The homozygous deletion of the 3′ enhancer of the SHOX gene causes Langer mesomelic dysplasia

To the Editor: A disturbance in the long-range regulation of gene expression has been proposed as the cause of a number of diseases, and the disruption of the transcriptional control through the deletion of cisacting enhancer elements has been associated to a loss of function and to phenotypes similar to those caused by inactivating gene mutations (1). Enhancer deletions have been observed, among the others, also for the short-stature-homeoboxcontaining (SHOX) gene, in which heterozygous deletions downstream the coding region causes the phenotype of the Leri–Weill syndrome (LWS) (2, 3). A similar mechanism has been suggested for the homozygous form of LWS, Langer mesomelic dysplasia (LMD, MIM 249700), a rare dwarfism with shortening of the middle segment of the limbs and hypoplasia of ulna and fibula: recently, a compound heterozygote with a deletion in the 3# SHOX flanking region in one allele and the loss of the entire SHOX gene in the other has been described with a mild’ LMD phenotype (4). In the present report, we characterize the first case of pure’ homozygous deletion of the enhancer region in LMD, thus confirming this mechanism as a possible cause of the SHOX pathies. We describe a consanguineous couple (first cousins) affected by LWS, referred to us for prenatal counseling because of a previous 22-week therapeutical abortion after the ultrasonographic diagnosis of LMD, subsequently confirmed at the pathological examination [Fig. 1(a)]. The husband and the wife presented the classical LWS phenotype with disproportionate short stature owing to mesomelic shortening of arms and legs (149 and 138.7 cm in height with an upper/lower segment ratio of 1.20 and 1.28, respectively), and with Madelung deformity (radial bowing, triangularization of the distal radial epiphysis and dorsal dislocation of the ulna). In the two parents, conventional gene testing for SHOX (sequencing and deletion analysis) (5) failed to detect any mutation: prenatal diagnosis was thus not performed and the follow-up was only ultrasonographical, as it was in the case of a third pregnancy, 2 years later: a boy and a girl were born of 48 and 47 cm of length (both between the second and ninth centile), respectively. After the evidence of the involvement of the SHOX enhancer in the pathogenesis of the LWS (2, 3), the family was reexamined, after obtaining an informed consent and with the approval by the local ethical committee. Deletion analysis of SHOX and the PAR1 was carried out by multiplex ligation-dependent probe amplification (MLPA), using theSHOX salsa P018B kit (MRC-Holland, Amsterdam, The Netherlands) (6), as detailed in Table S1 (supplementary material online) and by microsatellite analysis, using the known microsatellites DYS290, DXYS10093, DXYS233,DXYS234, andDXYS228, located distal to SHOX, plus two newly identified markers (DXS10150, intragenic, and DXS10149, 3# flanking the gene, http://www.gdb.org/gdb/). MLPA detected in both the LWS parents a large heterozygous deletion extending from exon 6b of the SHOX gene downstream for 1.1 Mb, encompassing the enhancer; the same deletion was found in homozygosity in the previously aborted LMD fetus, whereas in the other two children no deletion was found [Fig. 1(b)]. Microsatellite analysis confirmed the results by showing heterozygosity for CA SHOX, located 15 kb upstream of SHOX, for DXS10150, intragenic, and for DXYS228, located 2 Mb downstream of SHOX; the remaining five markers revealed only one allele in both the parents and the absence of any amplification product in the LMD fetus. The reported case is the first description of a homozygous deletion in a regulatory region causing disease in humans: the involvement in the deletion also of exon 6b of the SHOX gene coding region should not affect our conclusions, as no causative mutation in exon 6b, encoding for 14 aminoacids in the COOH terminal of the protein, has ever been described for the pathogenesis of LWS or LMD (7). The present report confirms the enhancer deletion as a possible mechanism of SHOX gene