Single median maxillary central incisor: new data and mutation review

BACKGROUND: Single median maxillary central incisor (SMMCI) is a rare anomaly that may occur alone or associated with other conditions, frequently as part of the holoprosencephaly (HPE) spectrum. However, it has been suggested that SMMCI alone, or associated with some midline defects, may be considered a different entity from HPE (OMIM: 147250). Families with SMMCI, without HPE cases, are difficult to counsel for the risk of HPE in future generations because the same midline defects described as part of the "SMMCI syndrome" can also be part of the HPE spectrum. METHODS: We screened five cases of SMMCI for mutations in three HPE genes, SHH, TGIF, and SIX3. RESULTS: A missense mutation c.686C>T was found in the gene SIX3 of one patient, which did not differ from the accepted 20% of known HPE gene mutations among all HPE cases. Our results and an extensive literature review of gene mutations in patients with SMMCI showed that 27/28 of them were in HPE genes: SHH (n = 21), SIX3 (n = 3), TGIF (n = 1), GLI2 (n = 1), and PTCH (n = 1), and only one in the SALL4 gene. CONCLUSIONS: The clinical findings in patients with SMMCI without HPE in families with mutations in HPE genes cannot be distinguished from the findings reported in the SMMCI syndrome. Therefore, persons with SMMCI and their relatives should be carefully investigated for related midline disorders, especially of the HPE spectrum, and all known HPE genes screened.     

TG-interacting factor 1 acts as a transcriptional repressor of sterol O-acyltransferase 2

Acat2 [gene name: sterol O-acyltransferase 2 (SOAT2)] esterifies cholesterol in enterocytes and hepatocytes. This study aims to identify repressor elements in the human SOAT2 promoter and evaluate their in vivo relevance. We identified TG-interacting factor 1 (Tgif1) to function as an important repressor of SOAT2. Tgif1 could also block the induction of the SOAT2 promoter activity by hepatocyte nuclear factor 1α and 4α. Women have ∼30% higher hepatic TGIF1 mRNA compared with men. Depletion of Tgif1 in mice increased the hepatic Soat2 expression and resulted in higher hepatic lipid accumulation and plasma cholesterol levels. Tgif1 is a new player in human cholesterol metabolism.     

Identification of novel mutations in SHH and ZIC2 in a South American (ECLAMC) population with holoprosencephaly

Holoprosencephaly (HPE) is genetically heterogeneous with four genes, SIX3, SHH, TGIF, and ZIC2 that have been identified to date and that are altered in 12% of patients. To analyze this prevalence in a South American population-based sample (57 HPE cases in 244,511 live and still births or 1 in 4300), we performed a mutational study of these genes in 30 unrelated children (26 newborns and 4 non-newborns) with HPE being ascertained by ECLAMC (Latin American Collaborative Study of Congenital Malformations). We identified three novel mutations: two were missense mutations of the SHH gene (Cys183-->Phe; His140-->Pro); the third mutation was a 2-bp deletion in the zinc-finger region of the ZIC2 gene. These molecular results explained 8% (2/26 newborn samples) of the HPE cases in this South American population-based sample, a proportion similar to our previously published data from a collection of cases.     

<Emphasis Type="Italic">SONIC HEDGEHOG</Emphasis> mutations causing human holoprosencephaly impair neural patterning activity

Holoprosencephaly (HPE) is a common forebrain malformation associated with mental retardation and craniofacial anomalies. Multiple lines of evidence indicate that loss of ventral neurons is associated with HPE. The condition is etiologically heterogeneous, and abnormalities in any of several genes can cause human HPE. Among these genes, mutations in SONIC HEDGEHOG ( SHH) are the most commonly identified single gene defect causing human HPE. SHH mediates a number of processes in central nervous system development and is required for the normal induction of ventral cell types in the brain and spinal cord. Although a number of missense mutations in SHH have been identified in patients with HPE, the functional significance of these mutations has not yet been determined. We demonstrate that two SHH mutations that cause human HPE result in decreased in vivo activity of SHH in the developing nervous system. These mutant forms of SHH fail to regulate genes properly that are normally responsive to SHH signaling and do not induce ventrally expressed genes. In addition, the immunoreactivity of the mutant proteins is altered, suggesting that the conformation of the SHH protein has been disrupted. These studies are the first demonstration that mutations in SHH associated with human HPE perturb the in vivo patterning function of SHH in the developing nervous system.     

The genomic structure, chromosome location, and analysis of the human DKK1 head inducer gene as a candidate for holoprosencephaly

Holoprosencephaly (HPE) is the most common developmental defect of the brain and face in humans. Here we report the analysis of the human ortholog of dkk-1 as a candidate gene for HPE. We determined the genomic structure of the human gene DKK1 and mapped it to chromosome 10q11.2. Functional analysis of four missense mutations identified in HPE patients revealed preserved activity in head induction assays in frogs suggesting a limited role for this gene in HPE pathogenesis.     

Holoprosencephaly: the Maastricht experience.

Holoprosencephaly (HPE) is a developmental field defect with impaired cleavage of the embryonic forebrain as the cardinal feature. The prevalence is about 1 in 11.000-20.000 in live births and 1 in 250 during embryogenesis. In most cases, craniofacial abnormalities are associated and reflect in 80% of cases the degree of severity. The severity is of marked variability and ranges from cyclopia to minimal craniofacial dysmorphism, such as mild microcephaly with a single central incisor. The etiology of HPE is very heterogeneous and comprises environmental factors (e.g. maternal diabetes) and genetic causes. Approximately 50% of HPE cases are associated with a cytogenetic abnormality (the most common of which is trisomy 13) or a monogenic syndrome. Based on recurrent cytogenetic abnormalities, there are at least 12 genetic loci that likely contain genes implicated in the pathogenesis of HPE. Currently, four human HPE genes are known: SHH at 7q36, ZIC2 at 13q32, SIX3 at 2p21 and TGIF at 18p11.3. Over the past 13 years, 16 patients with HPE have been observed at the Department of Clinical Genetics at Maastricht. Some of them are briefly presented in order to emphasize the spectral nature of HPE and the etiological heterogeneity. One patient appeared to have a partial 18p deletion due to a maternal cryptic translocation t(1:18) and, in addition, a SHH mutation. The mildest affected patient presented with microcephaly and a single maxillary incisor; she had a submicroscopic 7q deletion. Finally, we propose a protocol of etiological work-up of HPE cases.