Background
Human chromosome 15q is prone to cytogenetic rearrangements, in part due to repetitive elements located therein [1,2]. Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are two neurodevelopmental disorders caused by deletions of 15q11-q13, and neurodevelopmental abnormalities are associated with supernumerary chromosomes derived from inverted duplications of 15q [3]. In addition, an autistic disorder susceptibility locus has been localized to proximal 15q by linkage and association studies [4,5]. AS is associated with UBE3A mutations [6,7]. Strong candidates for PWS have recently emerged and are likely to have an additive effect in causing this disorder [8-12]. In particular, two members of the NDN/MAGE gene family, NDN and MAGEL2, are located in the PWS deletion region and are inactivated in individuals with PWS [8,11-14]. Respiratory and behavioral abnormalities in a mouse deleted for Ndn, the murine orthologue of NDN, may suggest that NDN is implicated in the PWS phenotype [15,16]. We have therefore investigated the possibility that other NDN/MAGE genes may also be present on proximal 15q and may be involved in neurodevelopmental disorders. Indeed, we identified a novel necdin-related gene, NDNL2, on proximal 15q within the critical region for autistic disorder susceptibility. We have examined the expression of NDNL2 and its murine orthologue, and placed the murine gene on Chr. 7. The proximal region of chromosome 15 is subject to genomic imprinting, the expression of a gene from only one allele depending on parent-of-origin. Furthermore, the maternal derivation of chromosome 15 abnormalities seen in individuals with autistic disorder suggests a parent-of origin effect [4,5]. We have therefore analyzed the imprinting of human and mouse NDNL2/Ndnl2.