ICR1 epimutations in llp15 are restricted to patients with Silver-Russell syndrome features

(Epi)mutations affecting chromosome llp15 are well known to be associated with growth disturbances. The finding of llp15 mutations in the overgrowth disease Beckwith-Wiedemann syndrome led to the identification of imprinted growth-promoting genes which are expressed paternally and of imprinted growth-suppressing genes in the same region that are expressed maternally. An opposite epimutation in the same region is associated with Silver-Russell syndrome (SRS), a growth retardation disorder characterised by a typical facial gestalt, clinodactyly V and asymmetry. In more than 30% of patients with SRS, hypomethylation of the telomeric llp15 imprinting domain (ICR1) can be detected. However, the general significance of this epimutation for human growth retardation was unclear. In a previous study' we showed that the ICR1 epimutation is not present in growth retarded patients with dysmorphisms not typical for SRS, but its role in the development of isolated growth restriction needed to be further elucidated. We therefore screened 30 patients with isolated pre- and postnatal growth retardation (IUGR/PNGR) and 65 patients diagnosed with SRS by external clinicians for ICR1I epimutations. In the latter group clinical data were rarely provided. These 65 'SRS' patients were additionally analysed for maternal uniparental disomy 7 (matUPD7). We excluded ICR1 hypomethylation in all 30 patients with isolated growth retardation. In the SRS group, we detected four cases with ICR1 epimutation and three with matUPD7. By combining our data with those from our previous study we could show that the hypomethylation of ICR1 in llp15 is indeed restricted to patients with SRS features and can be disregarded in isolated IUGR/PNGR. Thus, testing for the epimutation is indicated only in case of growth restriction in association with clinical signs reminiscent of SRS. The low detection rate of the ICR1 epimutation in our 'SRS' group can be explained by the clinical heterogeneity of cases referred by external institutions.