Maternal UPD causes people to have two active copies of some imprinted genes and no active copies of others. This phenomenon is called maternal uniparental disomy (UPD). In 7 percent to 10 percent of cases, people inherit both copies of chromosome 7 from their mother instead of one copy from each parent. A loss of methylation disrupts the regulation of these genes, which leads to slow growth and the other characteristic features of this disorder.Ībnormalities involving genes on chromosome 7 can also cause Russell-Silver syndrome. These genes are thought to be involved in directing normal growth. Russell-Silver syndrome has been associated with changes in methylation involving the H19 and IGF2 genes, which are located near one another at 11p15.
In genes that undergo genomic imprinting, methylation is one way that a gene's parent of origin is marked during the formation of egg and sperm cells. Methylation is a chemical reaction that attaches small molecules called methyl groups to certain segments of DNA. Researchers suspect that 30 to 50 percent of all cases of Russell-Silver syndrome result from changes in a process called methylation on the short (p) arm of chromosome 11 at position 15 (11p15). Abnormalities involving these genes appear to be responsible for many cases of Russell-Silver syndrome. Both chromosome 7 and chromosome 11 contain groups of genes that normally undergo genomic imprinting some of these genes are active only on the maternal copy of the chromosome, while others are active only on the paternal copy. These parent-specific differences in gene expression are caused by a phenomenon called genomic imprinting. For other genes, only the copy inherited from a person's mother (the maternal copy) is expressed. For some genes, however, only the copy inherited from a person's father (the paternal copy) is expressed. For most genes, both copies are expressed, or "turned on," in cells. People normally inherit one copy of each chromosome from their mother and one copy from their father. Research has focused on genes located in particular regions of chromosome 7 and chromosome 11. The disorder often results from the abnormal regulation of certain genes that control growth. The genetic causes of Russell-Silver syndrome are complex. Russell-Silver syndrome is also associated with an increased risk of delayed development, speech and language problems, and learning disabilities. Other features of this disorder can include an unusual curving of the fifth finger ( clinodactyly ), asymmetric or uneven growth of some parts of the body, and digestive system abnormalities. Many children with Russell-Silver syndrome have a small, triangular face with distinctive facial features including a prominent forehead, a narrow chin, a small jaw, and downturned corners of the mouth. Adults with Russell-Silver syndrome are short the average height for affected men is about 151 centimeters (4 feet, 11 inches) and the average height for affected women is about 140 centimeters (4 feet, 7 inches). Affected children are thin and have poor appetites, and some develop recurrent episodes of low blood sugar (hypoglycemia) as a result of feeding difficulties.
Head growth is normal, however, so the head may appear unusually large compared to the rest of the body. Babies with this condition have a low birth weight and often fail to grow and gain weight at the expected rate (failure to thrive). Russell-Silver syndrome is a growth disorder characterized by slow growth before and after birth.
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