An accomplice, by (Di)George!

There is nothing more frustrating than thinking you've got the answer to a problem only to find there is much more to it than you originally thought. The quest for the genetic cause of DiGeorge syndrome is one such vexing problem that now, thanks to Ingeborg Stalmans, Diether Lambrechts and colleagues' identification of a modifier of the syndrome, is one step closer to being solved.

When it was shown that most DiGeorge syndrome patients had a 3 million base-pair deletion in chromosome 22, it seemed pretty obvious that haploinsufficiency of one or more genes in the hemizygous region (22q11) was the cause. Mouse studies nailed down the transcription factor gene Tbx1 — orthologous to TBX1 in humans — as the prime suspect.

However, it turned out that the rare DiGeorge syndrome patients without the large deletion apparently had normal TBX1 genes. The syndrome's extreme phenotypic variability also suggested TBX1 might have an accomplice, or accomplices, that remained unknown. Some more detective work in mice was required.

DiGeorge syndrome is characterized by life-threatening cardiovascular birth defects, as well as craniofacial, thymic and parathyroid defects. Therefore, the authors targeted the gene encoding vascular enthothelial growth factor (VEGF) because its crucial role in angiogenesis marked it as a potential accomplice.

Promisingly, birth defects of homozygous Vegf-mutant mice (Vegf^120/120 and Vegf^188/188) were strikingly similar to those seen in DiGeorge patients. Moreover, the authors found that hot spots of Vegf expression correlated with sites affected in del22q11 individuals.

The mouse work provided good evidence that VEGF was involved in DiGeorge syndrome, but the most direct evidence came from zebrafish. Stalmans, Lambrechts and colleagues showed that progressive knock-down of vegf expression, against a background of a constant dose of tbx1-specific morpholino, resulted in increasingly severe heart defects.

The evidence from animal models was strong, but the authors needed some evidence to confirm that VEGF was similarly important in humans with DiGeorge syndrome. They targeted single nucleotide polymorphisms (SNPs) in the promoter and 5′ untranslated region of VEGF that are known to downregulate expression. Genotyping 91 DiGeorge cases and 316 unrelated controls for these polymorphisms showed a crucial role for one allele (−1154A) in determining the susceptibility of DiGeorge patients to cardiovascular defects, but didn't rule out the involvement of other SNPs.

The case is now sound: VEGF — which maps outside the chromosome 22 region that is deleted in most del22q11 patients — is a modifier of birth defects in the syndrome. However, perhaps the most interesting aspect of the study is the potentially general approach it models. Rather than using a genome-wide scan to track down modifiers of the DiGeorge phenotype, the authors used the evidence from mouse and fish to generate a specific hypothesis that they went on to test in humans.

Although their association data remains to be confirmed in additional larger study populations, the success of the authors' combined genetic approach raises the possibility that perhaps we should be using a similar approach to disentangle the genetic influences on other complex diseases. Regardless, we should keep in mind that no matter how obvious the genetic culprit responsible for a specific disorder seems we should never discount the possibility of an accomplice on the outside!