Facial abnormalities in the same patient
Facial abnormalities in the same patient
Between the fourth to sixth week of gestation, the forebrain (prosencephalon) is divided into the two hemispheres. Absence of this cleavage results in a spectrum of malformations called holoprosencephaly (HPE). Because the olfactory nerves which are part of the rhinencephalon are absent, the term arrhinencephaly has also been applied to this malformation. However, in HPE, there is much more missing than the olfactory brain. In the most severe form, alobar HPE, the brain consists of a single spherical forebrain structure with a single ventricle. A large cyst which communicates with the ventricle is present in the posterior-dorsal part of the brain. The brain in alobar HPE is small and the gyral pattern and cortical architecture are abnormal. The eyes, which evaginate from the forebrain in the fourth week, are small and malformed or there is only one eye (cyclopia). In milder forms (semilobar and lobar HPE), the brain is larger and there is partial separation of the hemispheres.
A specrum of midline facial anomalies accompanies the brain malformations. In alobar HPE, these are severe and include a proboscis (a trunk-like structure above the single eye), a single nostril, cleft lip, and cleft palate. Milder brain pathology is accompanied by milder or subtle facial defects such as a central incisor and hypotelorism. The correlation between the facial anomalies and HPE was pointed out by DeMeyer in a paper titled "The face predicts the brain" (Pediatrics1964;34:256-63). Alobar HPE is incompatible with survival. Milder forms are associated with variable psychomotor retardation depending on the pathology. Diabetes insipidus is frequent in these patients.
is rare among live born infants
but very common in embryogenesis.
It has genetic and environmental
causes. Most cases are sporadic
but there are also autosomal
dominant, recessive, and X-linked
forms. HPE also occurs as a component
of multiple malformation syndromes
and in several chromosomal abnormalities,
most commonly trisomy 13, trisomy
18, and triploidy. Genetic HPE
is associated with four genes
and has been linked to seven
additional chromosomal loci.
The best known HPE gene is the Sonic
Hedgehog (SHH) gene
on 7q36 which is important for
ventral patterning of the forebrain.
Mutations of this gene cause
autosomal dominant HPE. Defective
cholesterol synthesis inhibits
SHH signaling resulting in HPE-like
malformations. Retinoic acid
participates in the SHH system. Excess
retinoic acid during
embryogenesis (from administration
of Accutane for acne) inhibits
SHH and causes HPE and other
malformations. The HPE-associated
gene TG-interacting factor (TGIF)
on 18p11, regulates retinoic
acid. Mutations of TGIF result
in unrestrained retinoic acid
activity and HPE. The multitude
of genes and chromosomal loci
associated with HPE underlines
the complexity of genetic programs
that are involved in embryonic
patterning and the intricate
interaction between genes and
The chemical messages that induce the forebrain to divide into two hemispheres, including SHH, are first expressed in the prechordal plate, an area rostral to the notochord that gives rise to the facial mesoderm. SHH is also involved in craniofacial development.
Agenesis of the corpus callosum
Agenesis of the corpus callosum and pachygyria.
At about 10 weeks of gestation, a glial bridge (massa commisuralis) forms between the two hemispheres, at the bottom of the interhemispheric fissure. Soon after this, axons begin to cross this bridge, forming the corpus callosum (CC). This process is completed by 18 to 20 weeks gestation. Agenesis of the corpus callosum (ACC) develops either if the bridge does not form or if axons fail to cross it. HPE can be mistaken for ACC.
ACC is one of the most common and probably the most genetically diverse brain malformation. It occurs in many chromosomal abnormalities and malformation syndromes and in several inherited metabolic disorders. Among others, it is associated with mutations of the L1 cell adhesion molecule, a cell surface glycoprotein that is important for guidance of migrating neurons (see neuronal migration defects).
ACC may be complete or partial, involving only the posterior part of the CC (splenium). It may occur as an isolated defect. Many cases, however, have other associated CNS and non-CNS malformations. Most patients have psychomotor retardation. A small minority-usually patients with isolated ACC-have normal neuromotor skills. About one third of patients have seizures. When the CC is absent, the anterior horns of lateral ventricles have a bat-wing shape and the posterior horns are dilated and parallel to one another. The gap between the two hemispheres is filled sometimes by adipose tissue.
- Solomon BD, Rosenbaum KN, Meck JM, Muenke M. Holoprosencephaly due to numeric chromosome abnormalities. Am J Med Genet C Semin Med Genet. 2010 ;154C:146-8. PubMed
- Solomon BD, Mercier S, Vélez JI, et al.Analysis of genotype-phenotype correlations in human holoprosencephaly. Am J Med Genet C Semin Med Genet. 2010;154C:133-41. PubMed
- Roessler E, Muenke M.The molecular genetics of holoprosencephaly. Am J Med Genet C Semin Med Genet. 2010;154C:52-61. PubMed
Updated: February, 2012