Extended spectrum of MBD5 mutations in neurodevelopmental disorders

Abstract

Intellectual disability (ID) is a clinical sign reflecting diverse neurodevelopmental disorders that are genetically and phenotypically heterogeneous. Just recently, partial or complete deletion of methyl-CpG-binding domain 5 (MBD5) gene has been implicated as causative in the phenotype associated with 2q23.1 microdeletion syndrome. In the course of systematic whole-genome screening of individuals with unexplained ID by array-based comparative genomic hybridization, we identified de novo intragenic deletions of MBD5 in three patients leading, as previously documented, to haploinsufficiency of MBD5. In addition, we described a patient with an unreported de novo MBD5 intragenic duplication. Reverse transcriptase-PCR and sequencing analyses showed the presence of numerous aberrant transcripts leading to premature termination codon. To further elucidate the involvement of MBD5 in ID, we sequenced ten coding, five non-coding exons and an evolutionary conserved region in intron 2, in a selected cohort of 78 subjects with a phenotype reminiscent of 2q23.1 microdeletion syndrome. Besides variants most often inherited from an healthy parent, we identified for the first time a de novo nonsense mutation associated with a much more damaging phenotype. Taken together, these results extend the mutation spectrum in MBD5 gene and contribute to refine the associated phenotype of neurodevelopmental disorder.

Figure 1

(a) Frontal and lateral view of patients A and B (3 years 6 months), patient D (3 years 11 months) and patient E (10 years), demonstrating a broad nasal bridge and hypoplastic nares. (b) 105K array-based CGH results showing the extent of MBD5 intragenic deletion in patients A and B (7 probes) and for patient D (3 probes) and the extent of intragenic duplication for patient C (3 probes using 44K array-CGH) (c) Map of genomic alterations: deletions, duplication and nonsense mutation (snapshot of Database of Genomic Variants (http://projects.tcag.ca/variation/).

Figure 2

(a) Genomic sequencing results for patient E, his brother and parents showing (arrowhead) a de novo nonsense mutation in coding exon 4 (NM_018328.3:c.440C>G). Right panel: comparison of DNA and cDNA sequencing results in patient E showing that both normal and mutated alleles are expressed (arrowhead). (b) RT-qPCR results with primer set 1 (MBD5-coding exons 1–2) and primer set 2 (MBD5-coding exons 6–7): left and third panel, a decreased level of expression of MBD5 in patients A and B (compared with that healthy sister and parents), and in patient D (compared with that two controls), respectively, second panel, an increased level of expression for duplicated MBD5 exons in patient C (compared with that control), right panel, a normal level of expression of MBD5 in patient E (compared with that four controls). *indicates a significant difference. (c) Aberrant transcripts characterized by RT-PCR and sequencing analysis for patient C.