Disruption of the ATP8A2 gene in a patient with a t(10;13) de novo balanced translocation and a severe neurological phenotype

Abstract

Mental retardation is a frequent condition that is clinically and genetically highly heterogeneous. One of the strategies used to identify new causative genes is to take advantage of balanced chromosomal rearrangements in affected patients. We characterized a de novo t(10;13) balanced translocation in a patient with severe mental retardation and major hypotonia. We found that the balanced translocation is molecularly balanced. The translocation breakpoint disrupts the coding sequence of a single gene, called ATP8A2. The ATP8A2 gene is not ubiquitously expressed, but it is highly expressed in the brain. In situ hybridization performed in mouse embryos at different stages of development with the mouse homologue confirms this observation. A total of 38 patients with a similar phenotype were screened for mutations in the ATP8A2 gene but no mutations were found. The balanced translocation identified in this patient disrupts a single candidate gene highly expressed in the brain. Although this chromosomal rearrangement could be the cause of the severe phenotype of the patient, we were not able to identify additional cases. Extensive screening in the mentally retarded population will be needed to determine if ATP8A2 haploinsufficiency or dysfunction causes a neurological phenotype in humans.

Figure 1

Top panel: ideograms of human chromosomes 10 (Hs.10) and 13 (Hs.13). Middle panel: BAC clones and gene content in the two regions investigated by FISH and Southern blotting. Arrows indicate the direction of transcription for each gene in the region. The size of each gene and the number of exons are also indicated. Bottom panel: position of the two translocation breakpoints with respect to the exons of the WAC and ATP8A2 genes (large white arrows). The location and number of exons contained in the breakpoint region are indicated for each gene (exons 1–3 for the WAC gene, exons 26–28 for the ATP8A2 gene). The chromosome 10 breakpoint is located at nucleotide 28 846 023 and the chromosome 13 breakpoint at position 25 231 633 (according to reference sequence Ensembl54).

Figure 2

(A) Expression of ATP8A2 (top panels) and GAPDH (bottom panels) in 13 human tissues (left panels) and in 12 regions of the human brain (right panels). ATP8A2 is strongly expressed in all regions of the brain, in the retina and testis. A faint signal is also detected in the fetal kidney. (B) In situ hybridization performed with a mouse Atp8a2 probe on sections of embryos at E14.5 (a–c), E15.5 (d–e) and E18.5 (g–h). Atp8a2 is strongly expressed in the central nervous system. A faint background staining only is visible on the sections hybridized with an Atp8a2 sense probe (f and i). (C) In situ hybridization performed on postnatal day 3 (P3) and day 10 (P10) mouse brain revealing a strong expression of Atp8a2 in the cerebral cortex and the hippocampus. Sections hybridized with an Atp8a2 sense probe are shown (b and d).