Holoproesencephaly

Holoprosencephaly First described in 1963 Failure of proper formation of the midline structures of the forebrain is the common feature of the many variant forms. Occurs in about 1/10,000 liveborn infants but is much more frequent in prenatal studies.

Etiology of HPE Chromosomal Teratogens Syndromes Single gene disorders HPE is highly heterogeneous and likely cause depends to some extent on how it is ascertained. In prenatal diagnosis, the majority have chromosome abnormalities. Most of these die, so if you consider living children, the majority do not have chromosome abnormalities. Experience suggests that in postnatal cases, the majority are unexplained.

Famous Teratogens Alcohol Cyclopamine Your textbook indicates that fetal alcohol exposure is the number one cause of holoprosencephaly. As far as I’m concerned this is not true. Chromosome abnormalities of the number one cause of holoprosencephaly. The second-largest group is probably unknown etiology. Fetal alcohol exposure has cause of malformation is difficult to prove and to document. The story of cyclamate is very interesting. Ranchers noted that sheep grazing in certain areas had a high incidence of holoprosencephaly. This led to the discovery that the ingestion of a particular weed resulted in malformation, and this in turn led to the discovery of a small molecule that was named cyclopamine. Later it was discovered that this molecule specifically inhibits sonic hedgehog pathway.

HPE Genes Sonic Hedgehog (SHH); chromosome 7 ZIC2 (chromosome 13q32) SIX3; chromosome 2 TGIF, chromosome 18 Others The discovery that mutations in single genes can result in holoprosencephaly has been one of the success stories in the genetics of human malformation. The overall percentage of holoprosencephaly cases caused by mutations in single genes is probably about 10. The two biggest groups are SHH and ZIC2.

The point of this slide is to indicate that the three primary vesicles become further subdivided into five vesicles. The anterior most of these, the telencephalon, becomes divided in the midline to produce the the paired cerebral hemispheres.

This slide serves to show that a single telencephalic vesicle exists prior to the subdivision into paired cerebral hemispheres.

This slide indicates that the prechordal plate mesoderm represents a cephalic continuation of the notochord. Signals from the prechordal plate are necessary for the division of the single telencephalic vesicle to form the paired cerebral hemispheres. Exactly how this happens is shrouded in some mystery. It also happens very quickly - perhaps in 24 hours.

Holoprosencephaly can occur in many variant forms Holoprosencephaly can occur in many variant forms. In the most severe form there is no evidence of the midline and in less severe forms there are varying degrees of malformation of the midline. The classification scheme used in this slide is a rough guide. Many individual cases really do not fit perfectly into these categories.

This MRI is an example of lobar holoprosencephaly.

Alobar HPE. (A) lack of separation of the two hemispheres Alobar HPE. (A) lack of separation of the two hemispheres. Large dorsal cyst (dc) posteriorly. (B) reveals a midline ventricle, a monoventricle (mv), that communicates posteriorly with the dorsal cyst (dc).

Semilobar HPE. (C) separation of the hemispheres posteriorly but not anteriorly. There is incomplete separation of the basal ganglia. (D) reveals a lack of interhemispheric fissure and a monoventricle (mv)

Lobar HPE. (E) reveals that two hemispheres are separated by an interhemispheric fissure both anteriorly and posteriorly. (F) documents incomplete separation of the inferior frontal lobes near the midline.

This is an example of middle interhemispheric fusion defect, the mildest variant of holoprosencephaly.

These scans also show middle interhemispheric fusion defect These scans also show middle interhemispheric fusion defect. Note that there is a clear-cut midline both anteriorly and posteriorly but in the coronal sections in the midline there is no separation between right and left.

Face Predicts the Brain ~85% of HPE cases are associated with facial malformation of various types. Thus, brain imaging in the context of facial malformation is a good idea. Nonetheless, brain malformation can be severe even with a relatively normal face.

These are the faces of individuals with varying degrees of holoprosencephaly. The individual shown in the last three frames has a single central incisor as his only manifestation. He carries a mutation in the sonic hedgehog gene. There is no clear understanding of why some individuals have severe facial malformation and others do not.

13q32 deletion in a fetus in which HPE was the only malformation. My own research on deletions of chromosome 13 lead to the discovery of Zic2 as a holoprosencephaly gene. I present this to you not because of the tremendous importance of Zic2 mutations as a cause of holoprosencephaly, but rather to demonstrate how clinical investigations have led to huge advances in the understanding of the molecular pathogenesis of this brain malformation. For Zic2 story is similar to many other cases where chromosome abnormalities have led to the discovery of the genetic basis of a malformation or disease. The slide shows the chromosome 13s from an individual with HPE. Band q32 is largely missing, as is shown in the diagram. Deletions of chromosome 13 and q32 are frequently associated with brain malformations including holoprosencephaly.

This diagram depicts the location of deletions in a series of patients This diagram depicts the location of deletions in a series of patients. Those with brain malformations all had deletions that included all or part of band q32. This type of analysis of cytogenetic abnormalities ultimately led to the discovery of ZIC2 as an HPE gene. The same type of analysis is also what led to the discovery of sonic hedgehog as a holoprosencephaly gene.

Alobar HPE and alanine tract expansion in 2 sibs Alobar HPE and alanine tract expansion in 2 sibs. Father is a mosaic carrier of the mutation. Both of the children depicted in this picture have mutations in Zic2 and alobar holoprosencephaly. Note the nearly normal faces.

The slide shows them to the father of the two children in the previous slide is a mosaic carrier of the Zic2 mutation inherited by both of the affected children. As the sequence indicates, the mutation is a 30 base insertion.

De-novo 7 BP deletion in zinc finger region. Alobar HPE. This is another baby with a Zic2 mutation. Note the near normal face and a very highly abnormal brain. Thus far, none of the patients with mutations in Zic2 have had significant facial malformation. De-novo 7 BP deletion in zinc finger region. Alobar HPE.

The slide depicts the mutation in the baby from the previous slide

In this case, the patient has a mutation in Zic2 and the middle interhemispheric fusion defect form of holoprosencephaly. Despite his normal face and relatively normal looking brain, the degree of mental retardation in this child is extremely severe. He can’t sit upright without support and he has absolutely no communicative skills. 12 AA in frame deletion near carboxy terminus. Interhemispheric fusion defect.

De-novo alanine tract expansion. Semi-lobar HPE Again, a relatively normal looking face in conjunction with a severe brain malformation and very severe mental retardation.

De-novo 2 base deletion at AA 365. Stop at 366. Semi-lobar HPE. In this in the following slides I show more pictures of kids with holoprosencephaly and Zic2 mutations. Birth 21 Months De-novo 2 base deletion at AA 365. Stop at 366. Semi-lobar HPE.

Aspartic acid to Phenylalanine change caused by 2 base change Aspartic acid to Phenylalanine change caused by 2 base change. Lobar HPE. Inherited from mother who is normal except for hypotelorism.

De-novo single base deletion at AA 312. Stop at 413. Semi-lobar HPE.

Just for completeness is sake, this is a diagram of the amino acid sequence of the Zic2 genewith the various mutations summarized.

Day 15 coronal sections showing HPE like malformation in Zic2 mutant mice. Finally, these are sections from mice that had a engineered mutation in Zic2. The homozygous mutant mice have the functional equivalent of one working copy of Zic2. This study confirms the human finding that haploinsufficiency of Zic2 results in failure to adequately form the midline of the forebrain. From Nagai et al., PNAS, March 2000