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Cenani-Lenz Syndactyly (CLS) syndrome is a rare autosomal recessive disorder characterized by syndactyly and oligodactyly of fingers and toes, disorganization and fusion of metacarpals, metatarsals and phalanges, radioulnar synostosis and mesomelic shortness of the limbs, with lower limbs usually being much less affected than upper limbs.
HOXD13, the homeobox-containing gene located at the most 5' end of the HOXD cluster, plays a critical role in limb development. It has been shown that mutations in human HOXD13 can give rise to limb malformations, with variable expressivity and a wide spectrum of clinical manifestations. Polyalanine expansions in HOXD13 cause synpolydactyly, whereas amino acid substitutions in the homeodomain are associated with brachydactyly types D and E. We describe two large Han Chinese families with different limb malformations, one with syndactyly type V and the other with limb features overlapping brachydactyly types A4, D, and E and mild syndactyly of toes 2 and 3. Two-point linkage analysis showed LOD scores >3 (theta =0) for markers within and/or flanking the HOXD13 locus in both families. In the family with syndactyly type V, we identified a missense mutation in the HOXD13 homeodomain, c.950A-->G (p.Q317R), which leads to substitution of the highly conserved glutamine that is important for DNA-binding specificity and affinity. In the family with complex brachydactyly and syndactyly, we detected a deletion of 21 bp in the imperfect GCN (where N denotes A, C, G, or T) triplet-containing exon 1 of HOXD13, which results in a polyalanine contraction of seven residues. Moreover, we found that the mutant HOXD13 with the p.Q317R substitution was unable to transactivate the human EPHA7 promoter. Molecular modeling data supported these experimental results. The calculated interactions energies were in agreement with the measured changes of the activity. Our data established the link between HOXD13 and two additional limb phenotypes--syndactyly type V and brachydactyly type A4--and demonstrated that a polyalanine contraction in HOXD13, most likely, led to other digital anomalies but not to synpolydactyly. We suggest the term "HOXD13 limb morphopathies" for the spectrum of limb disorders caused by HOXD13 mutations.
Syndactyly type 1 (SD1) is an autosomal dominant limb malformation characterized in its classical form by complete or partial webbing between the third and fourth fingers and/or the second and third toes. Its four subtypes (a, b, c, and d) are defined based on variable phenotypes, but the responsible gene is yet to be identified. SD1-a has been mapped to chromosome 3p21.31 and SD1-b to 2q34-q36. SD1-c and SD1-d are very rare and, to our knowledge, no gene loci have been identified.
Fras1 encodes an extracellular matrix protein that is critical for the establishment of the epidermal basement membrane during gestation. In humans, mutations in FRAS1 cause Fraser Syndrome (FS), a pleiotropic condition with many clinical presentations such as limb, eye, kidney, and craniofacial deformations. Many of these defects are mimicked by loss of Fras1 in mice, and are preceded by the formation of epidermal blisters in utero.
Ectodermal dysplasias form a large disease family with more than 200 members. The combination of hair and tooth abnormalities, alopecia, and cutaneous syndactyly is characteristic of ectodermal dysplasia-syndactyly syndrome (EDSS). We used a homozygosity mapping approach to map the EDSS locus to 1q23 in a consanguineous Algerian family. By candidate gene analysis, we identified a homozygous mutation in the PVRL4 gene that not only evoked an amino acid change but also led to exon skipping. In an Italian family with two siblings affected by EDSS, we further detected a missense and a frameshift mutation. PVRL4 encodes for nectin-4, a cell adhesion molecule mainly implicated in the formation of cadherin-based adherens junctions. We demonstrated high nectin-4 expression in hair follicle structures, as well as in the separating digits of murine embryos, the tissues mainly affected by the EDSS phenotype. In patient keratinocytes, mutated nectin-4 lost its capability to bind nectin-1. Additionally, in discrete structures of the hair follicle, we found alterations of the membrane localization of nectin-afadin and cadherin-catenin complexes, which are essential for adherens junction formation, and we found reorganization of actin cytoskeleton. Together with cleft lip and/or palate ectodermal dysplasia (CLPED1, or Zlotogora-Ogur syndrome) due to an impaired function of nectin-1, EDSS is the second known "nectinopathy" caused by mutations in a nectin adhesion molecule.
Congenital hemidysplasia with ichthyosiform erythroderma and limb defects also known as CHILD syndrome is an X-linked dominant, male lethal genodermatosis with a prevalence of 1 in 100,000 live births. Mutations in NSDHL gene located at Xq28 potentially impair the function of NAD(P) H steroid dehydrogenase-like protein and is responsible for its pathogenesis.
Myhre syndrome is a rare multisystem genetic disorder that is caused by de novo heterozygous gain-of-function variants in SMAD4. Patients with Myhre syndrome exhibit several phenotypes at different ages such as small size, autism, developmental delay, left-sided heart defects, and hearing loss and often have a characteristic facial appearance. The early clinical diagnosis of Myhre syndrome remains a major challenge, particularly in the first year of life.
Timothy syndrome (TS) is a rare pleiotropic disorder associated with long QT syndrome, syndactyly, dysmorphic features, and neurological symptoms. Several variants in exon 8 or 8a of CACNA1C, a gene encoding the α-subunit of voltage-gated Ca2+ channels (Cav1.2), are known to cause classical TS. We identified a p.R412M (exon 9) variant in an atypical TS case. The aim of this study was to examine the functional effects of CACNA1C p.R412M on CaV1.2 in comparison with those of p.G406R. The index patient was a 2-month-old female infant who suffered from a cardio-pulmonary arrest in association with prolonged QT intervals. She showed dysmorphic facial features and developmental delay, but not syndactyly. Interestingly, she also presented recurrent seizures from 4 months. Genetic tests identified a novel heterozygous CACNA1C variant, p.R412M. Using heterologous expression system with HEK-293 cells, analyses with whole-cell patch-clamp technique revealed that p.R412M caused late Ca2+ currents by significantly delaying CaV1.2 channel inactivation, consistent with the underlying mechanisms of classical TS. A novel CACNA1C variant, p.R412M, was found to be associated with atypical TS through the same mechanism as p.G406R, the variant responsible for classical TS.
Polydactyly is a highly common congenital limb defect. Extra digits may appear as an isolated anomaly or as a part of a syndrome. Mutations in GLI3 have been shown to cause Greig cephalopolysyndactyly, Pallister-Hall syndrome and non-syndromic polydactyly. Genotype-phenotype correlation studies of GLI3 mutations suggest a model by which mutations in the zinc-finger domain (ZFD) of GLI3 likely lead to syndromic polydactyly. Here we describe a rare case of autosomal dominant heterozygous missense mutation in the ZFD of GLI3 leading to a variable polydactyly-syndactyly complex.
Laminins are the major noncollagenous glycoproteins of all basal laminae (BLs). They are alpha/beta/gamma heterotrimers assembled from 10 known chains, and they subserve both structural and signaling roles. Previously described mutations in laminin chain genes result in diverse disorders that are manifested postnatally and therefore provide little insight into laminin's roles in embryonic development. Here, we show that the laminin alpha5 chain is required during embryogenesis. The alpha5 chain is present in virtually all BLs of early somite stage embryos and then becomes restricted to specific BLs as development proceeds, including those of the surface ectoderm and placental vasculature. BLs that lose alpha5 retain or acquire other alpha chains. Embryos lacking laminin alpha5 die late in embryogenesis. They exhibit multiple developmental defects, including failure of anterior neural tube closure (exencephaly), failure of digit septation (syndactyly), and dysmorphogenesis of the placental labyrinth. These defects are all attributable to defects in BLs that are alpha5 positive in controls and that appear ultrastructurally abnormal in its absence. Other laminin alpha chains accumulate in these BLs, but this compensation is apparently functionally inadequate. Our results identify new roles for laminins and BLs in diverse developmental processes.
A failure in optic fissure fusion during development can lead to blinding malformations of the eye. Here, we report a syndrome characterized by facial dysmorphism, colobomatous microphthalmia, ptosis and syndactyly with or without nephropathy, associated with homozygous frameshift mutations in FAT1. We show that Fat1 knockout mice and zebrafish embryos homozygous for truncating fat1a mutations exhibit completely penetrant coloboma, recapitulating the most consistent developmental defect observed in affected individuals. In human retinal pigment epithelium (RPE) cells, the primary site for the fusion of optic fissure margins, FAT1 is localized at earliest cell-cell junctions, consistent with a role in facilitating optic fissure fusion during vertebrate eye development. Our findings establish FAT1 as a gene with pleiotropic effects in human, in that frameshift mutations cause a severe multi-system disorder whereas recessive missense mutations had been previously associated with isolated glomerulotubular nephropathy.
Isolated syndactyly in cattle, also known as mulefoot, is inherited as an autosomal recessive trait with variable penetrance in different cattle breeds. Recently, two independent mutations in the bovine LRP4 gene have been reported as the primary cause of syndactyly in the Holstein and Angus cattle breeds.
It has recently been attempted in the literature to analyze the aesthetic outcomes of syndactyly web space reconstruction utilizing dorsal pentagonal advancement flaps and dorsal rectangular flaps with skin grafting. The study utilized a categorical grading system for evaluating the aesthetic outcomes of reconstruction to be used in conjunction with a visual analog scale (VAS), which has yet to be validated in the assessment of aesthetic outcomes following web space reconstruction.
Mesoaxial synostotic syndactyly, Malik-Percin type (MSSD) (syndactyly type IX) is a rare autosomal-recessive nonsyndromic digit anomaly with only two affected families reported so far. We previously showed that the trait is genetically distinct from other syndactyly types, and through autozygosity mapping we had identified a locus on chromosome 17p13.3 for this unique limb malformation. Here, we extend the number of independent pedigrees from various geographic regions segregating MSSD to a total of six. We demonstrate that three neighboring missense mutations affecting the highly conserved DNA-binding region of the basic helix-loop-helix A9 transcription factor (BHLHA9) are associated with this phenotype. Recombinant BHLHA9 generated by transient gene expression is shown to be located in the cytoplasm and the cell nucleus. Transcription factors 3, 4, and 12, members of the E protein (class I) family of helix-loop-helix transcription factors, are highlighted in yeast two-hybrid analysis as potential dimerization partners for BHLHA9. In the presence of BHLHA9, the potential of these three proteins to activate expression of an E-box-regulated target gene is reduced considerably. BHLHA9 harboring one of the three substitutions detected in MSSD-affected individuals eliminates entirely the transcription activation by these class I bHLH proteins. We conclude that by dimerizing with other bHLH protein monomers, BHLHA9 could fine tune the expression of regulatory factors governing determination of central limb mesenchyme cells, a function made impossible by altering critical amino acids in the DNA binding domain. These findings identify BHLHA9 as an essential player in the regulatory network governing limb morphogenesis in humans.
Mesoaxial synostotic syndactyly with phalangeal reduction (MSSD) is a rare non-syndromic limb malformation with autosomal recessive inheritance. To date, only a few affected families with MSSD who had BHLHA9 mutations have been reported. The present report describes a consanguineous Pakistani family with five affected individuals with MSSD who exhibited an autosomal recessive pattern. Genotyping followed by Sanger sequencing was performed, and we identified a novel homozygous missense mutation (c.311T>C, p.Ile104Thr) in the BHLHA9 gene. This finding expands the spectrum of known mutations in the BHLHA9 gene that cause MSSD.
Syndactyly in Holstein cattle is an autosomal recessive abnormality characterized by the fusion of the functional digits. This disorder has been previously mapped to the telomeric part of bovine chromosome 15. Here, we describe the fine-mapping of syndactyly in Holstein cattle to a 3.5-Mb critical interval using a comparative mapping approach and an extended pedigree generated by embryo transfer. We report genetic evidence for the exclusion of two genes previously suggested as candidates (EXT2 and ALX4) and describe the identification of a doublet mutation in complete linkage disequilibrium with syndactyly in one gene of the critical interval: LRP4. Finally, based on recent discoveries concerning the mouse mutants dan and mdig and a mouse knockout for Lrp4, we present solid evidence that the subsequent substitution in LRP4 exon 33 is a strong candidate causal mutation for syndactyly in Holstein cattle.
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