Fragile X syndrome

Posted by e-Medical PPT
Fragile X syndrome is a genetic syndrome which results in a spectrum of characteristic physical and intellectual limitations and emotional and behavioral features which range from severe to mild in manifestation.The syndrome is associated with the expansion of a single trinucleotide gene sequence (CGG) on the X chromosome, and results in a failure to express the protein coded by the FMR1 gene, which is required for normal neural development. Fragile X is the most common known single gene cause of autism and the most common inherited cause of intellectual disability.
Aside from intellectual disability, prominent characteristics of the syndrome include an elongated face, large or protruding ears, flat feet, larger testes (macroorchidism), and low muscle tone. Speech may include cluttered speech or nervous speech.Behavioral characteristics may include stereotypic movements (e.g., hand-flapping) and atypical social development, particularly shyness, limited eye contact, memory problems, and difficulty with face encoding.
Fragile X syndrome was originally diagnosed by culturing cells in a folate deficient medium and then assessing the cultures for X-chromosome breakage by cytogenetic analysis of the long arm of the X chromosome. This technique proved unreliable for both diagnosis and carrier testing.
The fragile X abnormality is now directly determined by analysis of the number of CGG repeats and their methylation status using restriction endonuclease digestion and Southern blot analysis.
Fragile X syndrome is an X-linked dominant condition with variable expressivity and possibly reduced penetrance.Because males normally have only one copy of the X chromosome, those males with significant trinucleotide expansion at the FMR1 locus are symptomatic. Females have two X chromosomes and thus have an increased probability of having a working FMR1 allele. Females carrying one X chromosome with an expanded FMR1 gene can have some signs and symptoms of the disorder or be normal.


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