Full text
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| Author(s): |
Adriano Bonaldi
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Biociências (IBIOC/SB) |
| Defense date: | 2011-05-20 |
| Examining board members: |
Angela Maria Vianna Morgante;
Ester Silveira Ramos
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| Advisor: | Angela Maria Vianna Morgante |
| Abstract | |
Silver Russell syndrome (SRS) is characterized by severe intrauterine and postnatal growth retardation in association with a typical small triangular face and other variable features. Most cases are sporadic. Genetic and epigenetic disturbances on imprinted regions at chromosomes 7 and 11 are detected in about 50% of the patients. Most frequently, SRS is caused by altered gene expression on chromosome 11p15 due to hypomethylation of the telomeric imprinting center (ICR1) that is present in at least 40% of the patients. Maternally inherited duplications encompassing the centromic imprinting center (ICR2) domains at 11p15 are present in about 1-2% of cases. Maternal uniparental disomy of chromosome 7 (mUPD7) is identified in 5-10% of patients. More recently, chromosomal microdeletions and microduplications were detected in a small group of SRS patients, some of them with possible pathogenic effect. This leaves approximately half of the SRS cases without a genetic cause determined. Our cohort consisted of 64 unrelated Brazilian patients with clinical diagnosis of SRS. DNA copy number changes and the methylation pattern on chromosome 11p15 were investigated in 49 patients by MS-MLPA, and 21 (43%) presented with hypomethylation of ICR1. In one patient (2%), both centers (ICR1 and ICR2) were hypomethylated, a complex alteration that has been reported in ~4% of SRS patients that shows hypomethylation of ICR1. In a further patient (2%), we detected a ~1.6 Mb microduplication encompassing the whole ICR2 domain, but not the ICR1. This microduplication was shown to segregate in a three-generation family, and was associated with SRS whenever maternally transmitted: there were four instances of paternal transmissions of the microduplication from a single male uniformly resulting in normal offspring, and five maternal transmissions, via two clinically normal sisters, with all the children exhibiting SRS. A maternally inherited microduplication also restricted to the ICR2 domain and associated with SRS in a boy was described previously. Among the duplicated genes in both cases, CDKN1C is a likely candidate for the SRS phenotype, because it encodes a cyclin-dependent kinase inhibitor that negatively regulates cell proliferation and growth, and plays a crucial role in human fetal development. This new case brings confirmatory evidence that microduplications restricted to the ICR2 domain result in SRS when maternally transmitted. It also shows that no apparent phenotypic change is present when ICR2 duplication is paternally inherited. By genotyping chromosome 7 microsatellites, we identified three patients (4.7%) with mUPD(7), in the cohort of 64 patients. The frequencies of hypomethylation of ICR1 (43%) and mUPD(7) (4.7%) among our patients are in accordance with the literature, and point to a proper selection of patients with SRS, from the clinical point of view. The investigation of submicroscopic chromosomal imbalances by a-CGH was performed in 19 patients in whom (epi)genetic mutations at 11p15 and mUPD(7) had been excluded. Most patients showed no changes (n = 7) or had only CNV considered to be polymorphic (n = 8). Four potentially pathogenic microdeletions were detected, on chomosomes 2p23.3 (~320 Kb), 13q24 (~94.3 Kb), 15q11.2 (~320 Kb) and 16p13.11 (~95.8 Kb). In neither case we could establish a direct relationship between the imbalance and the phenotype, because it was not possible to investigate both parents or the change was present in a clinically normal parent or it had been reported in normal individuals, without, however, indication of being polymorphic. Incomplete penetrance or unmasking of a pathogenic recessive allele on the homologous chromosome are two possible explanations to the pathogenic effect of a microdeletion inherited from a clinically normal parent. Three recent studies that used microarrays to identify genes or chromosomal regions associated with SRS, wherein the genetic cause was unknown, detected microdeletions and microduplications, some of them potentially pathogenic: a microdeletion at 15q26.3, including the IGF1R gene, was identified in two patients; other microdeletions included the IGF2BP3 gene at 7p15, GPC5 gene at 13q31.3, MAPK1 gene at 22q11.2 and HMGA2 gene at 12q14, which were considered candidates, possibly influencing growth. This set of results, including ours, indicates that the investigation of submicroscopic chromosomal imbalances should be extended to a larger cohort of SRS patients, in the search for chromosomal regions and genes that may be causally associated with the syndrome. In 30 SRS patients, we searched for point mutations in the CDKAL1 gene by direct sequencing of coding regions. This gene was considered a candidate for SRS, after being disrupted in one of our SRS patients with a t(5;6). No pathogenic mutation was detected and, therefore, point mutations in the coding region of CDKAL1 do not appear to be a common cause of SRS. In 18 of the 30 patients, we investigated the presence of microdeletions and microduplications by a-CGH and found no changes encompassing CDKAL1 gene. Considering the small cohort size, we cannot definitely exclude the possibility that changes in CDKAL1 gene may contribute to the etiology of SRS. (AU) | |
| FAPESP's process: | 09/03341-8 - Genetic study on Silver-Russell syndrome |
| Grantee: | Adriano Bonaldi |
| Support Opportunities: | Scholarships in Brazil - Master |