Beckwith–Wiedemann Syndrome (BWS; OMIM#130650) is an overgrowth disorder characterized by heterogeneous clinical expression and complex molecular etiology. Distinctive clinical features are macroglossia, abdominal wall defects, lateralized overgrowth, enlarged abdominal organs, and increased risk of embryonal tumors during early childhood. The molecular defects consist in (epi)genetic deregulation of the 11p15.5 region containing two clusters of imprinted genes that play a key role in the regulation of fetal and postnatal growth: H19/IGF2:IG-DMR (IC1), methylated on the paternal allele, and KCNQ1OT1:TSS-DMR (IC2), methylated on the maternal allele which regulate the expression of IGF2 and CDKN1C genes, respectively. More than 50% of BWS patients display hypomethylation of IC2 (Loss of Methylation, LoM) on the maternal allele, and 5% show hypermethylation in the paternal IC1 (Gain of Methylation, GoM) [1].
IC2-LoM in BWS patients may be associated with epimutations in additional differentially methylated regions (DMRs) involved in other imprinting disorders, giving rise to a phenomenon defined as Multilocus Imprinting Disturbance (MLID). Several reports on MLID frequency in IC2-LoM BWS cases picture a variable but significant frequency, from 30 to 50%. However, knowledge on the etiology and the clinical impact of MLID is still under study [1].
Genomic sequencing of MLID patients’ mothers highlighted the occurrence of pathogenic variants in genes transcribed by the maternal genome and deposited in the oocyte, where they persist until the first phases of embryogenesis [2]. When the zygote genome is not yet transcriptionally active, these maternal genes contribute to the zygote genome activation (ZGA) and passage to early embryo, hence being named Maternal-effect Genes (MEGs). Out of MEGs, the subcortical maternal complex (SCMC), a large protein complex expressed in oocytes and preimplantation embryos and functionally conserved across mammals, turned out to be associated with MLID in humans [2]. The SCMC, located in the cortex of the oocyte cytoplasm and enduring in the embryo up to the blastocyst stage, plays a role in processes as meiotic spindle formation and positioning, regulation of translation, organelle redistribution, and epigenetic reprogramming. Even not all the SCMC players have been identified, at present the proteins proved to be part of the complex are NLRP2, NLRP4f, NLRP5, and NLRP9b, all belonging to the NLRP (nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain-containing protein) family, PADI6 (peptidyl arginine Deiminase 6), KHDC3L (KH domain containing 3), OOEP (oocyte expressed protein), TLE6, (transducin-like enhancer of Split 6), and ZBED3 (zinc finger, BED type containing 3). In animals first and recently in humans, it has been demonstrated that the destruction of the complex causes sterility/sub-sterility in females, often conceiving embryos unable to go beyond the first cell division [3].
Loss-of-function mutations in maternal trans-acting SCMC genes in healthy women with disturbed imprinting in their offspring or women with infertility or recurrent miscarriages rather than hydatidiform moles would point to a causative role of SCMC variants in the MLID onset [2].
To date, pathogenic variants in maternal-effect genes in BWS patients with an IC2-LoM and MLID genotype have been disclosed in a limited number of studies. The reported cases include both biallelic and monoallelic variants, though the pathogenicity of the latter remains uncertain. The first description of SCMC variants in IC2-LoM BWS was a homozygous NLRP2 frameshift in the mother of 2 BWS children, each inheriting a different NLRP2 allele: consistent with a trans-acting mechanism of the germline variant, analysis of methylation at additional loci identified loss of methylation in MEST:alt-TSS-DMR [4]. Another family has been then reported with the same homozygous NLRP2 variant, two BWS children, and a miscarriage history [5]. In both families, parents were consanguineous and Pakistani. A compound heterozygous NLRP7 genotype was shared by two sisters experiencing recurrent miscarriages before the 7th gestational week, one of whom conceiving a BWS IC2-LoM MLID fetus, voluntarily terminated at the 19th week [5]. Furthermore, an NLRP5 compound heterozygous genotype was reported in mothers of a BWS-MLID child in four independent families [6, 7] summing up to seven families documenting mothers with biallelic variants in NLRP genes and adverse reproductive outcomes and BWS MLID offspring.
Additionally, four families with BWS-MLID children and mothers, carriers of compound heterozygous PADI6 variants [5, 8, 9] have been described. These data show a spectrum of conditions caused by SCMC alterations that impact oocyte developmental events leading to infertility and miscarriages or biallelic hydatidiform moles, as well as live births though with Beckwith–Wiedemann and MLID.