Autosomal Dominant Holoprosencephaly Research Articles

SAT-288 Pituitary Developmental Defects Caused by Haploinsufficiency for the Transcription Factor SIX3 Are Worsened by POU1F1 Deficiency

Advances in genomic technologies are revolutionizing the practice of medicine by delivering molecular diagnoses that can be informative for prognosis and treatment of genetic disorders. Most of the known genetic causes of multiple pituitary hormone deficiency have been investigated as monogenic disorders. It can be challenging to predict clinical features from genetic data, as loss of function mutations in some genes can present with a spectrum of phenotypes ranging from craniofacial abnormalities, intellectual disability, and neurosensory and neuroendocrine defects to pituitary hormone deficiency with no other abnormalities. Although maternal exposures could be contributing factors, the contribution of rare, deleterious variation in other genes is a likely contributor. In humans, loss of function mutations in the transcription factor SIX3 cause variable, autosomal dominant holoprosencephaly with incomplete penetrance, and mouse models recapitulate some of the clinical features. Because Six3 and Pou1f1 gene expression patterns overlap in pituitary development, we hypothesized that doubly heterozygous mice (Six3+/-; Pou1f1+/dw) might have pituitary anomalies not present in singly heterozygous mice. We intercrossed Six3+/- and Pou1f1+/dw mice to produce doubly heterozygous animals. At e11.5, both Six3+/- and Six3+/-; Pou1f1+/dw exhibited abnormal morphology of the developing infundibulum and Rathke’s pouch, although ventral diencephalon expression of Tle4, Fgf10, and Nkx2.1 appeared normal. Both newborn Six3+/- and Six3+/-; Pou1f1+/dw littermates had abnormal pituitary gland morphology that resembled that of Aes-/-. AES is a co-repressor that interacts with SIX3. Specification of vasopressin neurons and anterior lobe hormone cell types appeared normal. Mice of all genotypes were born in expected Mendelian ratios (N=144, p=0.49), and there were no significant differences in body weight at 3 wks. A portion of the Six3+/- and doubly heterozygous mice developed hydrocephalus, exhibited failure to thrive, and died (6-9% of N=82, 85, respectively). At 6 wks, 25% (N=61) of the Six3+/-; Pou1f1+/dw animals exhibited striking pituitary dysmorphology in which the rostral aspect of the pituitary penetrated the palate. This was not observed in single heterozygotes. These results reveal that haploinsufficiency for Six3 affects Rathke’s pouch formation, resulting in pituitary gland dysmorphology in and around the stem cell niche. A significant portion of the Six3+/-; Pou1f1+/dw doubly heterozygous mice have a more pronounced pituitary phenotype than Six3+/-, supporting the possibility of digenic pituitary disease and highlighting phenotypic variability. Genetically engineered mice provide an excellent tool for assessing the possibility of gene-gene interactions that could enhance the severity of hypopituitarism and associated craniofacial development.

Clinical and experimental evidence suggest a link between KIF7 and C5orf42-related ciliopathies through Sonic Hedgehog signaling

Acrocallosal syndrome (ACLS) is an autosomal recessive neurodevelopmental disorder caused by KIF7 defects and belongs to the heterogeneous group of ciliopathies related to Joubert syndrome (JBTS). While ACLS is characterized by macrocephaly, prominent forehead, depressed nasal bridge, and hypertelorism, facial dysmorphism has not been emphasized in JBTS cohorts with molecular diagnosis. To evaluate the specificity and etiology of ACLS craniofacial features, we performed whole exome or targeted Sanger sequencing in patients with the aforementioned overlapping craniofacial appearance but variable additional ciliopathy features followed by functional studies. We found (likely) pathogenic variants of KIF7 in 5 out of 9 families, including the original ACLS patients, and delineated 1000 to 4000-year-old Swiss founder alleles. Three of the remaining families had (likely) pathogenic variants in the JBTS gene C5orf42, and one patient had a novel de novo frameshift variant in SHH known to cause autosomal dominant holoprosencephaly. In accordance with the patients’ craniofacial anomalies, we showed facial midline widening after silencing of C5orf42 in chicken embryos. We further supported the link between KIF7, SHH, and C5orf42 by demonstrating abnormal primary cilia and diminished response to a SHH agonist in fibroblasts of C5orf42-mutated patients, as well as axonal pathfinding errors in C5orf42-silenced chicken embryos similar to those observed after perturbation of Shh signaling. Our findings, therefore, suggest that beside the neurodevelopmental features, macrocephaly and facial widening are likely more general signs of disturbed SHH signaling. Nevertheless, long-term follow-up revealed that C5orf42-mutated patients showed catch-up development and fainting of facial features contrary to KIF7-mutated patients.

Familial diseases revealed by a fetal anomaly

The recognition of a fetal anomaly can lead to the same diagnosis being made in one of the asymptomatic parents unaware of the problem. We analyzed cases in which the discovery of a fetal anomaly led to the discovery of a genetic familial disorder. Families in which the recognition of a fetal anomaly led to the same diagnosis being made in one of the asymptomatic parents were included. Twenty couples were included in the study. The fetal anomalies were cleft lip and palate (4), cardiac anomalies (2), cerebral anomalies (1), bilateral club feet with polyhydramnios, akinesia or camptodactily (5), nuchal anomalies (2), micromelia (3), polydactyly (2), and limited elbow extension (1). Genetic counselling helped establish nine maternal diseases as follows: Steinert disease (3), spinal muscular atrophy (1), antecubital pterygium (1), DiGeorge (1), Wardenburg type II (1), Charge (1) and Greig syndromes (1). Eleven paternal diseases were discovered, which were Noonan-like syndrome (1), paternal cervical anomalies (1), Goldenhar syndrome (1), dominant autosomal arthrogryposis (1), osteogenesis imperfecta (3), tuberous sclerosis (1), dominant transposition of great vessels (1), Weyers acrofacial dysostosis (1), and autosomal dominant holoprosencephaly (1). Twelve couples continued with pregnancy and eight opted for termination of pregnancy. The fetus is central in giving the first insight into a familial disorder. It can reveal familial diseases undiscovered in the parent and help understand the mode of transmission of an anomaly, mainly the autosomal dominant diseases with variable expressions.

Extreme variability of expression of a Sonic Hedgehog mutation: attention difficulties and holoprosencephaly

Holoprosencephaly (HPE) is a clinically variable and genetically heterogeneous central nervous system (CNS) malformation. Alobar HPE, which is its most severe form, is associated with a poor prognosis. At the...

Mutational analysis of the Sonic Hedgehog gene in 220 newborns with oral clefts in a South American (ECLAMC) population.

Oral clefts generally have a multifactorial etiology. A number of genes contribute to the formation of the face and palate. Cleft lip and/or palate can occur in pedigrees with autosomal dominant holoprosencephaly due to mutations in Sonic Hedgehog (SHH). In addition, animal models have shown that SHH is involved in face development. We thus examined the human SHH gene in 220 newborn infants with nonsyndromic oral clefts registered by the Estudio Colaborativo Latinoamericano de Malformaciones Congenitas: ECLAMC (Latin American Collaborative Study of Congenital Malformations). We found 15 variant bands in 13 patients with oral clefts, representing five different base changes, all of which were found by sequencing to represent silent polymorphisms. Four occurred in introns. The alteration occurring in an exon, Ser190Ser, may create a consensus sequence for the 3'splice site 6 bp downstream of the original consensus sequence. Thus, we did not identify any clearly disease-causing mutation in SHH in these patients, and conclude that SHH mutations are not a frequent cause of isolated oral clefts in humans.

SHH mutation is associated with solitary median maxillary central incisor: a study of 13 patients and review of the literature.

Solitary median maxillary central incisor (SMMCI) or single central incisor is a rare dental anomaly. It has been reported in holoprosencephaly (HPE) cases with severe facial anomalies or as a microform in autosomal dominant HPE (ADHPE). In our review of the literature, we note that SMMCI may also occur as an isolated finding or in association with other systemic abnormalities. These anomalies include short stature, pituitary insufficiency, microcephaly, choanal atresia, midnasal stenosis, and congenital nasal pyriform aperture stenosis. SMMCI can also be a feature of recognized syndromes or associations or a finding in patients with specific chromosomal abnormalities. We performed a molecular study on a cohort of 13 SMMCI patients who did not have HPE. We studied two genes, Sonic Hedgehog (SHH) and SIX3, in which mutations have been reported in patients showing SMMCI as part of the HPE spectrum. A new missense mutation in SHH (I111F), segregating in one SMMCI family, was identified. Our results suggest that this mutation may be specific for the SMMCI phenotype since it has not been found in the HPE population or in normal controls. Published 2001 Wiley-Liss, Inc.

Solitary maxillary central incisor in the midline associated with systemic disorders

A single maxillary central incisor in the midline is a rare developmental anomaly. The appearance of a single incisor in place of two teeth may occur as an isolated dental finding that can be related to fusion of two neighboring teeth or to agenesis of a tooth germ. However, the condition has also been reported to occur in association with autosomal dominant holoprosencephaly, growth retardation, and midline developmental defects. This article reports on other systemic defects that can be found in association with a single maxillary central incisor.

Mutations in the human Sonic Hedgehog gene cause holoprosencephaly.

Holoprosencephaly (HPE) is a common developmental defect of the forebrain and frequently the midface in humans, with both genetic and environmental causes. HPE has a prevalence of 1:250 during embryogenesis and 1:16,000 newborn infants, and involves incomplete development and septation of midline structures in the central nervous system (CNS) with a broad spectrum of clinical severity. Alobar HPE, the most severe form which is usually incompatible with postnatal life, involves complete failure of division of the forebrain into right and left hemispheres and is characteristically associated with facial anomalies including cyclopia, a primitive nasal structure (proboscis) and/or midfacial clefting. At the mild end of the spectrum, findings may include microcephaly, mild hypotelorism, single maxillary central incisor and other defects (Fig. 1). This phenotypic variability also occurs between affected members of the same family. The molecular basis underlying HPE is not known, although teratogens, non-random chromosomal anomalies and familial forms with autosomal dominant and recessive inheritance have been described. HPE3 on chromosome 7q36 is one of at least four different loci implicated in HPE. Here, we report the identification of human Sonic Hedgehog (SHH) as HPE3-the first known gene to cause HPE. Analyzing 30 autosomal dominant HPE (ADHPE) families, we found five families that segregate different heterozygous SHH mutations. Two of these mutations predict premature termination of the SHH protein, whereas the others alter highly conserved residues in the vicinity of the alpha-helix-1 motif or signal cleavage site.

Linkage of a human brain malformation, familial holoprosencephaly, to chromosome 7 and evidence for genetic heterogeneity.

Holoprosencephaly (HPE) is a common malformation of the developing forebrain and midface characterized by incomplete penetrance and variable expressivity. Familial HPE has been reported in many families with autosomal dominant inheritance in some and apparent autosomal recessive inheritance in others. We have examined 125 individuals from nine families with autosomal dominant HPE. Expression in gene carriers varied from alobar HPE and cyclopia through microforms such as microcephaly or single central incisor to normal phenotype. We performed linkage studies by either Southern blot or polymerase chain reaction analyses with DNA markers (D7S22, D7S550, and D7S483) that are deleted from some patients with sporadic HPE and flank a translocation breakpoint in 7q36 associated with HPE. The strongest support for linkage was with D7S22, which was linked with no recombination to autosomal dominant HPE in eight of nine families with a combined logarithm of odds score of 6.4 with an affected-only model-free analysis and 8.2 with a reduced-penetrance model and all phenotypes. Close linkage to this region could be excluded in one family, and there was significant evidence of genetic heterogeneity. These results show that a gene for autosomal dominant HPE is located in a chromosomal region (7q36) known to be involved in sporadic HPE with visible cytogenetic deletions. They also demonstrate genetic heterogeneity in familial HPE. We hypothesize that mutations of a gene in 7q36, designated HPE3, are responsible for both sporadic HPE and a majority of families with autosomal dominant HPE.

Single maxillary central incisor in association with mid-line anomalies

A single maxillary central incisor may occur as an isolated dental finding. However, it has also been reported to occur in association with autosomal dominant holoprosencephaly, in association with growth retardation with or without growth hormone deficiency and occasionally in association with other mid-line developmental defects. Holoprosencephaly and pituitary dysfunction are two specific examples of mid-line defects and this report emphasises that other mid-line defects may be associated with a single maxillary central incisor. We recommend that patients with this dental anomaly should be referred for a detailed medical examination.