Disorders Involving Ion Transporters

Published on 22/03/2015 by admin

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Chapter 688 Disorders Involving Ion Transporters

William A. Horton, Jacqueline T. Hecht

In order of decreasing severity, the disorders involving ion transporters include achondrogenesis type 1B, atelosteogenesis type II, and diastrophic dysplasia. They result from the functional loss of the sulfate ion transporter called diastrophic dysplasia sulfate transporter (DTDST), which is also referred to as SLC26A2 (solute carrier family 26, member 2). This protein transports sulfate ions into cells and is important for cartilage cells that add sulfate moieties to newly synthesized proteoglycans destined for cartilage extracellular matrix. Matrix proteoglycans are responsible for many of the properties of cartilage that allow it to serve as a template for skeletal development. The clinical manifestations result from defective sulfation of cartilage proteoglycans.

A number of mutant alleles have been found for the DTDST gene; they variably disturb transporter function. The disorders are recessive traits requiring the presence of 2 mutant alleles. The phenotype is determined by the combination of mutant alleles; some alleles are present in more than one disorder.

Diastrophic Dysplasia

Diastrophic dysplasia (OMIM 22600) is a well-characterized disorder recognized at birth by the presence of very short extremities, clubfoot, and short hands, with proximal displacement of the thumb producing a hitchhiker appearance (Fig. 688-1). The hands are usually deviated in an ulnar direction. Bony fusion of the metacarpophalangeal joints (symphalangism) is common, as is restricted movement of many joints, including hips, knees, and elbows. The external ears often become inflamed soon after birth. The inflammation resolves spontaneously but leaves the ears fibrotic and contracted (cauliflower ear deformity). Many newborns have a cleft palate.

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Figure 688-1 Child with diastrophic dysplasia. The extremities are dramatically shortened (top). Clubfoot is commonly observed (middle left

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