[Abnormal electroencephalography results and specific language impairment: towards a theoretical neurodevelopmental model?]

BACKGROUND

Specific language impairment (SLI) is a primary developmental disorder in which language is significantly more impaired than other developmental domains. Abnormal electroencephalographic recordings without clinical seizures are often observed. The aim of this retrospective study was to characterize the frequency of these abnormalities, to describe them and to analyze their association with anamnestic, clinical, paraclinical and evolution characteristics.

METHODS

The cases of 35 children with a diagnosis of SLI, who also underwent electroencephalography and MRI, were systematically reviewed retrospectively.

RESULTS

In this population, aged between 4 and 7 years, 49% (n=17) of patients exhibited a specific expressive language disorder and 51% (n=18) a specific receptive disorder. Forty-nine percent of the children featured abnormal electroencephalography results. Abnormalities were essentially localized on the left side of the brain and in two specific regions: the temporo-occipital (60%) and the frontorolandic (30%) regions. The groups with and without abnormalities were compared statistically with each other in terms of clinical, paraclinical and evolution characteristics. Evolution data were available for 24 patients through a telephone interview and for nine patients through a new complete language evaluation. The comparison of the two groups showed significant differences in terms of severity of the phonological disorder, a higher number of delayed acquisition of walking and cleanliness and a higher range of non specific psychomotor difficulties.

DISCUSSION

A large proportion of children suffering from SLI present abnormal electroencephalography recordings with no clinical seizures. This rate is much higher than in the general population and the abnormalities are essentially localized on the left side of the brain in regions known for their specific role in language development. These abnormalities are more frequent in children with a severe phonological disorder, suggesting that they may share common pathophysiological features with SLI.

CONCLUSION

The presence of EEG abnormalities in a large group of patients suffering from SLI associated with minor neurological abnormalities suggests a possible theoretical neurodevelopmental model. Minor neurodevelopmental abnormalities, genetically transmitted or acquired during the pre- or perinatal period, may create vulnerability towards SLI. This vulnerability, in conjunction with environmental influences such as family environment, linguistic stimuli, exposure to multiple languages, or transitory hearing loss, might take the form of SLI. This hypothesis underlines the importance of prevention and early detection of SLI when identifying vulnerable subjects. Monitoring the family early through parental guidance and early school support would facilitate the acquisition of language.

Ectopic expression of Arabidopsis CYCD2 and CYCD3 in tobacco has distinct effects on the structural organization of the shoot apical meristem

Transgenic tobacco lines expressing Arath-CYCD2 or Arath-CYCD3 genes under a cauliflower mosaic virus 35S promoter are modified in the timing of their development, but not in the phenotype of their vegetative organs. They display an increased rate of leaf initiation, which is shown to be associated with distinct changes in the structural organization of their shoot apical meristem (SAM). Constitutive expression of Arath-CYCD2 leads to a progressive modification of the SAM structural organization with predominant periclinal divisions in the L3 layer and to the loss of the classical cytophysiological zonation, the central zone being reduced to the central cells of the L1 and L2 layers. These changes reveal a particular sensitivity of the corpus cells (L3) to Arath-CYCD2 over-expression and suggest a role for CYCD2 in controlling the planes of cell division in these cells. The SAM structural modifications in the Arath-CYCD3 over-expressing lines are less drastic; only an increased cell number together with a reduced cell size, particularly in the L1 layer, characterizes the peripheral zones. This could be related to the shortening of the G1-phase duration that renders cell growth incomplete between successive mitoses. Cell proliferation continues beyond the SAM in the developing internodes and confers a delayed senescence to Arath-CYCD3 over-expressing juvenile tissues. In addition, the ploidy levels of mature stem tissues in both types of transgenic lines are unaffected, suggesting that the studied G1 to S cell-cycle genes have no effect on the extent of endoreduplication in tobacco stem tissues.

Competency of Nicotiana tabacum L. stem tissues to dedifferentiate is associated with differential levels of cell cycle gene expression and endogenous cytokinins

The expression of the mitotic cyclin Arath; CYCB1;1 and of the cyclin-dependent kinase Arath; CDC2a was located by beta-glucuronidase histochemical detection and in situ hybridization, and was quantified by 4-methylumbelliferyl beta- D-glucuronide assay in tobacco stem tissues during both in vivo differentiation and in vitro dedifferentiation. The changes in localization of endogenous cytokinins were also determined during both processes using quantitative analysis and in situ immunocytochemistry. The CDC2a promoter was expressed continuously during stem development, with particularly high expression in the shoot apical meristem and in the internal and external primary phloem. CYCB1 expression was not restricted to the dividing cells; its expression in the shoot apical meristem was particularly high in the leaf-forming peripheral cells but the gene was also expressed throughout development in the internal and external phloem in which the rate of cell division was reduced or zero. Following in vitro culture, the internal phloem cells appeared to be particularly competent to re-enter the cell cycle within a short lag phase while the pith tissue reactivated later. In culture, cells that resumed division were found to accumulate cytokinins. The high competency of primary phloem to dedifferentiate was associated with its capacity to express CDC2a and CYCB genes and the presence of high cytokinin levels, providing some insights into the determinants of competency for resuming cell division.