Identification of a novel recessive RELN mutation using a homozygous balanced reciprocal translocation

Two siblings from a consanguineons Egyptian marriage showed an identical phenotype of cortical lissencephaly with cerebellar hypoplasia, severe epilepsy, and mental retardation. Examination of karyotype revealed 46, t(7;12) (q22;p13)mat (7;12)(q22;p13)pat in both affected children, suggesting a homozygous reciprocal balanced translocation. Each healthy parent was a carrier of the balanced translocation in the heterozygous state, suggesting homozygous disruption of a gents involved in brain development. There were early spontaneous abortions in this family, as would be expected from transmission of an unbalanced chromosome. A disruption of RELN at 7q22.1 with absence of encoded protein was identified, This is the first demonstration that such rare homozygous translocations can be used to identify recessive disease gene mutations. (C) 2007 Wiley-Liss, Inc.