Cytogenetic and molecular characterization of a de-novo t(2p;7p) translocation involving TNS3 and EXOC6B genes in a boy with a complex syndromic phenotype

We describe a premature newborn child with left renal agenesis, right low functional kidney, altered chemical-clinical parameters, neutropenia, recurrent pulmonary infections, long bone diaphysis broadening, growth and developmental delay. Postnatal cytogenetic analysis revealed a 46,XY,t(2;7)(p13;p12) de-novo karyotype. The chromosome breakpoints were defined by FISH using BAC probes and initially restricted to about 123,000 bp in 2p13 and delimited to 84,600 bp in 7p12. Bioinformatic analysis of these genomic regions showed two genes that are involved in the rearrangement: exocyst C6B (EXOC6B) for chromosome 2 breakpoint and tensin3 (TNS3) for chromosome 7 breakpoint. A EXOC6B-TNS3 fusion transcript together with a reciprocal TNS3-EXOC6B chimeric RNA have been detected by RT-PCR performed on skin fibroblasts RNA of the proband. These data localize the chromosome 2 breakpoint within the first intron of EXOC6B, while the translocation event on chromosome 7 occurred in intron 15 of TNS3. We hypothesize that the phenotype observed in the patient results from one or several mechanisms including: haploinsufficiency of EXOC6B and TNS3 genes; a dominant negative effect exerted by the chimeric transcripts; a disregulation in the expression of other genes adjacent the breakpoints. Although no clear evidences exist supporting a role of any of the above mentioned mechanisms in the pathogenesis of the complex phenotype, immunofluorescence analysis of tensin1 in the patient's fibroblasts suggests that the TNS3 gene haploinsufficiency results in a reduced expression of tensin1. These cells may be therefore a model for understanding the role and the organization of the tensin protein family.