Saturday, April 11, 2009

Fruit Flies and Tarsal Coaltion

Those of you who took genetics in college might recognize this little creature:

This is the fruit fly, Drosophila melanogaster, which has delighted geneticists and tortured premed students for decades. I still can smell the ether we used to anesthetize these buggers as we struggled to understand Mendelian inheritance in my college genetics class.

Far apart in the tree of life, humans and Drosophila nonetheless share major portions of DNA. One such DNA sequence is the homeobox, which encodes transcription factors that play a major role in limb development. These DNA sequences are conserved across vast distances in the phylogenetic tree— for example, a fly can function perfectly well with a chicken homeotic gene in place of its own.

Hox genes are a subgroup of homeobox genes. In vertebrates these genes are found in gene clusters on the chromosomes. In mammals four such clusters exist, on four different chromosomes.

Mutations in hox and other genes can cause multiple genetic anomalies, including segmentation errors. Segmentation errors can lead to fusion of bones in the foot, a phenomenon known as tarsal coalition.

Tarsal coalition has been know about for hundreds of years, although the genetic basis is only being investigated in the modern era. The first written description of tarsal coalition was by Buffon in 1769. The first radiologic depiction of tarsal coalition took place in 1898, only three years after Roentgen described x-rays.

The most common types of tarsal coaliton are calcaneonavicular and talocalcaneal coalitions, These variants are commonly seen by every busy radiologist that reads MRI scans of the foot and ankle.

39 year old female training for 10 mile run, who recently increased running up to seven miles a day, and complained of distal leg pain:

(A) Sagittal T1 and (B) Sagittal T2 fatsat images depict a stress fracture (red arrow) of the distal tibial metaphysis. Note the striking marrow edema, seen best on the T2 fatsat image.

One must be cautious about satisfaction of search, however, and examination of the remainder of the examination reveals a second finding:

(A) Sagittal T1 and (B) Sagittal T2 fatsat images display a predominantly fibrous coalition (red arrow) between the navicular (green arrow) and the cuboid (blue arrow) bones.

The coalition (red arrow) is nicely seen between the navicular (green arrow) and the cuboid (blue arrow) on this coronal intermediate image:

An oblique axial T2 fatsat image reveals marrow edema on both sides of the abnormal joint, reflecting abnormal stress:

Coalitions between the cuboid and navicular are rare, accounting for less than one percent of tarsal coalitions. Coalitions are often treated nonsurgically, but when necessary, they can be surgically resected.

Humans run and flies use their wings to get from place to place, but they both share common DNA. Errors in the DNA code in critical areas of either species can lead to segmentation anomalies in both.

Vic David MD


Craig said...

again, of course, very nice case with dual findings and elegant introduction

Anonymous said...

This is a wonderful blog, glad I just found it. Thanks GR MD

Felasfa Wodajo said...

What a joy to follow the lines of thought connecting coalitions and drosophila ! Thanks.

Felasfa Wodajo said...
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