Identification of balanced 4p16 paracentric inversions in three patients with Wolf-Hirschhorn phenotype not deleted for the WHS critical region

Wolf-Hirschhorn syndrome (WHS; OMIM 194190) is a complex variable malformation disorder characterized by growth and mental retardation, congenital hypotonia, seizures and a distinct facial appearance. WHS is caused by hemizygous deletions of chromosome 4p16.3 resulting in segmental aneuploidy of variable size with an unknown number of genes contributing to the clinical phenotype. We report on three patients presenting clinical signs highly suggestive for Wolf-Hirschhorn syndrome who were not found to carry the deletion of the WHS critical region by array CGH analysis (244K Agilent platform) that ruled out in all cases imbalances at both the 4p16.3 region and across the whole genome. In order to highlight a cryptic balanced rearrangement disrupting the 4p16.3 region, hypothesized on the basis of the patients’ clinical phenotype, interphase three color FISH analysis was carried out by using a panel of BAC probes. In each patient a paracentric inversion with breakpoints in the 4p16 region was identified. In detail, a 1,7 Mb inversion involving the 4p16.3 band, was found in patient 1, whereas a 5,4 Mb inversion with the telomeric and the centromeric breakpoint at 4p16.3 and 4p16.1 respectively, was observed in patient 2. Moreover, a 5,8 Mb inversion encompassing the region from 4p16.1 to 4p16. 3 was detected in a mosaic state (about 50% of cells) in patient 3. WHS patients who do not bear deletions in the WHS critical region are extremely rare. The identification in our patients of balanced inversions by means of molecular cytogenetic characterization, provides the opportunity to disclose new positional candidate genes involved in this complex syndrome. We hypothesize that the observed balanced rearrangements could contribute to the patients’ syndromic phenotype by either disrupting dosage sensitive genes or by altering the expression of the genes lying close to the breakpoints through a position effect. In order to assess how the inversions might contribute to the WHS phenotype, we are currently testing by quantitative expression studies whether the balanced rearrangements lead to halved expression of dosage-sensitive interrupted genes or to diminished expression of genes flanking the inversion breakpoints.