Members of the AFF (AF4/FMR2) family of putative transcription factors are involved in infant acute leukaemia and intellectual disability (ID), although very little is known about their transcriptional targets.
These results suggest that FMR2 is an upstream regulator of FOS and JUN, and further link deregulation of the immediate early response genes to the pathology of ID- and FRAXE-associated ID in particular.
AFF2/FMR2 is silenced in FRAXE intellectual disability, while the other three members have been reported to form fusion genes as a consequence of chromosome translocations with the myeloid/lymphoid or mixed lineage leukemia (MLL) gene in acute lymphoblastic leukemias (ALLs).
Although chromosome abnormalities that disrupt AFF2 have been reported in two individuals with mild-moderate intellectual disability, microdeletions of Xq28 that delete only AFF2 have not been described as a potential cause of FRAXE-intellectual disability.
With the forthcoming identification of the gene targets that trigger Purkinje cell death in the robotic cerebellum, and the functional conservation among the ALF proteins, the robotic mouse promises to deliver important insights into the pathogenesis of human ataxia, but also of mental retardation to which FMR2 and LAF4 have been linked.
Reverse transcriptase PCR studies on the FMR2 and FMR3 genes showed that only the FMR3 gene transcription was abolished, suggesting a possible causal relationship between the lack of FMR3 expression and mental retardation in this patient.
The presence of a phenotypically normal male with absent FMR2 expression in fibroblasts suggests that the relationship between the FRAXE mutation, FMR2 expression and MR needs to be further investigated.
This review summarises the new data on FRAXE associated mental retardation and the FMR2 gene in the light of the recent discoveries of new genes responsible for other forms of non-specific X-linked mental retardation.
So far, seven X-chromosomal genes mutated in nonspecific mental retardation (MRX) have been identified: FMR2, GDI1, RPS6KA3, IL1RAPL, TM4SF2, OPHN1 and PAK3 (refs 2-9).
We examined the prevalence of the fragile X mental retardation (FMR1) full mutation and fragile X E mutation (FMR2) among preschoolers evaluated for language delay.
Expansion of a (CCG)n repeat in the FMR2 gene corresponds to the FRAXE fragile site which lies distal to FRAXA and is also associated with mental retardation, but it is less frequent and lacks a consistent phenotype.
These results suggest that neither the FMR1 nor the FMR2 mutation is a common etiology of academic failure among school-age children without mental retardation and that the prevalence of the FMR1 premutation is no more frequent in children with academic failure than it is in the general population.