Homozygous PLCB1 deletion associated with malignant migrating partial seizures in infancy

Abstract

Malignant migrating partial seizures in infancy (MMPEI) is an early onset epileptic encephalopathy with few known etiologies. We sought to identify a novel cause of MMPEI in a child with MMPEI whose healthy parents were consanguineous. We used array comparative genomic hybridization (CGH) to identify copy number variants genome-wide and long-range polymerase chain reaction to further delineate the breakpoints of a deletion found by CGH. The proband had an inherited homozygous deletion of chromosome 20p13, disrupting the promoter region and first three coding exons of the gene PLCB1. Additional MMPEI cases were screened for similar deletions or mutations in PLCB1 but did not harbor mutations. Our results suggest that loss of PLCβ1 function is one cause of MMPEI, consistent with prior studies in a Plcb1 knockout mouse model that develops early onset epilepsy. We provide novel insight into the molecular mechanisms underlying MMPEI and further implicate PLCB1 as a candidate gene for severe childhood epilepsies. This work highlights the importance of pursuing genetic etiologies for severe early onset epilepsy syndromes.

Figure 1

A. The four-generation pedigree is notable for multiple consanguineous relationships. The proband’s parents are first cousins. B. An example of the ictal EEG from the proband demonstrates the migrating nature of the seizures typical of MMPEI. On the left side of the page, a right temporal seizure is shown. On the right, migration of the seizure to include the left temporal region is shown.

Figure 2

A. Proband, paternal, and maternal genomic DNA were isolated from peripheral blood and then fragmented, labeled, and hybridized for targeted array comparative genomic hybridization (array CGH). The proband’s study revealed a homozygous 476kb deletion on chromosome 20p12.3 as illustrated in the schematic (red band). This deletion corresponds to coordinates 8,099,741–8,575,520 on chromosome 20 (human genome build hg19). Parental studies revealed that each of the proband’s parents is heterozygous for the 20p12.3 deletion. B. The deletion occurs within the locus of the gene phospholipase C beta 1 (PLCB1). Comparison of genomic DNA with sequenced PLCB1 cDNAs revealed that the deleted region encompasses the first three coding exons of the gene. More precise deletion breakpoints were identified by long-range PCR of genomic DNA (8,094,049–8,094,072 to 8,580,261–8,580,284) and found to be flanked by repetitive long interspersed elements (LINE, red Xs). The exact deletion boundaries could not be resolved due to 23 nucleotides of 100% sequence homology between 5′ and 3′ breakpoints (shaded red).