Megalencephalic Leukoencephalopathy with subcortical Cysts type 1 (MLC1) due to a homozygous deep intronic splicing mutation (c.895-226T>G) abrogated by AMO treatment.

Megalencephalic leukoencephalopathy with subcortical cysts is an autosomal recessive disease characterized by early-onset macrocephaly, developmental delay, motor disability in the form of progressive spasticity and ataxia, seizures, cognitive decline and characteristic MRI findings. Mutations in two genes, MLC1(22q13.33; 75% of patients) or HEPACAM (11q24; 5-10% of patients) are associated with the disease. We describe an adult MLC patient with moderate clinical symptoms. MLC1 cDNA analysis from lymphoblasts showed a strong transcript reduction, and identified a 246-bp pseudoexon containing a premature stop codon between exons 10 and 11, due to a homozygous c.895-226T>G deep-intronic mutation. The role of this mutation on splicing was confirmed using a minigene assay, and an antisense morpholinated oligonucleotide (AMO) targeted to the aberrant splice site partially abrogated the mutation in vitro. The mutation c.895-226T>G has a leaky effect on splicing leaving part of the full length transcript and may explain the milder phenotype in our patient. This category of mutations is often overlooked, being outside of canonically sequenced genomic regions, and is particularly important because it can be corrected by antisense oligonucleotides therapy.