Cloning and characterization of an inversion breakpoint at 6q23.3 suggests a role for Map7 in sacral dysgenesis

Here we report on a male patient with sacral dysgenesis (SD) and constitutional pericentric inversion of chromosome 6 (p11.2;q23.3). SD is a heterogeneous group of congenital anomalies with complex genetic etiology. Previously, a patient with sacral abnormalities and an interstitial deletion of 6q23-->q25 region has been described. We speculated that a susceptibility gene for SD lies in 6q23.3 region (disrupted in both patients), and therefore, cloning of the breakpoint in our patient would lead to the identification of the disrupted gene. We performed FISH analysis followed by Southern blot analysis and inverse PCR to clone the breakpoint. The 6p11.2 breakpoint mapped very close to the centromere, and the 6q23.3 breakpoint localized in the ninth intron of the MAP7 gene. We then evaluated the involvement of MAP7 in SD by further screening of the gene in several patients with a similar phenotype. Two nucleotide changes causing Ile257Asn and Glu571Ala substitutions in the protein, both affecting amino acid residues conserved in the mouse homolog, were identified in two patients. Both changes are either very rare polymorphisms or true mutations, since they were not detected in 167 normal individuals nor found in the SNP database. Therefore, our study suggests MAP7 as a candidate gene for SD. However, we were unable to detect any sacral defects in the MAP7 knockout mice.