FRAXE mutation in a mentally retarded subject and in his phenotypically normal twin brother

AFF2 Is Associated With X-Linked Partial (Focal) Epilepsy With Antecedent Febrile Seizures

OBJECTIVE

AFF2 mutations were associated with X-linked intellectual developmental disorder-109 and in males with autism spectrum disorder (ASD). The relationship between AFF2 and epilepsy has not been defined.

METHOD

Trios-based whole-exome sequencing was performed in a cohort of 372 unrelated cases (families) with partial (focal) epilepsy without acquired causes.

RESULTS

Five hemizygous missense AFF2 mutations were identified in five males with partial epilepsy and antecedent febrile seizures without intellectual disability or other developmental abnormalities. The mutations did not present in the controls of general populations with an aggregate frequency significantly higher than that in the control populations. Previously, intellectual disability-associated AFF2 mutations were genomic rearrangements and CCG repeat expansion mutations mostly, whereas the mutations associated with partial epilepsy were all missense. Missense AFF2 mutations associated with epilepsy fell into the regions from N-terminal to the nuclear localization signal 1 (NLS1), while ASD-associated missense mutations fell in the regions from NLS1 to C-terminal.

CONCLUSION

AFF2 is potentially a candidate causative gene of X-link partial epilepsy with antecedent febrile seizures. The genotype-phenotype correlation and molecular sub-regional effect of AFF2 help in explaining the mechanisms underlying phenotypic variations.

Autism spectrum disorder: FRAXE mutation, a rare etiology

Autism spectrum disorder (ASD) is characterized by impaired social interaction and communication, restricted interests and repetitive behaviors. Fragile X E is associated with X-linked non-specific mild intellectual disability (ID) and with behavioral problems. Most of the known genetic causes of ASD are also causes of ID, implying that these two identities share common genetic bases. We present a child with an ASD with a normal range of intelligence quotient, that later evolved to compulsive behavior. FRAXE locus analysis by polymerase chain reaction revealed a complete mutation of the FMR 2 gene. This report stresses the importance of clinicians being aware of the association between a full mutation of FMR2 and ASD associated with compulsive behavior despite normal intellectual level.

Instability and chromatin structure of expanded trinucleotide repeats

Trinucleotide repeat expansion underlies at least 17 neurological diseases. In affected individuals, the expanded locus is characterized by dramatic changes in chromatin structure and in repeat tract length. Interestingly, recent studies show that several chromatin modifiers, including a histone acetyltransferase, a DNA methyltransferase and the chromatin insulator CTCF can modulate repeat instability. Here, we propose that the unusual chromatin structure of expanded repeats directly impacts their instability. We discuss several potential models for how this might occur, including a role for DNA repair-dependent epigenetic reprogramming in increasing repeat instability, and the capacity of epigenetic marks to alter sense and antisense transcription, thereby affecting repeat instability.

Transcription defects induced by repeat expansion: fragile X syndrome, FRAXE mental retardation, progressive myoclonus epilepsy type 1, and Friedreich ataxia

Fragile X mental retardation syndrome, FRAXE mental retardation, Progressive myoclonus epilepsy Type I, and Friedreich ataxia are members of a larger group of genetic disorders known as the Repeat Expansion Diseases. Unlike other members of this group, these four disorders all result from a primary defect in the initiation or elongation of transcription. In this review, we discuss current models for the relationship between the expanded repeat and the disease symptoms.

FMR2 function: insight from a mouse knockout model

The FMR2 gene is dysregulated by the fragile X E triplet repeat expansion in patients with FRAXE mental retardation syndrome. A CCG triplet, located in the 5' untranslated region of the FRAXE gene undergoes expansion and methylation in these patients, eliminating detectable gene transcription. FRAXE syndrome is distinct from fragile X syndrome, a more common genetic form of mental retardation caused by expansion and methylation of a similar repeat in the FMR1 gene located 600 kb proximal to FRAXE. FRAXE syndrome is rare, and patients' phenotypes are highly variable, leading to difficulties with predicting specific FMR2 functions based on the human disease. Recently, Lilliputian(Lilli), a Drosophila FMR2 orthologue, was identified; this gene has been linked with several signal transduction pathways, including the transforming growth factor-beta (TGF-beta) pathway, the Raf/MEK/MAP kinase (MAPK) pathway, and the P13K/PKB pathway. Mutation of Lilli shows defects in germinal band extension, cytoskeletal structure, cell growth, and organ development. The Lilli gene suggests possible functions for FMR2 (and related genes) in humans and mice, but cannot predict specific functions. Modeling FMR2 mutation in the mouse will be useful to understand specific functions of this gene in vertebrates. This review presents what has been learned thus far from the FMR2 knockout mouse model and suggests future studies on this model in order to compare it with the human FRAXE mental retardation disorder, Lilli mutants in Drosophila and other mouse models of genes in this family.

Clinical, cytogenetic, and molecular description of a FRAXE French family

BACKGROUND

FRAXE is a second locus associated with X chromosome fragility. Similar to FRAXA, the common mutation is a GCC expansion located in the 5' untranslated region, leading to the hypermethylation of the region and to the subsequent inactivation of specific genes (FMR1 and FMR2, respectively). Unlike FRAXA, FRAXE has a rare occurrence and is less currently studied in routine analyses. The phenotype associated with FRAXE is usually considered as mild or moderate mental retardation, with incomplete penetrance. However, phenotype/genotype relations have been less characterized.

OBJECTIVE

We report a French family with three members affected with mental retardation, including a female suffering from West syndrome, and two mentally retarded males.

METHODS

After exclusion of the FRAXA expansion by Southern blot analysis, we performed a karyotype using folate-thymidine-deficient medium and a southern blot to search for FRAXE expansion.

RESULTS

All three mentally retarded patients had a number of repeats over 800 GCC and expressed more than 20% of fragile sites in their leukocytes. Another carrier female with a full expansion had a subnormal mental impairment.

CONCLUSIONS

Clinical features and both the cytogenetic and molecular findings seem to correlate in this family. We discuss the bias encountered when studying such families and some of the mechanisms that may explain part of the clinical variability.