Under the Microscope – Brittney Chilton

In her presentation at ISHI, Brittney Chilton (Criminalist II at the Washoe County Sheriff’s Office Forensic Science Division) will share the story of Washoe County employee, Chris Long. In 2014, Chris was diagnosed with AML/MDS Leukemia. He underwent a stem cell/bone marrow transplant and volunteered to participate in a case study led by the Washoe County Sheriff’s Office Forensic Science Division. This case study is ongoing and monitors the fluctuation of DNA profiles from both the recipient and donor in various bodily fluids and tissues.


We sat down with Brittney to discuss the challenges that chimerism poses for forensic laboratories and any surprises that came up during the study.



Can you explain what a chimera is for those who may not be familiar with the term?

A Chimera is an organism in which cell populations’ house entirely different genomes.


How might a person become a chimera? Is it always permanent?

This can occur naturally and artificially and is both permanent and non-permanent. It can occur naturally and be permanent with fraternal twins in utero, when one twin absorbs the other during early embryonic development. One twin is born but both distinct genomes can be observed within the living twin. An example in which chimerism can be either permanent or non-permanent naturally, is fetal maternal transfusion. During pregnancy fetal cells can migrate into the mother via the placenta. This can be temporary during the pregnancy and seen in the blood. However studies have shown that these cells can integrate into the maternal organs and tissues and these genomes have been observed decades later.

A non-permanent example of artificially creating cherism is a blood transfusion. One individual is given blood from a second source. Both genomes would be present within the recipient but only as long as the life span of the donated blood cells. Chimerism can also be caused artificially and permanently by stem cell/bone marrow transplants. One individual is given blood stem cells from another individual. Those stem cells are accepted by the new host, but continue to create blood cells with the original donor’s genetic makeup. In this scenario, the recipient’s genetic profile in the blood will permanently be different from the other bodily tissues and fluids.


What challenges does chimerism pose for forensic laboratories? Are there ways for a lab to know that a sample has come from a chimera?

Chimerism although rare, poses challenges for forensic laboratories. Forensic DNA analysis and the utilization of DNA databases such as CODIS, are reliable based on the fact that, other than identical twins, no two people have the same DNA profile. However, a chimeric individual has two or more distinct DNA profiles. Biological evidence left behind at a crime scene may therefore differ from the source reference standard utilized to compare to the evidence. This could result in an incorrect exclusion/inclusion of an individual as a source of the evidence. Furthermore, this study shows that chimerism creates mixtures in biological tissues and fluids from a single source. This could lead to inconclusive mixture deconvolution, an incorrect lead for authorities, and pose implications for CODIS eligibility.


How did this case study come to fruition? How did you become involved?

The lab director and Chris worked together on several IT projects. When he shared about his illness and that he was going to have a stem cell replacement (bone marrow transplant).  She asked if the lab could monitor his DNA as he went through the process. We have continued to monitor Chris’ DNA since.


Can you briefly describe methods of the case study?

Blood, hair, semen, and buccal swabs were collected from the patient over a four year period. The samples were extracted using the Qiagen DNA Blood Mini Kit, quanted with PlexorHY on 7500s, amplified with both Powerplex16HS and Globalfiler on Eppendorff Mastercyler Pro S and Mastercycler epGradient S, and analyzed on 3130’s and Genemapper ID and IDX.


Were there any surprises that came up throughout the study?

The DNA results obtained from the semen samples were the most surprising. Blood stem cells should only be able to differentiate into blood cells. Previous literature has shown the donor profile in buccal swabs and so we expected to see it there. It’s hypothesized that injury to the mouth, or white blood cells in saliva could be the reason for the donor profile in buccal swab samples. Finding the donor profile in the recipient semen was a huge surprise. Even more so was observing that the amount of the donor profile fluctuated significantly over time. While chimerism is permanent, we learned through this sample that it is also fluid.


What has been your biggest takeaway from this experience? Do you have any advice for other laboratories who may process chimeric samples?

My biggest take away as a DNA analyst, is that we often get wrapped up in the certainty. We are trained to handle matches, no matches, and no conclusions. When we observe a match, we apply statistics to give a weight to the match and we move on. It’s auto pilot. Here, these possibilities don’t apply. Chimerism presents an opportunity for curiosity, it allows us to think beyond the normal and entertain the “what if” (which we usually don’t get within the means of our validated protocols). While striving to streamline and create efficient laboratories, chimerism shows me that we still have to be present in what we are doing. No computer system is going to be able to look past an association or exclusion and give pause. And question, “does this result fit?” Chimerism shows me we still need the human analyst, and we need to be open to those “what ifs”.

Our advice to other laboratories on the topic of chimerism is to not be too critical of the self. Our knee jerk reaction to an unexpected mixture in a sample is usually contamination. The rabbit hole of trying to trace the contamination likely follows. Knowing that chimerism is a possibility opens the idea that maybe the sample actually IS a mixture. Requesting additional reference samples and or sources of the reference sample is our biggest suggestion. How one would obtain “certain” bodily fluids as a reference standard is a problem we suggest leaving up to the detectives and lawyers!


What tips would you give to someone who is just starting out in the field of forensics, or what is the best advice that you’ve received?

I would tell a new analyst that nothing is permanent. Change in this field is to be expected, and change for people who are by nature really good at repetition can be uncomfortable. Don’t become too attached to anything in this field. You will surely have to let it go to make room for what better serves the progress.


As we celebrate the 30th anniversary of ISHI, do you have any predictions for what the future holds or do you have any fond memories of using older technologies/techniques?

I predict we will move faster, with less effort, and obtain massive amounts of genetic information that may or may not be necessary. I just wonder and worry as to what extent we will continue to push the bar. How much deeper do we really need to dive into the DNA to be able to discern individuals? What secrets might we find and how might those be misused? I worry pushing the limit might bring us more than we bargained for.


What’s one thing that others may not know about you?

Brittney : I have a huge love for food, yoga and travel. I started a blog called the “Chubby Yogi” and I write about my journey to true north which includes food, time on the mat, and life experience. Namaste.

Chris: I have a newfound freedom to not sweat the small stuff. I volunteer in my church. I love the outdoors and take time to camp and travel the back roads in my UTV. I am getting married this year and I will be going to Germany next year to meet my donor in person.