By David N. Leff
Among the rarer ¿ and nastier ¿ genetic diseases of infancy, Wiskott-Aldrich syndrome (WAS) owes its name to two pediatricians ¿ Alfred Wiskott (1898-1978), who first described it in a German family in the 1940s, and Robert Anderson Aldrich (1917-1999), who reported the disorder a decade later in an American family.
Educated guesstimates put the worldwide prevalence of WAS at anywhere from one case in 50,000 to one in 100,000. For one present American patient in particular, the dire disease has reached an unexpected and happy denouement.
His medical history began at the age of 10 months with encephalitis. Between 2 and 5 years of age, his eczema-ridden skin bruised easily, and middle-ear infections recurred frequently. At age 5, clinicians noticed that this patient¿s little brother had petechiae (tiny hemorrhages under his skin) and thrombocytopenia ¿ a dearth of platelets in the blood. When the doctors tested their young poster patient¿s platelet count, they found it in the range of 13,000 to 20,000 per cubic millimeter. Normal platelet levels are around 150,000.
At that point, they made a formal diagnosis of Wiskott-Aldrich syndrome.
Among the now grown-up patient¿s physicians is pediatric immunologist Fabio Candotti, who heads the Disorders of Immunity Section at the National Human Genome Research Institute in Bethesda, Md. ¿When WAS patients have a very low number of platelets,¿ he pointed out, ¿they start hemorrhaging spontaneously. They bleed from the nose, bloody diarrhea from the gastrointestinal tract, and of course they have hematomas and petechaie everywhere. But the most dangerous thing,¿ he added, ¿is that they can have intracerebral hemorrhages, which are mostly fatal.
¿So if there is no compatible sibling donor, or likelihood of finding one in the bone marrow donor program registry, then one option to prevent cerebral hemorrhages is removing the spleen. The spleen is the organ whose platelets in these patients are different from normal; it recognizes them as abnormal, and removes them from the circulation. If you ablate the spleen, that doesn¿t happen any more.¿
Free Of Disease ¿ But Why? How?
The medical team¿s young patient underwent a splenectomy soon after receiving his childhood diagnosis. For years after, he suffered bouts of pneumococcal meningitis, as well as his prior symptoms, until at age 12 he was hospitalized with thrombocytopenia and an illness resembling rheumatoid arthritis. At age 16, he developed mastoiditis.
However, since his 20s, the patient, now 43 years old, has been free of serious illnesses. His clinicians asked: ¿What has commuted his long-term sentence of recurrent diseases?¿
An answer to that question appears in the title to a paper in today¿s Proceedings of the National Academy of Sciences, (PNAS), dated July 17, 2001. It reads: ¿Somatic mosaicism in Wiskott-Aldrich syndrome suggests in vivo reversion by a DNA slippage mechanism.¿ In other words, the gene mutation that caused his WAS did a U-turn, reversed course, and went into remission.
¿Our finding,¿ Candotti told BioWorld Today, ¿was that this patient had been able to revert his mutation into the normal sequence of the gene. That is always rare, and in this particular case, the mechanism for the reversion was not obvious. What is obvious is that the mutation reverted to the normal sequence. In this case, because of the particular genomic sequence where the mutation exists, we were able to postulate that the DNA polymerase slippage mechanism caused this reversion to happen.
¿In the WAS gene, which is located on the X chromosome,¿ Candotti related, ¿there is a six-nucleotide sequence, ACGACT, followed by another identical ACGACT stretch. In the mutation of the gene there is a third repeat of those six nucleotides. What¿s happening in this patient is that some of the T cells that he now has in circulation have lost their third six-nucleotide stretch, and therefore the gene is no longer mutated in those cells.
¿As for DNA polymerase slippage,¿ Candotti went on, ¿when a cell divides, it has to double the amount of its DNA, which it shares into two daughter cells. To do that, an enzyme called DNA polymerase copies one of the DNA strands. It uses one strand as a template to synthesize exactly the complementary sequence. It does so by taking nucleotides from the milieu and synthesizing the strand.
¿What can happen is that there is a slippage forward or backward, so that one of the A¿s and one of the T¿s may not be really paired. If that happens, the polymerase sticks to that, and prevents mutations from occurring. But sometimes that doesn¿t happen. So this slippage is causing production of an extra T and an extra A on both strands, and that mutation gets passed on to the daughter cells. And if it takes place in a germ line cell, it will be passed on to the offspring.
¿We think that in an ancestor of this patient¿s family, DNA polymerase slippage has caused generation of the mutation in some of the germ line cells, and since that time, a series of male patients harbor these mutations, while carrier females carry them.¿
Mutation Reversion Sets Gene Therapy Stage
¿In this 43-year-old man,¿ Candotti said, ¿the DNA polymerase has slipped forward, and caused the patient to be a mosaic for the mutation itself. The mosaicism¿ he explained, ¿describes the presence of a combination of cells with different genetic characteristics. This patient has most of his T cells carry the correct sequence, but any other blood cells ¿ like B cells and K cells and monocytes and leukocytes ¿ all have the wrong sequence. So that¿s why he¿s a mosaic.
¿We are interested in gene therapy for WAS,¿ Candotti volunteered. ¿If we can correct the T cells we stand a chance of achieving clinical benefit. In this patient, all the T cells were corrected, and improved clinically.
¿The design of such a gene therapy trial,¿ he continued, ¿would be similar to previous trials that have been performed for immunodeficiency disease. We would take bone marrow cells from the patient, use a retroviral vector to correct the disease, or add a copy of the normal gene to these cells and then infuse them back to the patient.¿
He foresees that a gene therapy trial ¿will be at least five years from now.¿