Alterations in the relative amounts of specific mRNA species in the developing human brain in Down's syndrome.

Total cellular polyadenylated RNA [poly(A)+ RNA] was prepared after guanidinium thiocyanate extraction of frozen brain tissue from age-matched normal and Down's-syndrome (trisomy 21) human foetuses. Poly(A)+ RNA populations were analysed by translation in vitro, followed by two-dimensional gel analysis by using both isoelectric focusing (ISODALT system) and non-equilibrium pH-gradient electrophoresis (BASODALT system) as the first-dimension separation. The relative concentrations of poly(A)+ RNA species coding for seven translation products were significantly altered in Down's syndrome, as determined by both visual comparisons of translation-product fluorograms from normal and Down's-syndrome samples and by quantitative radioactivity determination of individual translation products. The relative concentrations of mRNA species coding for two proteins (68 kDa and 49 kDa) were increased in Down's syndrome and may represent genes located on chromosome 21. The relative concentrations of mRNA species coding for five proteins (37 kDa, 35 kDa, 25.5 kDa, 24.5 kDa, 23 kDa) were decreased in Down's syndrome, these probably representing secondary effects of the trisomy. Six Down's-syndrome-linked translation products (49 kDa, 37 kDa, 33 kDa, 25.5 kDa, 24.5 kDa, 23 kDa) did not migrate with appreciable amounts of cellular proteins on two-dimensional gels and hence may represent either proteins of high turnover rates or those that are post-translationally modified in vivo. One translation product (68 kDa) comigrated with a major cellular protein species, which was identified as a 68 kDa microtubule-associated protein by limited peptide mapping. The significance of these changes is discussed in relation to the mechanisms whereby the Down's-syndrome phenotype is expressed in the human brain.