Commentary

Don’t forget the chromosomes


 

The past year has seen tremendous media coverage surrounding genetic testing, sparking conversations regarding who should consider genetic testing – and with which genetic test – to determine if they have an increased risk for a particular condition such as cancer ("The Jolie effect on BRCA risks," Internal Medicine News, July 2013, p. 13).

Added to this is the recent Supreme Court ruling that overturned some of Myriad Genetics’ patents on BRCA1 and BRCA2 testing, arguably the best known hereditary cancer genes by patients and physicians ("Keep your patents off my genes!" Internal Medicine News, Oct. 1, 2013, p. 18). Many laboratories have jumped into the cancer testing arena, offering their version of genetic testing panels that examine a variety of genes based on cancer type. Some of these panels test for more than 50 genes related to hereditary cancers – and they go well beyond BRCA1 and BRCA2 testing, using next-generation sequencing (NGS) technology.

The emergence, promise, and application of NSG in cancer genetics have been discussed from numerous angles in this column, but it’s important not to forget chromosomal disorders.

Approximately 1 in 300 live births will carry chromosome abnormalities, and as medical care improves for these conditions, primary care physicians will see more children, such as those with Down syndrome, "graduate" from their pediatricians and require care from primary care physicians. New insight into adult-onset health issues (e.g., early-onset Alzheimer’s disease in Down syndrome) is emerging, but our understanding of the pathology and mechanism of disease remains a challenge. Although animal models exist, it has been a major challenge to recapitulate the effects of having an "extra copy" of an entire chromosome in order to study the biological effects, let alone develop a strategy to silence an entire extra chromosome in order to provide a potential therapy.

Exciting research by Jeanne Lawrence, Ph.D., and her colleagues was presented at the recent American Society of Human Genetics meeting in Boston regarding trisomy 21 (Down syndrome), and it caught my attention despite the onslaught of NSG data presented related to single-gene disorders. By using genome editing, they were able to silence the extra copy of chromosome 21 in Down syndrome pluripotent stem cells by taking advantage of the known genetic processes that silence X chromosomes.

In a normal female cell, there are two copies of the X chromosome; however, only one copy is active, because the other copy is "silenced" through a process called X-inactivation or Lyonization. This is driven by the X-inactivation gene (XIST), which produces a noncoding RNA that covers the entire X chromosome and essentially silences it – making it inactive and condensing it into what is termed a Barr body.

The investigators set out to determine if they could insert a copy of the XIST gene into the extra chromosome 21 – at a very specific location so as not to disrupt any known functional genes – and thereby silence the extra chromosome without affecting other chromosomes. After preliminary success in other cell lines, they attempted this in pluripotent stem cells from a Down syndrome patient, so that differentiation and different tissue subtypes could be studied.

Remarkably, Dr. Lawrence and her colleagues were able to accomplish this integration and overcome two major obstacles: the challenge of working with pluripotent stem cells, and inserting such a large amount of DNA (the XIST gene) via genomic editing. They demonstrated their successful silencing of the extra chromosome 21 through a variety of molecular, cellular, cytological, and genomic assays. Added to the elegance of their work, the strategy they used to insert the XIST gene created an inducible system – meaning they could turn on or off the chromosome 21 silencing based on whether the cells were exposed to doxycycline.

In essence, the investigators created a model to study human chromosome inactivation, which opens the doors to understanding the pathology and molecular pathways involved in chromosome abnormalities such as Down syndrome. This is the potential first step toward developing targeted "chromosomal therapies," something that was not thought to be feasible given the difficulty surrounding gene therapy for single-gene disorders.

While there have been many exciting advances in single-gene disorders and NGS technology, it is refreshing to see exciting research advance in other areas of genetics. I encourage readers to explore this groundbreaking research further.

Dr. Hulick is a medical geneticist at NorthShore University HealthSystem, Evanston, Ill., and a clinician educator at the University of Chicago. He reported having no conflicts of interest.

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