Mice heterozygous for the mld mutation have intermediate levels of myelin basic protein mRNA and its translation products.

Myelin-deficiency (mld) is an autosomal recessive mutation in mice exhibiting a severe deficit in the synthesis of myelin basic protein (MBP). In order to understand the mechanisms involved in the regulation of MBP synthesis in the mld mutation, we examined the amount of MBP and MBP-specific mRNA in control, heterozygous and homozygous mld brains. In vitro translation of poly(A)+ RNA in a cell-free system, in situ hybridization, and filter hybridization with a radiolabelled probe pMBP-1 after dot or Northern blotting were used in this study. The levels of MBP and MBP-specific mRNA were very low but detectable in mld homozygotes, and intermediate in heterozygotes. MBP specific mRNA from mutants, and its translation products, were of normal size. These results show that the mld mutation is expressed co-dominantly in heterozygotes and affects a cis-acting regulatory element controlling the MBP gene.