Holoprosencephaly-Polydactyly syndrome: in search of an etiology

Abstract

Holoprosencephaly-Polydactyly (HPS) or Pseudotrisomy 13 syndrome are names conferred to clinically categorize patients whose phenotype is congruent with Trisomy 13 in the context of a normal karyotype. The literature suggests that this entity may be secondary to submicroscopic deletions in holoprosencephaly (HPE) genes; however, a limited number of investigations have been undertaken to evaluate this hypothesis. To test this hypothesis we studied a patient with HPE, polydactyly, and craniofacial dysmorphologies consistent with the diagnosis of Trisomy 13 whose karyotype was normal. We performed mutational analysis in the four main HPE causing genes (SHH, SIX3, TGIF, and ZIC2) and GLI3, a gene associated with polydactyly as well as fluorescent in situ hybridization (FISH) to search for microdeletions in these genes and two candidate HPE genes (DISP1 and FOXA2). No mutations or deletions were detected. A whole genome approach utilizing array Comparative Genomic Hybridization (aCGH) to screen for copy number abnormalities was then taken. No loss or gain of DNA was noted. Although a single case, our results suggest that coding mutations in these HPE genes and copy number anomalies may not be causative in this disorder. Instead, HPS likely involves mutations in other genes integral in embryonic development of the forebrain, face and limbs. Our systematic analysis sets the framework to study other affected children and delineate the molecular etiology of this disorder.

Figure 1

The radiographic and clinical phenotypes. MRI of the head revealed: (A) a large cystic region (red arrow) and agenesis of the corpus callosum on sagittal sections and (B) a single ventricle containing a large dorsal cyst (red arrow) on the axial view. Some separation of the two cerebral hemispheres can be seen in the axial view. The radiographic findings are consistent with semi-lobar HPE. (C)Frontal view of the face reveals a midline frontal capillary hemangioma (red arrowhead), bilateral epicanthal folds, exotropia, and hypoplastic ala nasae. (D,E)The right hand exhibits 6 digits (a post-axial pedunculated digit) and syndactyly of digits 4 and 5. (F)The left hand has 8 digits, syndactyly of digits 1 to 4, 5 and 6 and a cleft between the fused digits. (G,H) The left foot has 6 digits, the supernumerary digit is a small pedunculated. (I) the right foot also has 6 digits and a broad bifid great toe.

Figure 2

Cytogenetic studies. (A) A high-resolution karyotype without evidence of chromosome 13 aberrations. (B) Multicolor fluorescence in situ hybridization analysis performed on metaphase chromosomes. A panel of six probes (BAC clones) containing four HPE genes (SHH, ZIC2, SIX3, TGIF) and two candidate genes (DISP1, FOXA2) were used and image processed. No deletions were noted. (The probe or BAC clone name is under the gene name for easy correlation)

Figure 3

CGH array results (chromosome 13). Genome wide analysis did not reveal any gain or loss of chromosomal material implying 2 or more tailing probes on chromosome 13, as well as on all the other chromosomes (data not shown).