Silver-Russell syndrome and human growth: genetic and epigenetic studies

A large part of the genetic and epigenetic changes contributing to human height and growth remain unknown. Silver-Russell syndrome (SRS) is a rare human growth disorder, where maternal uniparental disomy of chromosome 7 [UPD(7)mat] and loss of methylation at 11p15 (11p15 LOM) are the two major findings. However, a substantial proportion of the SRS patients remain without a molecular diagnosis. SRS is caused by a disturbance of imprinted genes that are expressed solely or predominantly from one parent and are important for growth and development. Imprinted genes may also be especially susceptible to environmental factors, which can cause persistent changes in methylation patterns and affect development of adult-onset diseases. This thesis aims to explore new genetic and epigenetic changes in SRS, as well as epigenetic changes of imprinted genes in growth-restricted children, children of normal growth and individuals born pre-term. In Study I, genomic structural variation of 22 SRS patients was studied with the Affymetrix 250K Sty microarray. Several copy number changes were found, including a heterozygous deletion of 15q26.3 including the insulin-like growth factor 1 receptor (IGF1R) gene. In Study II, CpGs in the insulin-like growth factor 2 (IGF2) gene (located in the 11p15 region) were tested in pre-term and very low birth weight born individuals with elevated levels of cardiovascular risk factors. DNA methylation changes were found in one CpG site of the pre-term born individuals compared to controls. In Study III, genome wide comparisons between the DNA methylation levels of SRS patients that have UPD(7)mat, controls, and an individual with paternal uniparental disomy of chromosome 7 [UPD(7)pat] were done. DNA methylation was studied using the Illumina Infinium HumanMethylation450K BeadChip technology, which is capable of measuring methylation level of more than 450 000 CpG sites across the genome. The study provided new information on the DNA methylation landscape of chromosome 7, suggesting new differentially methylated regions (DMRs) and imprinted genes. In Study IV, DNA methylation levels were compared between three different subgroups of SRS: SRS with UPD(7)mat, SRS with 11p15 LOM and clinical SRS without a known molecular etiology. The promoter region of homeobox A4 (HOXA4) gene was found hypomethylated in all of the subgroups of SRS. This region was subsequently tested in other severely growth-restricted patients and was found hypomethylated. Additionally, the methylation level of the HOXA4 promoter region was found to be associated with the height of school-aged children, suggesting that HOXA4 plays a role not only in SRS but also in the regulation of height in general. These studies found molecular changes in SRS, new differentially methylated regions in chromosome 7 and epigenetic findings potentially relevant for regulation of human growth. The findings provide potential targets for further studies in human growth and development.