Dental DNA, Tooth Sampling and Molecular Identification

Tooth enamel is approximately 96% mineral by weight (hydroxyapatite crystals in an organic matrix) and ranks around 5 on the Mohs hardness scale. Dentine, which makes up the bulk of the tooth crown and root, is softer (around 3-4 Mohs) but still substantially harder than cortical bone. Together, these tissues create a closed mineral chamber around the pulp cavity that physically blocks most of the environmental insults that destroy DNA in other tissues.

The protective mechanism operates at several levels. Against heat, the mineral shell acts as an insulator: in a house fire, the temperature inside an intact tooth remains below the DNA denaturation point (roughly 90-100°C) even when the external temperature exceeds 500-700°C, until the enamel itself cracks. Against water, the tight mineral matrix limits diffusion of water and the nucleases carried in it. Against microorganisms, the sealed pulp chamber is not accessible to bacteria until the tooth fractures, cracks at the gum line, or has existing caries providing an entry point.

This combination of hard mineral outer layers and enclosed biological contents is the reason that studies in ancient DNA have recovered amplifiable sequence from teeth tens of thousands of years old. For forensic purposes the relevant range is more modest (days to decades), but the principle is the same: a tooth recovered from a fire scene, a body recovered from water, or a set of skeletal remains found years after burial will often yield DNA from an intact tooth when every other tissue has failed.

Not all teeth offer the same DNA yield, and in a partial or degraded skeleton, the correct selection can determine whether a profile is obtained at all. The guiding principle is to maximise the volume of protected biological material while avoiding teeth that are compromised by caries, restorations, or root canal treatment.

• Canines: preferred first choice in most protocols. The large single root with a broad apex, the thick layer of dentine, and the absence of caries in many adults makes them the highest-yield tooth for both pulp and dentinal DNA. The upper canine has the longest root of any tooth in the permanent dentition.
• Second molars: multi-rooted with a large pulp volume, making them good backup candidates. Their position at the back of the jaw means they are often the last teeth to be lost in decomposition or skeletonisation.
• Premolars: intermediate choice; single-rooted (usually) with moderate pulp volume. Selected when canines are absent, heavily restored, or carious.
• Incisors: thinner root and smaller pulp, last choice among intact teeth. Acceptable when other options are unavailable.
• Avoid: teeth with obvious caries extending to the pulp, previously root-filled teeth (pulp absent or dead), cracked or fractured roots, or teeth with visible periapical pathology. These yield little or no viable DNA.

Three main approaches are used in forensic dental DNA extraction, and they map roughly to three categories of sample condition. The goal in all three is to maximise DNA yield while minimising degradation from heat, oxidation, or handling.

1. Pulp extraction (fresh to moist teeth)
   The tooth is surface-decontaminated (UV irradiation, bleach wash, or both), then cut at the cementoenamel junction using a low-speed diamond saw under constant water cooling. The root is opened longitudinally and the pulp tissue is scooped or dissolved out using a standard soft-tissue lysis buffer. This yields intact nucleated cells with high-molecular-weight nuclear DNA suitable for full STR profiling. It works best when the tooth is less than 6-12 months post-mortem in temperate conditions or when the body has been refrigerated.
2. Cryogenic grinding (moderately to severely degraded teeth)
   The whole tooth or crown section is submerged in liquid nitrogen for 2-5 minutes until fully frozen, then transferred to a steel cryogenic mill and pulverised for 30-60 seconds. The powder is removed and placed directly into extraction buffer. The liquid nitrogen freeze prevents frictional heat during the high-speed grinding, which would fragment already-short DNA. The powder presents both dentinal tubule material and pulp remnants in a fine dispersion that maximises buffer contact and extraction efficiency.
3. Cementum sampling (ancient, charred, or extremely degraded teeth)
   The root surface is mechanically scraped to remove cementum, which is then processed using demineralisation protocols similar to those used for ancient DNA. Because cementum binds DNA from extracellular sources and retains it within its mineral matrix, it can yield amplifiable sequence even when pulp is carbonised or absent. Low-copy-number methods or mtDNA profiling are typically used when cementum is the only substrate.

Each human somatic cell contains two copies of nuclear DNA but hundreds to thousands of mitochondrial genomes. This copy-number advantage means that in conditions of severe nuclear DNA degradation, there may still be enough mitochondrial sequence to amplify and type. Teeth are an especially good source because the odontoblast processes in dentinal tubules, though lacking nuclei in mature tissue, retain mitochondria with intact mtDNA.

The hypervariable regions (HV1 and HV2) of the mitochondrial control region are the targets most commonly sequenced in forensic casework. These regions vary significantly between unrelated individuals but are shared by all maternal-line relatives. A profile from a degraded tooth can be compared against a maternal-line reference sample from a known relative (mother, maternal sibling, maternal aunt) even when no direct ante-mortem sample from the victim exists.

Bone is the most common substrate for DNA identification in skeletonised remains because it is abundant and femoral cortical sampling is well-established in most forensic DNA laboratories. There are, however, specific conditions under which intact teeth reliably produce a better result than available bone.

• Fire and heat exposure: bones in a fire first crack and then calcine, causing progressive DNA loss through denaturation, strand breakage, and oxidative damage. The enamel-dentine shell protects tooth pulp from this damage longer than periosteal bone is protected. Published case series document tooth-derived STR profiles from skeletons where all available bone failed.
• Waterlogged remains: prolonged immersion in water, especially warm or flowing water, accelerates bone DNA degradation through hydrolysis and nuclease action from aquatic microorganisms. Intact teeth with sealed pulp chambers resist this better, particularly if the water is cold.
• Long-interval burials (10+ years): in acidic soils and warm climates, bone can be completely demineralised and DNA destroyed within a decade. Tooth enamel is more resistant to acidic hydrolysis than bone hydroxyapatite, allowing teeth to survive intact in conditions that have destroyed all bone structure.
• Mass disaster fragmentation: in aircraft accidents, explosions, or mass grave disarticulation, body parts may be severely fragmented and mixed. A tooth fragment can be a complete sampling unit even at 1 cm of root. A fragment of cortical bone of the same size may not contain enough DNA for a full profile.

The sensitivity that makes dental DNA so useful in degraded samples makes it equally sensitive to contamination. Touch DNA from the recovery team, DNA from other bodies in a mass grave, or laboratory contamination from staff or previous extractions can all produce a false profile from a well-preserved tooth. The contamination risk does not diminish just because the sample is from a tooth; it increases because the same methods used to extract small quantities of endogenous DNA are just as efficient at amplifying exogenous contamination.

• All personnel handling unprocessed teeth should wear double nitrile gloves, face masks, and full gown. No ungloved contact with any tooth surface.
• Surface decontamination of the tooth before extraction (5% bleach wash followed by UV irradiation) removes exogenous DNA from the tooth surface without penetrating the mineral interior where endogenous DNA is protected.
• All recovery and laboratory personnel who handled the sample should be on an elimination DNA database so that any contamination contribution can be identified and excluded.
• Chain of custody from tooth extraction at the scene to laboratory receipt must be documented continuously: who handled it, when, and what procedures were performed. This documentation is required for court admissibility.

In large-scale disaster victim identification operations, a portion of extracted material is retained as an archival sample so that if the initial profile is inconclusive or challenged, re-analysis is possible. Destructive sampling of the only available tooth without archiving is regarded as poor practice by most national laboratory standards organisations.