Dominant Mutant Phenotypes

Mutations in human Kv-channel genes have been detected that are associated with hereditary diseases ranging from heart arrythmia (long QT-syndrome) and deafness to epilepsy and ataxia (see Table 2). Typically, many Kv-channel related channelopathies are correlated with a mutant phenotype that is episodic in nature and appears as a dominant hereditary trait. 

I now want to return to the transcriptional cascade and the Clk gene. As mentioned previously. Drosophila circadian transcription studies in vivo have relied heavily on a single dominant negative allele of Clock, ClkJ ) and one in mouse (Allada et al 1998). This is a precarious situation for the field, because some of the mutant phenotypes could be due to effects on other transcription factors and systems rather than just to low activity of the CLK—CYC complex. For this reason, we characterized a second allele of the Clk gene, which turned out to be a real recessive allele. 

Dominant mutations lead to a mutant phenotype in the presence of a normal allele of the gene. The phenotypes associated with dominant mutations often represent a gain of function but in the case of some genes result from a loss of function. 

A FIGURE 9-2 Effects of recessive and dominant mutant aiieies on phenotype in dipioid organisms. Only one copy of a... 

In the case of rapamycin, it was possible to demonstrate that TOR is the target using a dominant mutant of TOR that is resistant to FKBP-rapamycin inhibition. These resistance mutations are the single most definitive means of demonstrating the phenotypically relevant target of a small molecule. Unfortunately, the availability of resistance mutations can be limiting attempts to engineer such a mutation may adversely affect the function of the protein... 

If we try to enlarge the list of dominant mutants to be searched for in a screening system, we quickly discover that the phenotypes become less sharp, more difficult for the nonspecialist to recognize, and more ambiguous as to interpretation. Increasing the scope of biochemical testing systems to include mutations at many loci requires methods which are not yet available and which would be enormously expensive. 

The constitutive mutants so far isolated as resistant to D-fucose inhibition [6] or as revertants of D-ribulokinaseless mutants [54] all map within the araC gene as defined by araC point mutations or are closely linked to the farthest left C" point mutation at the operator end of the ara OIBAD operon (see Section VI,B, and Fig. 5). A severe test of the negative control-internal inducer model was performed with the isolation and characterization of revertants of deletion 719 that excises araO and araC [7,8] (see Section VII). Nineteen revertants analyzed were shown to map within the aral region and produced a cis-dominant constitutive phenotype I ). There were no revertants of this deletion with properties o R mutations. It has been shown in other systems lac and gal), where negative control has been established, that mutations to constitutivity in the repressor gene occur at a relatively high frequency [63,85]. 

The three Squat (sqt) mutants have a characteristic dominant-dumpy/ recessive-roller phenotype, and of the two genes characterized sqt-1 and sqt-3), both encode cuticle collagens (Kramer, 1997). 

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