Homeobox genes, and the proteins they encode, the homeodomain proteins, play important roles in the developmental processes of many multicellular organisms. A subgroup of these genes, the Hox genes, are found in clusters on the chromosomes. Within a cluster the genes are arranged in the order of their function along the anteroposterior body axis. Some of these genes have been shown to play important roles in limb development. Combinations of Hox proteins are thought to specify individual segments of the appendicular skeleton. For example, Hox10 paralogous genes are required to pattern the stylopod, Hox11 paralogous genes are required to pattern the zeugopod, and Hox13 paralogous genes were shown to be important for patterning of the autopod. However, it has remained a continuous challenge in the field to establish where Hox genes fit into the molecular-genetic program of patterning and organogenesis of the limb elements.

In humans, mutations in HOXD13 results in synpolydactyly, a limb malformation characterized by an additional finger between digits 3 and 4 and a fusion of these three digits.

Synpolydactyly phenotype with mutation in HOXD13 (+10 Ala expansion) in heterozygous (left) and homozygous (hand and foot) individuals.

The mutations that cause this condition are rather unusual as the comprise expansions of a polyalanine tract in the N-terminal region of the HOXD13 protein. Polyalanine expansion comprise a new type of mutation that is distinct from other expansion mutations. Our studies show that expansion beyond a certain threshhold (more than 21 alanines) result in degradation of the protein. We have studied the nature of this mutation in a mouse mutant that carries the exact same mutation found in humans (spdh) as well as in other Hox mutant mice.

Skeletons of wt (right) and spdh mutant (left) fore limbs. Note brachydactyly and polydactyly in mutant.

We showed that Hoxd13 regulates Raldh2, an enzyme critical for retinoid acid (RA) synthesis in the limb and that, consequently, RA is reduced in mutant limbs. RA is produced in the interdigital space where it suppresses chondrogenesis. The relevance of this finding was supported by the fact that treatment of pregnant mice with RA resulted in restoration of pentadactyly in spdh mice. Furthermore, we showed that Hox genes control bone formation in the limbs by directly activating Runx2, the transcription factor essential for bone formation. In addition, we demonstrate that Hox genes determine the shape and identify of limb bones and that their inactivation causes a homeotic transformation of long bones (metacarpals) into round bones (carpals). Together, our findings show that Hox genes are essential modifiers of shape and limb gestalt by controlling stem cell differentiation into chondrocytes or osteoblasts.

Selected publications:

Kuss P, Villavicencio-Lorini P, Witte F, Klose J, Albrecht AN, Seemann P, Hecht J, Mundlos S. Mutant Hoxd13 induces extra digits in a mouse model of synpolydactyly directly and by decreasing retinoic acid synthesis. J Clin Invest. 2009 Jan;119(1):146-56.

Innis JW, Mortlock D, Chen Z, Ludwig M, Williams ME, Williams TM, Doyle CD, Shao Z, Glynn M, Mikulic D, Lehmann K, Mundlos S, Utsch B. Polyalanine expansion in HOXA13: three new affected families and the molecular consequences in a mouse model. Hum Mol Genet. 2004 Nov 15;13(22):2841-51.

Albrecht AN, Kornak U, Böddrich A, Süring K, Robinson PN, Stiege AC, Lurz R, Stricker S, Wanker EE, Mundlos S. A molecular pathogenesis for transcription factor associated poly-alanine tract expansions. Hum Mol Genet. 2004 Oct 15;13(20):2351-9

Albrecht AN, Schwabe GC, Stricker S, Böddrich A, Wanker EE, Mundlos S. The synpolydactyly homolog (spdh) mutation in the mouse -- a defect in patterning and growth of limb cartilage elements. Mech Dev. 2002 Mar;112(1-2):53-67.