Title: Epigenotype-phenotype correlations in Beckwith-Wiedemann syndrome
Abstract: Beckwith-Wiedemann syndrome (BWS) is a model imprinting disorder resulting from mutations or epigenetic events involving imprinted genes at chromosome 11p15.5. Thus, germline mutations in<i>CDKN1C</i>, uniparental disomy (UPD), and loss of imprinting of <i>IGF2</i> and other imprinted genes have been implicated. Many familial BWS cases have germline<i>CDKN1C</i> mutations. However, most BWS cases are sporadic and UPD or putative imprinting errors predominate in this group. We have identified previously a subgroup of sporadic cases with loss of imprinting (LOI) of <i>IGF2</i> and epigenetic silencing of <i>H19</i> proposed to be caused by a defect in a distal 11p15.5 imprinting control element (designated BWSIC1). However, many sporadic BWS patients show biallelic<i>IGF2</i> expression in the presence of normal<i>H19</i> methylation and expression patterns. This and other evidence suggested the existence of a further imprinting control element (BWSIC2) at 11p15.5. Recently, we showed that a subgroup of BWS patients have loss of methylation (LOM) at a differentially methylated region (KvDMR1) within the<i>KCNQ1</i> gene centromeric to the<i>IGF2</i> and <i>H19</i>genes. We have now analysed a large series of sporadic cases to define the frequency and phenotypic correlates of epigenetic abnormalities in BWS. LOM at KvDMR1 was detected by Southern analysis or a novel PCR based method in 35 of 69 (51%) sporadic BWS without UPD. LOM at KvDMR1 was often, but not invariably associated with LOI of<i>IGF2</i>. KvDMR1 LOM was not detected in BWS patients with putative BWSIC1 defects and cases with KvDMR1 LOM (that is, putative BWSIC2 defects) invariably had a normal<i>H19</i> methylation pattern. The incidence of exomphalos in putative BWSIC2 defect patients was not significantly different from that in patients with germline<i>CDKN1C</i> mutations (20/29 and 13/15 respectively), but was significantly greater than that in patients with putative BWSIC1 defects (0/5, p=0.007) and UPD (0/22, p<0.0001). These findings are consistent with the hypothesis that LOM of KvDMR1 (BWSIC2 defect) results in epigenetic silencing of<i>CDKN1C</i> and variable LOI of<i>IGF2</i>. BWS patients with embryonal tumours have UPD or a BWSIC1 defect but not LOM of KvDMR1. This study has further shown how (1) variations in phenotypic expression of BWS may be linked to specific molecular subgroups and (2) molecular analysis of BWS can provide insights into mechanisms of imprinting regulation.