants and intragenic deletions. Missense variants clustered in two mutation hot-spots, i.e., exons 15-17 and 25-31. We found that patients carrying missense mutations had more frequently epilepsy and showed a more severe phenotype. This study ascertains missense variations in MED13L as a cause for MED13L-related intellectual disability and improves the clinical delineation of the condition.