Fragile X syndrome pathophysiology: Difference between revisions

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Revision as of 17:56, 17 July 2021

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Pathophysiology

Fragile x syndrome has an x-linked dominant inheritance. It is caused by an expansion of CGG trinucleotide repeat within FMR1 gene on X chromosome. Due to high number of CGG repeats (>200), this leads to methylation of part of gene on X chromosome that codes for Fragile X Mental retardation protein (FMRP), which is required for proper development of connections between neurons.

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 == Pathophysiology ==
-[[Image:XlinkRecessive.jpg|frame|left|X-linked recessive inheritance]]
+Fragile x syndrome has an x-linked dominant inheritance. It is caused by an expansion of CGG trinucleotide repeat within FMR1 gene on X chromosome. Due to high number of CGG repeats (>200), this leads to methylation of part of gene on X chromosome that codes for Fragile X Mental retardation protein (FMRP), which is required for proper development of connections between neurons.
-The diagram (left) of X-linked recessive inheritance is not entirely inappropriate but it markedly oversimplifies the situation and does not provide a sufficient foundation for genetic counseling about the fragile X syndrome. Technically, fragile X syndrome is an X-linked dominant condition with 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. They are intellectually disabled and may show various physical features of the fragile X syndrome.
-Females have two X chromosomes and thus have double the chance 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. Although the extra X chromosome can serve as a backup, only one X chromosome is active at a time due to [[X-inactivation]].
-Males with the fragile X cannot transmit it to any of their sons (since males contribute a Y chromosome, not an X, to their male offspring), but will transmit it to all of their daughters, as males contribute their X to all of their daughters.
-Females carrying one copy of the fragile X can transmit it to their sons or daughters; in this case each child has a 50% chance of inheriting the fragile X. Sons who receive the fragile X are at high risk of intellectual disability. Daughters who receive the fragile X may appear normal or they may be intellectually disabled, usually to a lesser degree than boys with the syndrome.  The transmission of fragile X often increases with each passing generation.  This seemingly anomalous pattern of inheritance is referred to as the [[Sherman paradox]].
 == References ==

Fragile X syndrome pathophysiology: Difference between revisions

Revision as of 17:56, 17 July 2021

Overview

Pathophysiology

References

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