Genetics of mental retardation 2012

Author: F. Forzano
Submitted: Tuesday 26th of June 2012 02:44:26 PM
Submitted by: egf
Language: English
Content type: Learning resource
Educational levels: expert, qc2, qc3


Mental retardation (MR) is a common condition which affects 2-3% of people worldwide, and is currently defined as “a significant impairment of cognitive and adaptive functions, with onset before age 18 years”. It is classified in four degrees of severity according to the Intelligence Quotient level (IQ): mild MR (IQ 50-70), moderate MR (IQ 35-50), severe MR (IQ 20-35), profound MR (IQ below 20). A genetic cause can be found roughly in a half of the cases, being much more likely as the IQ progressively decrease. Among the genetic causes it is possible to make a gross distinction between multiple genes defects and single genes defects. The first group include genomic imbalance (deletion or duplication) which involves various contiguous genes and which can be identified through standard karyotyping (resolution 3-5 Mb) or molecular karyotyping (FISH and array-CGH, resolution 25Kb-1Mb). The contribution of each of the genes located within the critical region to the phenotype can be different, and sometimes it is possible to identify one major gene responsible for the core phenotype. Almost 10-15% of the patients affected by genetic related MR carry an overt or subtle chromosomal abnormality. The second group includes a few hundreds of genes spread throughout all chromosomes. More than 200 genes are located on the X chromosome. Among these, FMR1 is responsible for the most frequent inherited form of MR, Fragile-X syndrome, which affects 1 in 6000 people. On the contrary, all the other genes individually account for a very small proportion of MR disorders. The functions of the genes involved in MR can be very diverse and include the structure, the function or the metabolic environment of neurones. To search the causes of mental retardation is important for many reasons: to define a prognosis, to start a proper care plan, to provide a specific recurrence risk and to get a proper support to the family. Guidelines on the evaluation of mental retardation have been established through Consensus Conferences, one of the foremost has been from the American College of Medical Genetics in 1997. As the research advances, new genes are identified and new techniques available, thus improving both knowledge and tools that can drive clinicians in the diagnostic process. It’s now emerging that mental retardation can be the end result of a number of different abnormal pathways, no one of them overriding the others, which underlie the huge complexity of our intellectual processing. So unraveling the causes of mental retardation phenotypes will ultimately be important to understand how the brain develops and works and eventually to find out possible specific treatments.


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Original version - English

abstract FORZANO_Genetics_of_MR_Ronzano_2012_april_5_STUDENTS_5202.pdf

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F. Forzano. Genetics of mental retardation 2012. EUROGENE portal. June 2012. online:



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