Lp is one of only two known mouse mutants that exhibit craniorachischisis as a major phenotype. Two other mutations have been reported to give an occasional fetus with craniorachischisis (the noggin knockout, and the disorganisation mutant) but these are
rare events, suggesting possible environmental effects or generalised embryonic disruption. In comparison, defects in 23 known genes (Table 1.1) and 19 mouse mutants (Table 1.2) cause either spina bifida or exencephaly. This information can be interpreted to mean several different things.
Firstly, it can be taken to suggest that the majority of genes involved in Closure 1
exhibit functional redundancy, such that mutations that affect Closure 1 are rare. Only a few genes are essential for this site of neural tube closure, exemplified by the small number of genes and mutants that affect Closure 1. Another interpretation is that very few genes are involved in Closure 1, so that gaining a mutation in one of these genes is a rare event. The existence of only one allele of loop-tail may imply either that the gene is very small, giving only a low possibility of a random mutation affecting it, or that a mutation that causes loss of function of the gene does not have a developmental
phenotype; perhaps the rare Lp mutation is due to an unusual mutation that creates some gain-of-function.
The Lp mutation exhibits a semi-dominant effect. Although this could be caused by a gain-of-function mutation, dominant effects are also observed in mutants with the loss of function of a gene. This effect is known as haploinsufficiency; a mutant phenotype is generated owing to the low level of a particular gene product. Examples of mutations that exhibit haploinsufficiency are the GU3 mutation responsible for the extratoes
phenotype and the mutation of the hematopoietic growth factor KL, encoded by the
Steel locus (Huang et a l, 1990). GU3 encodes a zinc finger transcription factor, and at least one allele of Xt disrupts the transcription of GU3, creating a null mutant. The homozygous lack of GU3 is lethal, but the heterozygote exhibits limb deformities; this genetic dominance is the result of haploinsufficiency. Similarly, the steel mutant contains a deletion and is therefore a null allele (Huang et al., 1990), but heterozygotes exhibit a phenotype owing to haploinsufficiency.
1.9.1 Spontaneous mutations encompass many different genomic
alterations
Loop-tail is a spontaneous mutant. Many different genomic changes have been identified in spontaneous mouse and human mutations, including single nucleotide substitutions which either change the encoded amino acid (missense mutations), convert
a coding triplet into a STOP codon (nonsense mutations), or disrupt intron-exon splicing patterns (Hastbacka et al, 1994; Wuyts et al, 1996; Carstea et a l, 1997; Everett et al,
1997; Nagamine et al, 1997; Sauer et al, 1997; Sidow et a l, 1997; Zuo et a l, 1997; Savukoski et a l, 1998; Varon et a l, 1998). Other mutations have been identified that consist of deletions or insertions of a few (1-5) base pairs, changing the reading frame (Hastbacka fl/., 1994; Carstea gf a/., 1997; Everett era/., 1997; Sauer a/., 1997; Biervert et al, 1998; Savukoski et al, 1998; Varon et a l, 1998). A few human disease genes have been identified with larger deletions, such as one or more exons (Hardtke and Berleth, 1998) or 73 bp deletion (Carstea et a l, 1997).
A significant number of mouse and human mutations have recently been shown to be caused by gene disruption due to the insertion of a transposable element. Insertion into an exon disrupts the transcript directly, while other cases cite insertion into an intron, leading to a mutant transcript by generating splicing defects. A major class of transposons are the long interspersed nuclear elements (LINEs or Lis), a family of autonomous retrotransposons that comprise approximately 15% of the mammalian genome (Kazazian and Moran, 1998). Autonomous retrotransposable elements are mobilized via an RNA intermediate; the RNA they transcribe encodes a reverse
transcriptase, which acts on its RNA template to create a DNA copy that can reintegrate into a new location in the genome. Examples of mutations that are caused by LI
insertion events include a muscular dystrophy patient with a disruption of the dystrophin gene (Holmes et al, 1994), LI insertions in the factor VIU gene that lead to haemophilia A (Kazazian et al, 1988), disruption of the glycine receptor p subunit (Glyrb) gene in the spastic mouse mutant (Miilhardt et a l, 1994) and an intronic insertion in the RP2
gene, causing retinitis pigmentosa (Schwahn et al, 1998).
Insertion of Alu repeats (another class of retrotransposons) has also been identified in a number of human mutations (reviewed by Kazazian & Moran, 1998). Several
spontaneous mouse mutants are due to the insertion of other, related, types of
retrotransposons. One example is the pale ear (ep) mutant, which has an intracistemal A particle (LAP) element insertion near the 3’ end of the coding sequence (Gardner et al,
1997). The nu-Bc allele of the nude mutant has an intronic integration of an early transposon (ETn) element in the nude gene (whn) (Hofmann et a l, 1998). Another ETn integration has been identified within an exon of the tyrosinase gene, in the mutant
allele c-3Bc (Hofmann et a l, 1998). Although lAPs and ETns are both types of autonomous retrotransposons, all the lAPs & ETns identified to date have mutations in their reverse transcriptase gene that make this enzyme defective; they require the action of a cellular reverse transcriptase for their mobilisation (Kazazian and Moran, 1998). It has recently been estimated that 10% of spontaneous mouse mutants are caused by retrotransposition events (Kazazian and Moran, 1998), 60-fold greater than the
proportion observed in humans, owing primarily to the increased mobility of defective retrotransponsons in mice.