- Explain that genetic analysis of three families with lower extremity and joint calcification found mutations in the NT5E gene encoding an enzyme converting AMP to adenosine.
- Note that the deficiency in this metabolic pathway suggests a genetic basis for the disorder and possible treatment options such as bisphosphonates, which provide an “adenosine rescue.”
Mutations in the NT5E gene have been identified as the cause of symptomatic lower extremity arterial calcifications in three families, researchers reported.
Nine patients in these families have now been found to have an adult-onset disorder that results from missense and nonsense mutations in NT5E, according to William A. Gahl, MD, PhD, of the National Human Genome Research Institute in Bethesda, Md., and colleagues.
This gene encodes the enzyme CD73, which is involved in a metabolic pathway by which AMP is converted to adenosine on the surface of cells, the researchers explained in the Feb. 3 New England Journal of Medicine.
One family that included five siblings with intermittent claudication and abnormal ankle-brachial blood pressure index values was enrolled in the National Institutes of Health (NIH) Undiagnosed Diseases Program for assessment.
The proband in this family was a 54-year-old English woman whose parents were third cousins.
Radiography showed calcification with arteriomegaly in the legs, plus calcifications of the joint capsules in the ankles, feet, wrists, and fingers. Magnetic resonance angiography confirmed occlusion of several lower extremity arteries.
Her siblings also had femoropopliteal occlusion with arteriomegaly and remodeling, plus calcified obstructive lesions.
Genetic analysis identified a 22.4 Mb region on chromosome 6q14, homozygous in all the siblings but heterozygous in the parents, that contained 92 genes.
Three of these genes were known to be involved in degenerative calcification, and direct sequencing found a nonsense mutation, C.662C→A, in exon 3.
In the second family, the proband was a 68-year-old woman from northern Italy with possible consanguinous marriage in an earlier generation.
She had experienced unremitting pain in the joints of her hands for many years as an adolescent and young adult. Calcifications found in her legs had originally been diagnosed as chondrocalcinosis.
Two of her sisters, both in their seventies, had similar calcifications and pain in the lower extremities.
In this family, the mutation was found to be a missense mutation, c.1073G→A, in exon 5.
The proband in the third family was a 44-year-old woman who had been born to a French mother and an English father.
Diagnostic tests for lower extremity paresthesias revealed distal artery calcifications, without involvement of the coronary or carotid arteries. She had undergone a femoral-popliteal bypass one year earlier.
Analysis determined that this patient was a compound heterozygote for the same nonsense mutation found in the first family that resulted in a premature stop codon in exon 9.
Molecular and cellular studies revealed nearly absent expression of the CD73 protein, along with high levels of tissue-nonspecific alkaline phosphatase (TNAP), which is a crucial enzyme involved in calcification.
Only one previous inherited disorder of vascular calcification has previously been identified, which is a mendelian abnormality associated with a mutation in ENPP1that results in inadequate levels of pyrophosphate, causing vascular calcification, myocardial infarction, and usually death in childhood.
“In our adult patients, CD73 deficiency may not lead directly to decreased pyrophosphate levels, but the consequent reduction in extracellular adenosine levels apparently enhances TNAP activity,” the researchers explained.
“We hypothesize that increased TNAP activity reduces pyrophosphate levels leading to calcification,” they wrote.
Understanding of the genetic defect in these patients suggests the possibility of corrective interventions, such as with bisphosphonates, according to Gahl and colleagues.
These drugs inhibit tissue calcification, are analogs of pyrophosphate, and have been used for the childhood-onset ENPP1 deficiency.
In addition, according to the researchers, adenosine-receptor agonists or TNAP inhibitors such as lansoprazole (Prevacid) also could be investigated, initially in cell cultures and then in an animal model, if a suitable one can be developed.