It is now widely accepted that holoprosencephaly is an example of a multi-factorial trait requiring the synergy between novel mutations in key genes, the interaction of these mutations with endogenous host variants, and the likely additional effect of environmental insults. A difference between human HPE and its closest mouse model was apparent from the first example. When mice are deleted for both copies of the Sonic hedgehog
gene, the animals display uniformly severe HPE-like features, growth retardation, limb anomolies, extreme cyclopia and defective axial patterning throughout the entire neuraxis. However, murine Shh +/-
heterozygotes are phenotypically normal [Chiang et al., 1996
]. Although this degree of clinical severity, evident in the homozygous null mouse embryos, can be seen in humans, it is not typical for these cases to survive to term. On the other hand, heterozygous variations in the SHH
gene are the most commonly detected mutations in a live-born collection of HPE probands [Roessler et al., 1996
]. Furthermore, instances of two mutations in the human SHH
gene in the same individual HPE patient have not been reported. With the passage of time, this gene dosage discrepancy has never been fully explicated. In one scenario, this paradox would be explained by invoking multiple different genetic alterations. However, these mutations, in our current view, would likely occur in several independent genes (in humans) instead of two identical mutations in the same gene (in mice).
Subsequent studies of model systems confirmed that dysfunction of hedgehog signaling was a common mechanism for the production of HPE-like phenotypes [reviewed in Roessler et al., 2003
; Ingham 2008
]. Three additional genes in the human SHH signaling pathway have been described as mutational targets in HPE patients, including the SHH receptor PTCH1
[Ming et al., 2001; Ribiero et al., 2006
], the ligand transporter DISP1
[Ma et al., 2002
; Roessler et al., 2009b
] and the transcription factor GLI2
[Roessler et al., 2003
; Rahimov et al., 2007]. Again, we detect salient differences between the mouse models and the human phenotypes of HPE probands with heterozygous mutations. These phenotypic differences suggest that the consequences of diminished hedgehog signaling are similar between mice and humans, but that number and types of genetic alterations that accomplish them are different.
A recurrent theme emerging from the comparison of mouse models of HPE with human pathologies is the notion that homozygous null animals serve as proof and illustration of the more severe phenotypic extremes but do not reliably reconstruct the genetic architecture of human HPE cases [Hayhurst and McConnell, 2003
]. For example, mice lacking the key transducer of hedgehog signals, Smo, arrest early in embryonic development due to the elimination of all hedgehog signals [e.g. see 1B, fails to proceed to 1B′; see Zhang et al., 2003]. Similarly, mice homozygous null for Disp
arrest at a nearly identical stage [Ma et al., 2002
]; however, in both cases the murine heterozygotes were phenotypically normal. These differences between murine and human phenotypic pathologies from a given gene variant are common to almost all murine HPE models and suggest: 1) humans with two lesions in the same gene are likely to be uncommon, and, 2) if present, it would be expected to reflect only the severe end of the spectrum [reviewed in Krauss 2007
]. The extreme variability that is characteristic of HPE is difficult to explain if both alleles of a key gene (genetically recessive) must be significantly impaired in most cases. Furthermore, heterozygous carriers should be prevalent in a control population; yet, this is contrary to the experience of molecular diagnostic centers. In contrast, gene-gene interactions between mutations and functionally linked factors and/or gene-environmental interactions would not be precluded in a model of novel heterozygous mutations interacting with other factors.