The researchers then tested the hypothesis that Kabuki syndrome is more genetically heterogeneous than originally assumed, and that multiple genes could potentially cause the disorder. Looking for novel variants in genes that were shared among subsets of the 10 patients' exomes, they found novel, matching variants that were shared in three genes in nine of the patient's exomes, shared in six genes in at least eight exomes, and shared in 16 genes among seven exomes.
With no obvious way to rank these gene candidates, the researchers categorized each Kabuki case based on a subjective assessment of how well the patient matched the characteristics of Kabuki syndrome. They also looked in particular for variants in genes that led to a loss of function. The combined analysis pointed to gene called MLL2.
The researchers found novel variants which lead to a loss of function in the MLL2 gene in the four highest ranked cases, and in three of the remaining six cases. The variants were nonsense and frameshift mutations. The nonsense mutation substituted a single nucleotide (a molecule that links to form DNA) in the gene code, while the frameshift mutation resulted in a four nucleotide deletion in the gene.
In these cases, each nonsense and frameshift mutation resulted in the production of a shortened, nonfunctional protein. The MLL2 gene normally encodes a protein important in the regulation of chromatin, a protein that helps package the DNA in a compact form and allows the chromosome to fit in the cell nucleus. Changes in chromatin structure are associated with DNA replication and turning genes on or off.
Once the researchers suspected the MLL2 gene, follow-up sequencing using the traditional and highly accurate Sanger DNA sequencing method, more capable of reliably detecting frameshifts, was used to identify
|Contact: Omar McCrimmon|
NIH/National Human Genome Research Institute