Limb development: molecular dysmorphology is at hand!

Proximal radio-ulnar synostosis with bone marrow failure syndrome in an infant without a HOXA11 mutation

This report summarizes the clinical management of an infant with a proximal radio-ulnar synostosis and inherited bone marrow failure syndrome (PRUS/IBMFS). Molecular studies were negative for the characteristic HOXA11 mutation described earlier. He was successfully treated with a non-myeloablative hematopoietic stem cell transplantation from an human leukocyte antigen-identical sibling donor at the age of 3 months. We reviewed the literature on PRUS/IBMFS with an emphasis on the current understanding of the molecular mechanisms involved in the disease pathogenesis. Absence of the HOXA11 mutation in this case implies that molecular mechanisms beyond the HOXA11 gene, yet to be discovered, may contribute for the development of PRUS/IBMFS.

p63 in epithelial survival, germ cell surveillance, and neoplasia

The p53homolog p63has emerged as a gene with an enormously complex function that is distinct from that of p53. It encodes two distinct transcript isoforms that have a dramatic impact on replenishment of cutaneous epithelial stem cells and on ovarian germ cell survival. However, although these two fundamental roles of p63 attest to its powerful place in development, its other functions-specifically the apparent capacity of p63, when induced, to supervise the emergence of new cell populations in the breast, prostate, cervix, and upper reproductive tract-are shared by embryo and adult. These observed functions may only scratch the surface of a repertoire that has been postulated to encompass a range of cellular activities, as evidenced by the fact that p63 proteins have been shown to potentially bind to over 5800 target sites. Whether tumorigenic pathways are also involved, and to what extent, is a subject of both promise and controversy that remains to be resolved.

Société Française d’Orthopédie Pédiatrique

Limb malformations are frequent. These malformations are isolated or associated with anomalies of other developmental fields and accurate diagnostic is essential for prognosis evaluation, treatment and genetic counseling. Animal embryology and molecular biology techniques, have given us a better understanding of the processes of growth and patterning of the limb buds. The key genes that are involved in these processes have been identified and their interactions recognized. Human genetics has been able to identify, or at least localize, several genes implicated in limb development. We here review the present knowledge on these genes and their mutations responsible for limb anomalies.

Why study human limb malformations?

Congenital limb malformations occur in 1 in 500 to 1 in 1000 human live births and include both gross reduction defects and more subtle alterations in the number, length and anatomy of the digits. The major causes of limb malformations are abnormal genetic programming and intra-uterine disruption to development. The identification of causative gene mutations is important for genetic counselling and also provides insights into the mechanisms controlling limb development. This article illustrates some of the lessons learnt from the study of human limb malformation, organized into seven categories. These are: (1) identification of novel genes, (2) allelic mutation series, (3) pleiotropy, (4) qualitative or (5) quantitative differences between mouse and human development, (6) physical and teratogenic disruption, and (7) unusual biological phenomena.

p63 Coordinates anogenital modeling and epithelial cell differentiation in the developing female urogenital tract

p63 is a p53 homologue required for cutaneous development that is expressed in immature squamous epithelium and reserve cells of the cervix. Humans with p63 mutations exhibit defects in limb, accessory organ (skin appendage, breast, prostate), and genitourinary development. Because p63 expression patterns imply a strong role of the gene in the female genital tract development, newborn female p63-/-, +/-, and +/+ mice were examined in situ, dissected, and compared. Nuclear p63 protein was localized to the skin, vagina, bladder, urethra, and basal columnar cells of the caudal uterus in p63+/+ and +/- animals. p63-/- mice exhibited abnormal genital morphogenesis with hypoplastic genitalia, a single cloacal opening, and persistence of columnar epithelium at lower genital tract sites that normally undergo squamous and urothelial differentiation. The defects observed support p63-dependent pathways of genital tract development that permit externally, ectodermal basal cell replenishment integral to reciprocal epithelial stromal signaling, urorectal septation, and modeling of the external genitalia; and internally, the emergence of basal epithelial cell populations capable of divergent epithelial cell differentiation in the vagina, cervix, and urinary tract. Defects in the first pathway explain imperforate anus, vaginal septum, genital hypoplasia, and micropenis reported in humans with p63 mutations. The second is necessary for the generation of multipotential reserve cells in the cervix and may be operative in other epithelial stromal interactions integral to the emergence of uterine basal cells later in life.

Integrative biology and the developing limb bud

The identification or selective construction of mutations within genes has allowed researchers to explore the downstream effects of gene disruption. Although these approaches have been successful, a limitation in our assessment of the consequences of conditional changes, and thereby our understanding of roles or function of genes, limits the degree to which we examine the effects of our manipulations. It is also clear that linear associations are incorrect models for describing development, and newer methods now give us an opportunity to practice an integrative biology. In our attempts to explore the consequences of Hoxa13 disruption in mice and humans, it has become clear that a better understanding of the consequences of gene alteration may be achievable by taking a broader approach with a long-term view. Fundamental questions regarding Hox gene function in vertebrates, including those related to the number of target genes; the degree of overlap of target gene regulation among paralogs; the magnitude of modulation exerted; and the identity of genes that are activated versus repressed need to be explored if a more thorough mechanistic understanding is to be achieved. To begin to address these questions, we undertook a comprehensive analysis of the expression of genes within developing limb buds of mice, and here we present some of our preliminary results. Our efforts will further (1) the exploration of the broader genetic relationships of expressed genes, (2) the determination of parallels or variations in target usage for a given gene in different tissues and between different organisms, (3) the evaluation of limb patterning mechanisms in other animal model systems, and (4) the exploration of gene expression hierarchies regulated by HOX proteins in developmental systems.

Role of the subchondral vascular system in endochondral ossification: endothelial cell-derived proteinases derepress late cartilage differentiation in vitro

Endochondral ossification in growth plates proceeds through several consecutive steps of late cartilage differentiation leading to chondrocyte hypertrophy, vascular invasion, and, eventually, to replacement of the tissue by bone. The subchondral vascular system is essential for this process and late chondrocyte differentiation is subject to negative control at several checkpoints. Endothelial cells of subchondral blood vessels not only are the source of vascular invasion accompanying the transition of hypertrophic cartilage to bone but also produce factors overruling autocrine barriers against late chondrocyte differentiation. Here, we have determined that the action of proteases secreted by endothelial cells were sufficient to derepress the production of the hypertrophy-markers collagen X and alkaline phosphatase in arrested populations of chicken chondrocytes. Signalling by thyroid hormones was also necessary but endothelial factors other than proteinases were not. Negative signalling by PTH/PTHrP- or TGF-beta-receptors remained unaffected by the endothelial proteases whereas signalling by FGF-2 did not suppress, but rather activated late chondrocyte differentiation under these conditions. A finely tuned balance between chondrocyte-derived signals repressing cartilage maturation and endothelial signals promoting late differentiation of chondrocytes is essential for normal endochondral ossification during development, growth, and repair of bone. A dysregulation of this balance in permanent joint cartilage also may be responsible for the initiation of pathological cartilage degeneration in joint diseases.

Congenital thrombocytopenia and radio-ulnar synostosis: a new familial syndrome

The association of bone marrow failure and skeletal defects has been frequently noted, however, the genetic basis for most of these syndromes remains unclear. We describe a previously uncharacterized autosomal dominant syndrome of amegakaryocytic thrombocytopenia associated with radial-ulnar synostosis. The clinical features of this syndrome appear to be distinct from other similar conditions, including Fanconi's anaemia and thrombocytopenia-absent radii (TAR). The physical findings at diagnosis and clinical management of each case are detailed, as well as a discussion of this disorder in the context of other syndromes in which marrow failure and skeletal defects are prominent features. We also review recent developments in molecular genetics that may provide important clues to the underlying aetiology of this condition.

Ectodermal dysplasias: not only 'skin' deep

The ectodermal dysplasias (EDs) are a large and complex nosologic group of diseases; more than 170 different pathologic clinical conditions have been identified. Despite the great number of EDs described so far, few causative genes have been identified. We review EDs in the light of the most recent molecular findings and propose a new classification of EDs integrating both molecular-genetic data and corresponding clinical findings of related diseases.

Distinct mutations in the receptor tyrosine kinase gene ROR2 cause brachydactyly type B

Brachydactyly type B (BDB) is an autosomal dominant skeletal disorder characterized by hypoplasia/aplasia of distal phalanges and nails. Recently, heterozygous mutations of the orphan receptor tyrosine kinase (TK) ROR2, located within a distinct segment directly after the TK domain, have been shown to be responsible for BDB. We report four novel mutations in ROR2 (two frameshifts, one splice mutation, and one nonsense mutation) in five families with BDB. The mutations predict truncation of the protein within two distinct regions immediately before and after the TK domain, resulting in a complete or partial loss of the intracellular portion of the protein. Patients affected with the distal mutations have a more severe phenotype than do those with the proximal mutation. Our analysis includes the first description of homozygous BDB in an individual with a 5-bp deletion proximal to the TK domain. His phenotype resembles an extreme form of brachydactyly, with extensive hypoplasia of the phalanges and metacarpals/metatarsals and absence of nails. In addition, he has vertebral anomalies, brachymelia of the arms, and a ventricular septal defect-features that are reminiscent of Robinow syndrome, which has also been shown to be caused by mutations in ROR2. The BDB phenotype, as well as the location and the nature of the BDB mutations, suggests a specific mutational effect that cannot be explained by simple haploinsufficiency and that is distinct from that in Robinow syndrome.

Dominant mutations in ROR2, encoding an orphan receptor tyrosine kinase, cause brachydactyly type B

Inherited limb malformations provide a valuable resource for the identification of genes involved in limb development. Brachydactyly type B (BDB), an autosomal dominant disorder, is the most severe of the brachydactylies and characterized by terminal deficiency of the fingers and toes. In the typical form of BDB, the thumbs and big toes are spared, sometimes with broadening or partial duplication. The BDB1 locus was previously mapped to chromosome 9q22 within an interval of 7.5 cM (refs 9,10). Here we describe mutations in ROR2, which encodes the orphan receptor tyrosine kinase ROR2 (ref. 11), in three unrelated families with BDB1. We identified distinct heterozygous mutations (2 nonsense, 1 frameshift) within a 7-amino-acid segment of the 943-amino-acid protein, all of which predict truncation of the intracellular portion of the protein immediately after the tyrosine kinase domain. The localized nature of these mutations suggests that they confer a specific gain of function. We obtained further evidence for this by demonstrating that two patients heterozygous for 9q22 deletions including ROR2 do not exhibit BDB. Expression of the mouse mouse orthologue, Ror2, early in limb development indicates that BDB arises as a primary defect of skeletal patterning.

Genetics of limb anomalies in humans

The limbs have an essential function in all vertebrates. In animals, the key genes that are involved in the growth and patterning of the limb buds, and of the development of the complex extremities, have been identified and their interactions recognized. Aided by these discoveries, human genetics has also been able to identify, or at least localize, certain genes responsible for anomalies of the limbs. These malformations are isolated or associated with anomalies of other developmental fields, according to the expression domain of the gene involved. Increasing knowledge of the embryology and genes involved has lead to a regrouping of malformation manifestations in genetics terms. Clear genotype-phenotype correlations are difficult to establish owing to the interlinking network of genetic signals underlying limb development.

Tibial agenesis, femoral duplication, and caudal midline anomalies

Tibial agenesis with femoral duplication (Gollop-Wolfgang complex) and cloacal exstrophy are each rare malformations. Thus, their concurrence in an individual is an extremely rare event. We report on a patient born with distal duplication of the right femur, agenesis of the right tibia and hallux, cloacal exstrophy, and sacral defects. Review of the small group of cases reported with femoral duplication and tibial agenesis in association with caudal midline defects demonstrated a pattern of anomalies that while varying in presentation and severity was quite specific. We postulate that this disorder is related to misexpression of one or more distal HOX genes, potentially HOX10 or HOX11, leading to abnormal induction and proliferation of caudal mesenchyme.

Reconstructing the history of human limb development: lessons from birth defects

A major goal of biology has been to understand the developmental mechanisms behind evolutionary trends. This has led to a growing interest in studying the molecular basis of the evolution of developmental programs such as those mediating the diversification of tetrapod limbs. Over the last 10 y, it has become clear that the genes and general developmental programs used to build a limb are strongly conserved among widely disparate species. This finding suggests that altered regulation of the timing and locations of developmental events may be responsible for the morphologic variation observed among some species. However, genetic analyses of the regulatory regions of genes controlling vertebrate developmental programs are very limited. Characterization of the genetic basis of human birth defects of the limb provides an opportunity to dissect the developmental programs used to modify the architecture of the hominoid limb. This may allow us to assess the relative contributions of altered gene regulation to morphologic variation among species and reconstruct the evolutionary history of the hominid limb. Such insight is also important because morphologic differences in the hominid upper limb have been correlated with the use of tools, and tool making is often regarded as the milestone that marked the emergence of the genus Homo.