The molecular mechanisms underlying the HPS brain, craniofacial and limb phenotypes in this entity are unclear. In order to determine the molecular mechanisms responsible for HPE other disorders with similar phenotypes should be considered in the differential diagnosis and ruled out. The major considerations involved in the differential diagnosis involve disorders with HPE, limb anomalies and those with both, taking into consideration phenotypic heterogeneity in a number of disorders. This disorders involve chromosomal aberrations, classic HPE and those disorders with polydactyly.
We began our search for the molecular basis of HPS in our patient based upon HPE. Because HPE is a major diagnostic criterion for this syndrome we first screened for coding mutations in four main genes known to cause HPE (SHH, ZIC2, SIX3
, and TGIF
). We then tested for deletions and duplications in these genes as well as in two candidate genes involved in the Sonic Hedgehog signaling pathway (DISP1
. No abnormalities were identified, therefore we used aCGH for a pan-genomic screening for DNA gains and losses. No genomic copy number changes were detected. In particular, the 5q35.1 cytoband (FBXW11
gene locus) which was elegantly reported as duplicated in one case of HPE with preaxial polydactytly, was not duplicated in our patient [9
]. Next, we focused on GLI3
as a candidate gene because the polydactyly was more characteristic of that seen in patients with GLI3
mutations such as Greig cephalopolysyndactyly or Pallister-Hall syndromes than typically seen in Trisomy 13. No mutations in the coding sequence were found in GLI3
, suggesting that coding mutations in this component of the hedgehog signaling pathway is unlikely to be causative. Cholesterol studies were normal, ruling out Smith-Lemli-Opitz syndrome, in which HPE in observed in approximately 5% of cases and exhibit a similar limb phenotype.
Explanations for our results include: the mutation is intronic in the genes we screened or does not involve the genes we analyzed; copy number aberrations do not lead to HPS or the resolution of the FISH and array probes were not sufficient for their detection; the developmental anomalies seen in this syndrome results from a elaborate interplay between genetic and environmental factors which are yet to be determined.
Our results underscore the huge void in our knowledge of the underlying molecular mechanisms leading to HPS, as well as a number of other developmental disorders involving multiple anatomic structures. The complexity of these molecular mechanisms in this disorder is exemplified by the brain and limb malformations. At first the molecular mechanisms underlying HPS appears counter-intuitive because classical HPE appears to be due to a loss-of-function whereas polydactyly results from gain-of-function. Possible explanations for this paradox include: a single gene mutation results in the dysregulation of a signaling pathway which exhibits bifunctional characteristics leading to gain-of-function in some embryonic structures and loss-of-function in other structures during development. This may result because the brain and limb have different evolutionary origins and although they may use the same molecular pathways during embryonic development their response to a given signaling pathway differs. Alternatively, the phenotype is not the result of a single gene mutation (monogenic) but involves mutations in two different yet undetermined genes (digenic) such as described in other human diseases including HPE [10
Although definitive conclusions regarding the molecular etiology can not be made based upon a single case, our approach to the clinical workup of patient’s with this HPS sets the stage for the evaluation of other children with this syndrome. Additional patients are needed to elucidate the genetics leading to the abnormal brain, face and limb phenotypes in HPS. Advances in our understanding of brain, face and limb development may also provide new candidate genes and novel avenues in which to direct research efforts and diagnostic testing of this elusive syndrome.