Emily S. Paper

Syndactylism

Imagine being born with fused fingers and toes or not having any real fingers at all and just a palm with stubs where there should be fingers and toes. Having a condition such as the one stated above is hard for the majority of people to imagine having, because of its rarity. Although, in actuality people do have syndactylism or syndactly, which is what the condition described above is. Throughout this, paper a discussion of the various types of syndactly, including type one, two, three, four, and five.
Syndactlyism is the fusing together of the fingers and toes as mentioned up above. There are five different types of syndactly, but before we can discuss all of them in detail, we need to know what all the disease encompasses. Syndactly is the result of an abnormality or malformation of the digits, meaning the fingers or toes ( Sato, D., Liang, D., Wu, L., et. al, 1). The condition itself deals with the webbing of the fingers and toes, so that they are close enough together as just one digit, instead of individual digits (Perkoff, David, 1). Therefore, syndactly is the fusing of certain digits or fingers, and not necessarily, all of the digits. This means that there can be just one or two fused digits, and the rest of the digits are independent of each other.
Knowing that there is only the fusing of certain digits and others are not leads me to the various varieties of syndactly. There are three main types of syndactly, which include, combination by fibrous tissue and skin, combination by skin, and the combination of skin, fibrous tissue, and bone (Perkoff, David, 1). Each of these three basic types underlines the basis for the five different types of syndactly. Type one syndactly or zgodactyly is the webbing between the second and third toes and/or the third and fourth fingers (Bosse, K., Betz, R., Lee, Y., Wienker, T. et al., 1). Synpodactly or type two has webbing between the middle finger and ring finger and the fourth and fifth toes as well (Cross, H., Lerberg, D., and McKusick, V., 1). Then we have type three, which refers to having only the webbing of the fourth and fifth fingers and nothing else (Cross, H., Lerberg, D., and McKusick, V., 1). This means that type three syndactly does not involve webbing on the toes or any of the other digits, besides the fourth and the fifth digits. The fourth type of syndactly means that one has webbing through all of their fingers and toes or complete syndactly (Cross, H., Lerberg, D., and McKusick, V., 1). Type four, is the rarest of all of the types of syndactly and has only be reported a couple of times since 1940 (Sato, D., Liang, D., Wu, L., et. al, 2). Lastly, we have type five, which is when syndactly is associated with the metatarsals and metacarpals fusing together with the webbing of the digits (Cross, H., Lerberg, D., and McKusick, V., 1). Type five, is the only type that deals with the fusing together of the hand and feet bones, otherwise known as the metacarpals and metatarsals. These are the five different types of syndactly, which will have more in depth discussion in the following paragraphs.
To begin the more in depth discussion, we will start with type one syndactly, otherwise known as zgodactly. As previously mentioned, zgodactly refers to the webbing of the second and third toes and/or the third and fourth fingers. In zygodactly, sometimes only the fingers are affected and other times only the toes are affected (Percin, F., Percin, S., Egilmez, H., Sezgin, I., et. al., 2). This just means that in order to classify for type one, a person does not have to have both webbing on the fingers and toes. Instead, they may have webbing between their third and fourth fingers or second and third toes. This type of syndactly is the most common of the five different types ((Percin, F., Percin, S., Egilmez, H., Sezgin, I., et. al., 6). There has been linkage of type one to certain markers on chromosome 2q34-q36 (Bosse, K., Betz, R., Lee, Y., Wienker, F. et. al., 1). This has made huge strides in being able to identify if a child will have type one syndactly or not. However, knowing that there is a linkage on chromosome 2q34-q36, does not always guarantee that this will always be where the gene for type one syndactly will be inherited (Bosse, K., Betz, R., Lee, Y., Wienker, F. et. al., 3). It could also be the case that another gene could be involved in the development of the limb that is located on chromosome two (Bosse, K., Betz, R., Lee, Y., Wienker, F. et. al., 3). That is why when mapping genes, one can never is completely sure that the gene for a certain disease will express itself.
Unlike type one syndactly, type two or synpodactyly is when the middle and ring fingers are fused together as well as the fourth and fifth toes (Merlob, P., Grunebaum, M., 1). Synpodactyly, on many occasions has polydactyly along with it, making it one of the rarest of the five types of syndactly (Merlob, P., Grunebaum, M., 1). Polydactyly is a condition that refers to having multiple digits or fingers and toes. The metatarsal and metacarpal bones may also be involved with type two syndactly as well, even though it is very common in type five (Merlob, P., Grunebaum, M., 4). One can tell the difference between the two types of syndactly simply that type two has polydactyly associated with it, unlike type five, which are strictly the metatarsals and metacarpals.
Synpodactyly in the hands, usually has a very thick webbing between the middle and ring fingers, which makes the two fingers completely fused together (Cross, H., Lerberg, D., McKusick, V.,2). However, there may be a groove that extends longitudinally between the third and fourth fingers, making it appear as if one had a double nail (Cross, H., Lerberg, D., McKusick, V.,4). As for the feet, the fourth and fifth toes may appear duplicated, and have an extra metatarsal (Cross, H., Lerberg, D., McKusick, V., 6). Therefore, this means that it is very common to have fusing of multiple toes recognized as the fourth and fifth toes. As mentioned before this is polydactyly, meaning having an extra finger or toe, instead of the normal five digits on a single hand or foot in humans. This apparently does not cause the person any discomfort and many attempts to separate the fused toes and fingers have been attempted during the first years of life and have failed because the toes and fingers then start to grow at an angle (, H., Lerberg, D., McKusick, V., 7). Therefore, it is in the best interest of the person with type two syndactly to leave the fused digits together instead of separating them to avoid angled growth.
In addition, synpolydactyly includes three types of SPD conditions that are genetically determined as SPD1, SPD2, and SPD3 (Malik, S., Girisha, K., Wajid, M., and Roy, A., et. al., 1). The SPD1 type is linked with the mutations in the HOXD13 genes (, S., Girisha, K., Wajid, M., and Roy, A., et. al., 1). However, no one with this phenotype has ever been reported, even though it has been found to be involved with type two syndactyly (S., Girisha, K., Wajid, M., and Roy, A., et. al., 1). The three types of synpodactyly have been located on chromosomes 2q31, 22q13, and 14q11.2q12 (S., Girisha, K., Wajid, M., and Roy, A., et. al., 1). This is how the genes for type two synpodactyly are in the human genome, as to which chromosome it is located.
Along with being located on these chromosomes synpolydactyly is shown to by connected with the HOXD13 gene through defective trinucleotide sequence that repeats by encoding a polyalanine tract in a 15-residue (Goodman, F., Mundlos, S., Donnai, D., Giovannucci, M., 1). These HOXD13 proteins correlate with the SPD1, SPD2, and SPD3 respectively. Therefore, since the trinucleotide is defective, which makes the slippage of replication highly unlikely (Goodman, F., Mundlos, S., Donnai, D., Giovannucci, M., 6). If not due to the slipping of replication then what could be the cause of the expansion of the trinucleotide repeat when it is defective? The answer to this question is that the replication slippage is due to a misalignment during replication or nonreciprocal recombination, with different segments of the trinucleotide repeat that are mediated (Goodman, F., Mundlos, S., Donnai, D., Giovannucci, M., 6). For example, in chicks and mice the Hoxd13 gene is at a larger expression than any of the other hoxd genes at the five prime end and is also expressed first (Goodman, F., Mundlos, S., Donnai, D., Giovannucci, M., 6). The hoxd13 gene helps to form the arms, legs, fingers and toes of the chicks and mice. In addition, it is very common to have mutations frequently in chickens that result in polydactyly as mentioned previously.
However, synpodactyly or type two syndacyly is quite different from type three syndactyly, which is what we will be discussing next. Type three syndactyly is different from type one and type two, because it requires the fusing of the fourth and fifth fingers and that is it (Cross, H., Lerberg, D., McKusick, V., 1). This is very different from type one syndactyly, which includes the webbing or fusing of the third and fourth fingers, and/or the fusing of the second and third toes. Synpodactyly is also very different, because it requires the fusing of the third and fourth fingers with the addition of polydactyly. However, only type three syndactyly is an allelic disorder in oculodentodigital dysplasia (Sato, D., Liang, D., Wu, L., et. al, 1). The causation for this allelic disorder is by mutations for the gene of the gap junction protein, known as alpha-1 (Sato, D., Liang, D., Wu, L., et. al, 1). This gene is the only gene that has been identified for type three syndactyly, and no other genes have thus far been identified as being the genes that are responsible for other types of syndactyly (Sato, D., Liang, D., Wu, L., et. al, 1). Thus far, this is all that is known about type three syndactyly, since researchers have put more effort on the other four types of syndactyly.
This now concludes the discussion of syndactyly types one through three and is time for type four. Type four syndactyly is one of the rarest types of syndactyly that a person can have (Sato, D., Liang, D., Wu, L., et. al, 2). As mentioned previously, type four has only been documented twice since the 1940’s. People with this type of syndactyly have complete syndactyly or the fusing of all fingers and toes along with polydactyly (Sato, D., Liang, D., Wu, L., et. al, 2). Therefore, type four syndactyly lacks fingers or toes that are separated from each other, instead each finger and toe is fused with the next. This makes the persons hand scooped or cup shaped, and often leads to difficulty picking things up, such as loose change. When a person picks up a quarter or a penny they always use the index finger and thumb to pick it up, but a person with type four syndactyly cannot acquire this technique do to the fusing of all of the fingers.
In addition, the localization of genes for type four syndactyly at this time is currently unknown (Sato, D., Liang, D., Wu, L., et. al, 2). This is because the gene for type four’s location does not exist. Although with patience and time, maybe someday we will know the exact location of the gene for type four syndactyly.
Type five syndactyly is last on the list of the various types of syndactylism. It entails the fusing together of the hand and feet bones or the metacarpals and metatarsals (Percin, F., Percin S, Egilmez, H, Sezgin, I., 2). Thus far, type five is the only type that is strictly associated with the fusing of the metacarpals and the metatarsals. Although, in multiple cases they have found people with the fusing of the hand and feet bones along with the fourth and fifth fingers and toes (Percin, F., Percin S, Egilmez, H, Sezgin, I., 2). The fusing of the hand and feet bones in this type of syndactyly is different from any other type of syndactyly, because the combination of bones is different from cartilage or skin. Instead of separate metacarpals and metatarsals, a person with type five syndactyly gets these bones all fused together as one bone.
Like type four, type five syndactyly is also considered a rare, because it is the only one that directly deals with the fusing of the metacarpals and the metatarsals (Zhao, X., Sun, M., Zhao, J., Alfonso, J., et. al. 2). Type five sydactyly is one of the few types to be mapped out, even though it is rare. The mutations that are responsible for this type are the HOXD13, which are responsible for multiple malformations of the limbs (Zhao, X., Sun, M., Zhao, J., Alfonso, J., et. al. 6). The hoxd13 deals with the SPD as I have already mentioned and the cause is by p.Q317R and BDA4/BDD/BDE that requires the syndactyly of toes two and three, in which a polyalanine contraction is the main cause (Zhao, X., Sun, M., Zhao, J., Alfonso, J., et. al. 6). This limb malformation is the one that causes type five syndactyly. However, many malformations of the limbs are due to the many different classes of the hoxd13 mutations (Zhao, X., Sun, M., Zhao, J., Alfonso, J., et. al, 8).
In conclusion, syndactylism is a very common disease that affects many people throughout the world. The five different types of syndactyly make it almost nearly impossible for any two people to have exactly the same type of syndactyly. With type one syndactyly dealing with webbing between the third and fourth fingers and/or the second and third toes. Type two with webbing between the fourth and fifth toe and middle and ring fingers and type three with webbing only between the fourth and fifth fingers. Then we have the two rarer types of sydactyly, which include type four with complete syndactyly of the fingers and toes, and type five with the fusing of the metacarpals and metatarsals. Therefore, each type of syndactyly includes features, which are unique and completely separate from all of the other types of syndactylism.

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