A physical and transcriptional map of the preaxial polydactyly locus on chromosome 7q36.

Preaxial polydactyly is a congenital hand malformation that includes duplicated thumbs, various forms of triphalangeal thumbs, and duplications of the index finger. A locus for preaxial polydactyly has been mapped to a region of 1.9 cM on chromosome 7q36 between polymorphic markers D7S550 and D7S2423. We constructed a detailed physical map of the preaxial polydactyly candidate region. With a combination of methods we identified and positioned 11 transcripts within this map. By recombination analysis on families with preaxial polydactyly, using newly developed polymorphic markers, we were able to reduce the candidate region to approximately 450 kb. The homeobox gene HLXB9, a putative receptor C7orf2, and two transcripts of unknown function, C7orf3 and C7orf4, map in the refined candidate region and have been subjected to mutation analysis in individuals with preaxial polydactyly.

Clinical and genetic studies on 12 preaxial polydactyly families and refinement of the localisation of the gene responsible to a 1.9 cM region on chromosome 7q36.

Polydactyly is the most frequently observed congenital hand malformation with a prevalence between 5 and 19 per 10000 live births. It can occur as an isolated disorder, in association with other hand/foot malformations, or as a part of a syndrome, and is usually inherited as an autosomal dominant trait. According to its anatomical location, polydactyly can be generally subdivided into pre- and postaxial forms. Recently, a gene responsible for preaxial polydactyly types II and III, as well as complex polysyndactyly, has been localised to chromosome 7q36. In order to facilitate the search for the underlying genetic defect, we ascertained 12 additional families of different ethnic origin affected with preaxial polydactyly. Eleven of the kindreds investigated could be linked to chromosome 7q36, enabling us to refine the critical region for the preaxial polydactyly gene to a region of 1.9 cM. Our findings also indicate that radial and tibial dysplasia/aplasia can be associated with preaxial polydactyly on chromosome 7q36. Combining our results with other studies suggests that all non-syndromic preaxial polydactylies associated with triphalangism of the thumb are caused by a single genetic locus, but that there is genetic heterogeneity for preaxial polydactyly associated with duplications of biphalangeal thumbs. Comparison of the phenotypic and genetic findings of different forms of preaxial polydactyly is an important step in analysing and understanding the aetiology and pathogenesis of these limb malformations.

Psychomotor development in children with triphalangeal thumbs. A preliminary study.

In order to explore the influence of an isolated congenital hand malformation on psychomotor development, we performed an exploratory, observational study on 18 children with triphalangeal thumbs. The investigative procedure consisted of a hand function examination, a semi-structured interview with the mother about the development of the child, the so-called "Hand test", and the "Child Behaviour Check List". Our observations suggest specific developmental difficulties in fine motor skills and language development, but the children showed no signs of behavioural psychopathology.

Genetics of limb development and congenital hand malformations.

The vertebrate limb bud develops along three different axes: proximodistal, anteroposterior, and dorsoventral. Several genetic factors responsible for control of each of the three limb axes have been identified. The genes involved interact in complex feedback loops to achieve proper arrangement and differentiation of tissues. Most of the available information on limb development and patterning has come from studies carried out in the lower vertebrates. In recent years, an increasing number of studies have been unraveling the genetic basis of human hand malformation phenotypes. At present, genes responsible for preaxial polydactyly, split hand/split foot malformation, and brachydactyly type C have been localized, and the gene responsible for synpolydactyly has been identified. In this paper, we present an overview of the genetic factors involved in limb development, followed by summarized discoveries in the genetics of human congenital hand malformations.

Metacarpophalangeal pattern (MCPP) profile analysis in a family with triphalangeal thumb.

Triphalangeal thumb (TPT) is a rare congenital disorder characterised by a long, finger-like thumb with three phalanges instead of two. It can occur as an isolated defect, in association with other abnormalities of the hands and feet, or as a part of a syndrome. Sporadic cases have been described, but it is usually inherited as an autosomal dominant trait. In order to examine skeletal morphology in different phenotypic variations of this disorder, we performed metacarpophalangeal pattern profile analysis in one kindred with this disorder. A characteristic profile occurred in all affected people, based on the individual lengthening or shortening of the thumb bones. Comparison of the affected and unaffected people from this family with people with a different genetic background suggests that the described profile is specific for TPT and could be used as a helpful diagnostic tool in syndromes which include TPT.

The role of metacarpophalangeal pattern (MCPP) profile analysis in the treatment of triphalangeal thumbs. Description of a method and a case report.

The metacarpophalangeal pattern (MCPP) profile analysis is a method of comparing the length of each of the 19 tubular bones of the hand on the X-ray with the standard length in the normal population according to age and sex. An MCPP plot is a graphic illustration of the MCPP analysis. It is not the exact height of the curve on the MCPP plot which is most important, but the profile which occurs because of the individual lengthening or shortening of the bones. This pattern profile appears to be specific for several congenital malformation syndromes. We have recently used MCPP analysis in planning surgery for triphalangeal thumbs. The percentage of excessive or reduced length of each individual bone of the hand can be read from the MCPP plot and is helpful in calculating a more accurate length for the newly created thumb. MCPP analysis can be used as a diagnostic tool in a number of congenital hand malformations, but may also be helpful in planning surgical treatment of congenital hand malformations when abnormal bone length is involved.

Genetic aspects of polydactyly.

The early limb development follows the similar pattern in all vertebrates since different species develop using the same regulatory genes in the formation of the body plan. Some of these genes remained well conserved during evolution and can be traced back as far as Drosophila--while some others changed their structure or developed new functions. This is why the limbs from different animals still look different from one another. However, all existing tetrapods have, like Homo Sapiens, limbs with five, or fewer digits. It has been argued that the interplay of the factors controlling the patterning and differentiation during the embryonal limb development can provide five distinct "genotypes" allowing development of only five different digits. This would imply that the Greek definition of polydactyly, namely "duplication of the finger or a part of it", is correct, not only in morphological but also in a genetical sense. Genes involved in the determination of the outline of the limb are candidates for disorders like polydactyly and syndactyly. Recently, we have localised the gene for triphalangeal thumb (TPT) on chromosome 7q. As almost 50% of our patient population also had rudimentary postaxial polydactyly and/or syndactyly, the interesting question arose whether the TPT gene also was responsible for isolated post-axial polydactyly. Our preliminary evidence suggests, however, that different gene(s) are involved in the pathomorphogenesis of postaxial polydactyly. Studies of human congenital hand malformations--combined with genetic studies in lower vertebrates--will help us to understand not only the molecular basis of these disorders, but also to get insight into the fascinating mechanisms involved in the normal development of the human hand.

Phenotypic analysis of triphalangeal thumb and associated hand malformations.

Triphalangeal thumb (TPT), a long, finger-like thumb with three phalanges instead of two, is regarded as a subtype of preaxial polydactyly. It can occur as a sporadic disorder, but is more often seen as a dominant familial trait. We describe four white Dutch families in which triphalangeal thumb has variable expression and is sometimes associated with preaxial extra rays, rudimentary postaxial polydactyly, cutaneous syndactyly of the hands, and, rarely, postaxial polydactyly and syndactyly of the feet. A comparison with similar familial conditions reported during the past 10 years is provided. The potential significance of linkage and molecular genetic analysis for better insight into the pathogenesis of complex hand malformations is discussed.

The gene for triphalangeal thumb maps to the subtelomeric region of chromosome 7q.

Triphalangeal thumb is a developmental anomaly, sometimes dominantly transmitted, characterized by a long, finger-like thumb with three phalanges instead of two. The underlying genetic defect is unknown, but presumably involves genes that regulate the differentiation of the developing forelimb. In two large kindreds with triphalangeal thumb, evidence for linkage to the long arm of chromosome 7 was obtained with a maximum lod score of 12.61. Multipoint linkage and haplotype analysis placed the gene close to the telomere of the long arm. To our knowledge this is the first time that a human gene involved solely in the pathologic morphogenesis of the hand and feet has been localized.