What's in a face?

Genetic syndromes causing human facial abnormalities have been extensively studied from the clinical point of view. In these syndromes, distinctive facial features are frequently associated with specific defects of other organ systems, and the genes responsible must all play a significant part in normal development. In this paper genes causing craniofacial abnormalities, that have either been mapped or isolated, are reviewed.

Mutations in the fibroblast growth factor receptor 2 gene cause Crouzon syndrome.

Crouzon syndrome is an autosomal dominant condition causing premature fusion of the cranial sutures (craniosynostosis) and maps to chromosome 10q25-q26. We now present evidence that mutations in the fibroblast growth factor receptor 2 gene (FGFR2) cause Crouzon syndrome. We found SSCP variations in the B exon of FGFR2 in nine unrelated affected individuals as well as complete cosegregation between SSCP variation and disease in three unrelated multigenerational families. In four sporadic cases, the normal parents did not have SSCP variation. Finally, direct sequencing has revealed specific mutations in the B exon in all nine sporadic and familial cases, including replacement of a cysteine in an immunoglobulin-like domain in five patients.

The detection of subtelomeric chromosomal rearrangements in idiopathic mental retardation.

A major challenge for human genetics is to identify new causes of mental retardation, which, although present in about 3% of individuals, is unexplained in more than half of all cases. We have developed a strategy to screen for the abnormal inheritance of subtelomeric DNA polymorphisms in individuals with mental retardation and have detected three abnormalities in 99 patients with normal routine karyotypes. Pulsed-field gel electrophoresis and reverse chromosome painting showed that one case arose from an interstitial or terminal deletion and two from the de novo inheritance of derivative translocation chromosomes. At least 6% of unexplained mental retardation is accounted for by these relatively small chromosomal abnormalities, which will be an important resource in the characterization of the genetic basis of neurodevelopment.

Identical mutations in the FGFR2 gene cause both Pfeiffer and Crouzon syndrome phenotypes.

Mutations in the fibroblast growth factor receptor 2 (FGFR2) gene have been identified in Crouzon syndrome, an autosomal dominant condition causing premature fusion of the cranial sutures (craniosynostosis). A mutation in FGFR1 has been established in several families with Pfeiffer syndrome, where craniosynostosis is associated with specific digital abnormalities. We now report point mutations in FGFR2 in seven sporadic Pfeiffer syndrome patients. Six of the seven Pfeiffer syndrome patients share two missense mutations, which have also been reported in Crouzon syndrome. The Crouzon and Pfeiffer phenotypes usually breed true within families and the finding of identical mutations in unrelated individuals giving different phenotypes is a highly unexpected observation.

A promoter mutation in the XIST gene in two unrelated families with skewed X-chromosome inactivation.

X-chromosome inactivation is the process by which a cell recognizes the presence of two copies of an X chromosome early in the development of XX embryos and chooses one to be active and one to be inactive. Although it is commonly believed that the initiation of X inactivation is random, with an equal probability (50:50) that either X chromosome will be the inactive X in a given cell, significant variation in the proportion of cells with either X inactive is observed both in mice heterozygous for alleles at the Xce locus and among normal human females in the population. Families in which multiple females demonstrate extremely skewed inactivation patterns that are otherwise quite rare in the general population are thought to reflect possible genetic influences on the X-inactivation process. Here we report a rare cytosine to guanine mutation in the XIST minimal promoter that underlies both epigenetic and functional differences between the two X chromosomes in nine females from two unrelated families. All females demonstrate preferential inactivation of the X chromosome carrying the mutation, suggesting that there is an association between alterations in the regulation of XIST expression and X-chromosome inactivation.

Mutation and deletion of the pseudoautosomal gene SHOX cause Leri-Weill dyschondrosteosis.

Leri-Weill Dyschondrosteosis (LWD; OMIM 127300) is a dominantly inherited skeletal dysplasia characterized by disproportionate short stature with predominantly mesomelic limb shortening. Expression is variable and consistently more severe in females, who frequently display the Madelung deformity of the forearm (shortening and bowing of the radius with dorsal subluxation of the distal ulna). The rare Langer Mesomelic Dysplasia (LD; OMIM 249700), characterized by severe short stature with hypoplasia/aplasia of the ulna and fibula, has been postulated to be the homozygous form of LWD (refs 4-6). In a six-generation pedigree with LWD, we established linkage to the marker DXYS6814 in the pseudoautosomal region (PAR1) of the X and Y chromosomes (Z max=6.28; theta=0). Linkage analysis of three smaller pedigrees increased the lod score to 8.68 (theta=0). We identified submicroscopic PAR1 deletions encompassing the recently described short stature homeobox-containing gene SHOX (refs 7,8) segregating with the LWD phenotype in 5 families. A point mutation leading to a premature stop in exon 4 of SHOX was identified in one LWD family.

Animal models for dysmorphology.

Areas where studies of animal models have been vital to the understanding of human malformation syndromes include the dissection of complex genetic mechanisms such as imprinting, the creation of animal models of known human conditions, and the provision of further clues involving gene families and pathways.

European Cytogeneticists Association Register of Unbalanced Chromosome Aberrations (ECARUCA); an online database for rare chromosome abnormalities.

During recent years a considerable improvement in diagnostic techniques has enabled cytogeneticists to find more and smaller chromosomal aberrations. However, accurate clinical knowledge about rare chromosome disorders is frequently lacking, mostly due to a significant decline in publishable cases. On the other hand, there is an increasing demand from parents and physicians for reliable information. In order to improve the quality and the quantity of data available, we designed a new database named the European Cytogeneticists Association Register of Unbalanced Chromosome Aberrations (ECARUCA) at http://www.ecaruca.net. This Internet-database contains cytogenetic and clinical data of patients with rare chromosome abnormalities, including microscopically visible aberrations, as well as microdeletions and -duplications. Cases with certain breakpoints collected in the Zurich Cytogenetic Database were transferred to ECARUCA. The advantages of ECARUCA compared to existing sources are that ECARUCA is interactive, dynamic and has long-term possibilities to store cytogenetic, molecular and clinical data. Professionals can login to submit new cases and perform searches in the database through the Internet. Currently the database contains 1500 unique chromosomal aberrations from almost 4000 patients. A frequent submission of new data ensures the up-to-date quality of the collection. Individual parent accounts allow parents to inform the ECARUCA team about the follow-up of their child. The ECARUCA database provides health care workers with accurate information on clinical aspects of rare chromosome disorders. Additionally, detailed correlations between chromosome aberrations and their phenotypes are of invaluable help in localising genes for mental retardation and congenital anomalies.

Clinical studies on submicroscopic subtelomeric rearrangements: a checklist.

BACKGROUND: Submicroscopic subtelomeric chromosome defects have been found in 7.4% of children with moderate to severe mental retardation and in 0.5% of children with mild retardation. Effective clinical preselection is essential because of the technical complexities and cost of screening for subtelomere deletions. METHODS: We studied 29 patients with a known subtelomeric defect and assessed clinical variables concerning birth history, facial dysmorphism, congenital malformations, and family history. Controls were 110 children with mental retardation of unknown aetiology with normal G banded karyotype and no detectable submicroscopic subtelomeric abnormalities. RESULTS: Prenatal onset of growth retardation was found in 37% compared to 9% of the controls (p<0.0005). A higher percentage of positive family history for mental retardation was reported in the study group than the controls (50% v 21%, p=0.002). Miscarriage(s) were observed in only 8% of the mothers of subtelomeric cases compared to 30% of controls (p=0.028) which was, however, not significant after a Bonferroni correction. Common features (>30%) among subtelomeric deletion cases were microcephaly, short stature, hypertelorism, nasal and ear anomalies, hand anomalies, and cryptorchidism. Two or more facial dysmorphic features were observed in 83% of the subtelomere patients. None of these features was significantly different from the controls. Using the results, a five item checklist was developed which allowed exclusion from further testing in 20% of the mentally retarded children (95% CI 13-28%) in our study without missing any subtelomere cases. As our control group was selected for the "chromosomal phenotype", the specificity of the checklist is likely to be higher in an unselected group of mentally retarded subjects. CONCLUSIONS: Our results suggest that good indicators for subtelomeric defects are prenatal onset of growth retardation and a positive family history for mental retardation. These clinical criteria, in addition to features suggestive of a chromosomal phenotype, resulted in the development of a five item checklist which will improve the diagnostic pick up rate of subtelomeric defects among mentally retarded subjects.

Kohlschutter syndrome in siblings.

Kohlschutter syndrome is a rare neurodegenerative disorder presenting with intractable seizures, developmental regression and characteristic hypoplastic dental enamel indicative of amelogenesis imperfecta. We report a new family with two affected siblings.

Mutational analysis in X-linked spondyloepiphyseal dysplasia tarda.

Spondyloepiphyseal dysplasia tarda (SEDT) is an X-linked recessive disorder characterized by short stature due to defective growth of the vertebral bodies. In addition, deformities of the femoral heads result in early onset secondary osteoarthritis of the hips. The disorder affects males only with heterozygous female carriers showing no consistent abnormalities. The gene causing SEDT, which is located on Xp22.12-p22.31, consists of 6 exons of which only exons 3, 4, 5, and 6 are translated to yield an 140 amino acid protein, referred to as SEDLIN. SEDLIN mutations have been observed in SEDT patients, and we have undertaken studies to characterize such mutations in four unrelated SEDT kindreds by DNA sequence analysis. We identified two nonsense and two intragenic deletional frameshift mutations. The nonsense mutations occurred in exons 4 (TGG-->TGA, Trp70Stop) and 6 (CGA-->TGA, Arg122Stop). Both of the intragenic deletions, which were approximately 750 bp and 1300-1445 bp in size, involved intron 5 and part of exon 6 and resulted in frameshifts that lead to premature termination (Stop) signals. Thus, all four mutations are predicted to result in truncated proteins. The results of our study expand the spectrum of SEDLIN mutations associated with SEDT, and this will help to elucidate further the role of this novel protein in the etiology of this form of osteochondrodysplasia.

Syndactylies and polydactylies: embryological overview and suggested classification.

In 1978, Temtamy and McKusick classified isolated, non-syndromic polydactyly and syndactyly, using a logical anatomical approach, into five distinct types for each group. Since then, there have been considerable advances in the molecular embryology of the developing limb bud. These include the proposal that retinoic acid and/or related retinoids are the morphogens responsible for the morphogenetic gradient giving rise to anterior-posterior pattern formation of the limb bud, the suggestion that the HOX4 complex and other homeotic genes may also be involved in patterning, and a greater understanding of other mechanisms such as programmed cell death in the shaping of the final hand and foot. This paper briefly reviews the molecular embryology of limb development and outlines the 'end-organ responsiveness' of the limbs to a variety of single-gene mutations. An alternative classification of syndactylies and polydactylies is suggested. It is still too early to match specific defects to individual genes with precision, and it is obvious that many important developmental genes remain to be identified; nevertheless, it is envisaged that clues from molecular embryological studies will become increasingly more useful.

Spectrum of craniosynostosis phenotypes associated with novel mutations at the fibroblast growth factor receptor 2 locus.

The causative relationship between several of the syndromic forms of craniosynostosis and mutations in the fibroblast growth factor receptor (FGFR) loci is now well established. However, within the group of patients with craniosynostosis, there are several families and sporadic cases whose clinical features differ in variable degrees from the classically described syndromes of craniosynostosis. In this communication we present novel FGFR2 mutations associated with a spectrum of craniosyostosis phenotypes in 4 sporadic cases and in one family in which craniosynostosis segregates. The mutation and phenotype data presented emphasise the clinical variability of mutations at this locus and underline the plasticity of the phenotype-genotype relationship in this important group of congenital malformation syndromes. Mutations found were tyrosine 105 to cysteine, glycine 338 to glutamic acid, serine 351 to cysteine and glycine 384 to arginine. These are the first reported mutations in the first immunoglobulin-like loop (tyrosine 105 to cysteine) and the transmembrane domain (glycine 384 to arginine) of FGFR2, providing further insights into the mechanism of abnormal receptor function in FGFR2 mutations.

The estimation of phenotype distributions from pedigree data.

A method is described to estimate from pedigree data that probability density function of a quantitative phenotype, given a specific genotype. Theoretically, pedigrees of any size or complexity that are segregating for conditions of known Mendelian inheritance can be used. Under certain circumstances, the estimates are unbiased by the method of ascertainment.

A mouse malformation mutant supplement to the London Dysmorphology Database.

A mouse malformation mutant supplement to the London Dysmorphology Database is described. Using a microcomputer, the mouse malformation database can be searched by asking for all mutants with a specific combination of physical abnormalities. The database is available free to interested researchers on receipt of two 5 1/4" floppy disks.

Distinctive autosomal or X-linked dominant syndrome of microcephaly, mild developmental delay, short stature, and distinctive face.

We report on a mother and two sons with a syndrome of microcephaly, short stature, a distinctive face, broad thumbs and great toes, and mild developmental delay. There are similarities to the patients reported by Bawle and Horton [Am J Med Genet 33:382-384, 1989] and Evans [Clin Genet 39:178-180, 1991] but it is not certain whether the patients have the same condition. Inheritance could either be autosomal or X-linked dominant.

Greig cephalopolysyndactyly syndrome: a possible mouse homologue (Xt-extra toes).

Greig cephalopolysyndactyly syndrome is an autosomal dominant form of complex polydactyly in man. Attention is called to the evidence that, on both morphological and comparative gene mapping grounds, this defect is homologous to Xt-extra toes in the mouse. The pattern of polydactyly in both species is very similar. In addition, both conditions probably map close to the T-cell receptor gamma polypeptide at 13 A2-3 in mouse and 7p15 in humans.

A child with sclerocornea, short limbs, short stature, and distinct facial appearance.

We describe a boy with sclerocornea, short limbs, short stature and a distinct facial appearance. The resemblance to other reports of affected sibs suggests that this is a newly recognised, autosomal recessive syndrome.

Two brothers with facial anomalies, microcephaly, hypoplastic genitalia, and a failure of psychomotor development.

We report on 2 brothers from a consanguineous Moslem family with prenatal onset of growth deficiency, microcephaly, hypoplastic genitalia, and convulsions from birth. Phenotypic anomalies consisted of a prominent glabella, arched eyebrows, a low upswept frontal hairline, a small pinched nose, large posteriorly rotated ears with overfolded upper helices, partial camptodactyly, and widespaced nipples. Psychomotor development was absent, and there was marked failure to thrive. Death occurred at ages 21 days and 7 months, respectively. Postmortem examination on one child showed dilated cerebral ventricles and hydronephrosis. Microcephaly was detectable by fetal ultrasound in one brother at 17 weeks of gestation.

Radial ray defect and Duane anomaly: report of a family with autosomal dominant transmission.

We report on a family in which a mother and her 2 offspring presented with bilateral radial defects and absent thumbs. In addition, Duane anomaly was present in both offspring, one of whom also had anal stenosis. Clinical investigation showed no skeletal, cardiac, or urogenital abnormalities. Autosomal dominant transmission of radial defects is suggested, with Duane anomaly and anal stenosis representing manifestations of the same gene.