High-throughput targeted paleoproteomics sex estimation on medieval Great Moravia individuals using MALDI-CASI-FTICR mass spectrometry

This study introduces and validates a novel high-throughput targeted paleoproteomics method using MALDI-CASI-FTICR mass spectrometry for rapid and accurate biological sex estimation, successfully applying it to 130 medieval Great Moravia individuals to improve efficiency and correct previous osteomorphological errors.

The Gist

Imagine you are a detective trying to solve a mystery from 1,000 years ago. You have a pile of old bones and teeth from a medieval village, and you need to know: Who was a man, and who was a woman?

For decades, scientists have tried to answer this by looking at the shape of the bones (like the pelvis or skull). But this is like trying to guess someone's height by looking at a blurry photo—it often fails, especially with children whose bones haven't fully grown into their adult shapes yet.

This paper introduces a new, super-powered detective tool that uses tooth enamel and mass spectrometry (a fancy machine that weighs molecules) to solve the mystery quickly and accurately.

Here is the story of how they did it, explained in simple terms:

1. The "Sexual ID Card" Inside Your Teeth

Every human has a protein called Amelogenin in their teeth. Think of this protein as a biological ID card.

• Females have a version of this card from their X chromosome (AMELX).
• Males have both the X version (AMELX) and a slightly different Y version (AMELY).

The problem is that these proteins are tiny and hidden inside the hard shell of the tooth. In the past, scientists had to dissolve the tooth in acid and run it through a long, slow machine (like a liquid chromatography column) to find them. It was like trying to find a specific needle in a haystack by slowly sifting through the hay one grain at a time.

2. The New "Speed Trap" Method

The researchers in this paper invented a new way to catch these proteins. They call it MALDI-CASI-FTICR. That's a mouthful, so let's break it down with an analogy:

• The Old Way (LC-HRMS): Imagine a long, winding highway where cars (molecules) have to drive one by one through a toll booth. It takes a long time, and you can only check a few cars an hour.
• The New Way (MALDI-CASI-FTICR): Imagine a high-speed camera at a racetrack. Instead of making the cars stop, you take a snapshot of the whole track in a split second. The machine uses a laser to zap the tooth dust, turning the proteins into a cloud of ions, and then uses a super-strong magnet (the FTICR part) to sort them by weight instantly.

The "CASI" trick: This is the secret sauce. Usually, when you zap a sample, you get a messy cloud of thousands of different proteins. The CASI method acts like a VIP bouncer at a club. It only lets the two specific "VIPs" (the X and Y proteins) into the VIP lounge to be weighed, ignoring all the other noise. This makes the signal incredibly clear.

3. The "Gold Standard" Check

To make sure they aren't making mistakes, the scientists added a "heavy" version of the protein to every sample. Think of this like adding a gold coin to a bag of silver coins.

• If the machine sees the gold coin, it knows the bag is working correctly.
• By comparing the weight of the "light" ancient protein to the "heavy" gold coin they added, they can calculate exactly how much protein was there. This prevents false alarms.

4. The Great Moravia Test

They tested this new method on 130 people from medieval Great Moravia (in today's Czech Republic).

• The Speed: They could process samples in under a minute each. That's like checking 1,400 people in a single day!
• The Accuracy: They got the right answer for 129 out of 130 people.
• The Surprise: In three cases, the old "bone-shape" method said the person was female, but the teeth said male (and vice versa). The new method proved the teeth were right.
• The Kids: They successfully identified the sex of children, something the old bone method couldn't do at all.

Why This Matters

This isn't just about sorting bones; it's about rewriting history.

• For Archaeologists: They can now look at entire villages and say, "Here is the ratio of men to women," or "Did women and children get buried differently?" without guessing.
• For Forensics: If a body is found that is too damaged to tell by looking at the bones, this method can still tell you if it was a man or a woman.
• For Efficiency: It turns a process that used to take days into a process that takes minutes, allowing scientists to study thousands of people instead of just a lucky few.

The Bottom Line

The researchers built a high-speed, ultra-precise scanner that reads the "sex code" hidden in our teeth. It's faster, cheaper, and more accurate than looking at bones, and it works even on children. It's like upgrading from reading a blurry map to using a GPS that tells you exactly where everyone is.

Technical Summary

1. Problem Statement

Estimating the biological sex of archaeological and forensic remains is a fundamental step in bioarchaeology and forensic identification. However, traditional osteomorphological methods (analyzing skeletal features) face significant limitations:

• Reliability: They are highly sensitive to population variation, taphonomic damage, and pathological conditions.
• Subadults: They are generally ineffective for fetal, infant, and juvenile remains because sexually dimorphic skeletal traits are not fully developed before puberty.
• Throughput: Existing molecular methods (like DNA analysis) are often destructive, time-consuming, or expensive for large-scale studies.
• Current Proteomics Limitations: While proteomic analysis of amelogenin (AMELX/AMELY) peptides from tooth enamel is a promising minimally invasive alternative, current high-throughput Liquid Chromatography-High Resolution Mass Spectrometry (LC-HRMS) methods still require significant processing time (minutes to hours per sample) and can be sensitive to instrumental variations.

The authors aimed to develop a high-throughput, targeted, and minimally invasive proteomic method capable of processing large archaeological assemblages rapidly while maintaining high accuracy and resolving errors inherent in morphological assessments.

2. Methodology

The study introduces a novel workflow combining Matrix-Assisted Laser Desorption/Ionization (MALDI) with Continuous Accumulation of Selected Ions (CASI) and Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometry.

A. Sample Preparation

• Source: Tooth enamel was used due to its superior protein preservation compared to DNA.
• Extraction: A minimally invasive acid etching protocol was used (0.6 N HCl, 1 hour at room temperature) to extract peptides from the enamel surface.
• Purification: Peptides were purified using C18 96-well plates.
• Internal Standards: To ensure quantitative accuracy and correct for instrumental variations, synthetic isotope-labeled peptides (AQUA™ Peptides) were added to every sample. These heavy peptides (AMELX2H and AMELYH) have a known mass shift (+10 Da for Arginine, +16 Da for Methionine oxidation) relative to the native "light" peptides.

B. Mass Spectrometry Analysis

• Instrument: A 9.4 Tesla Bruker SolariX XR FTICR mass spectrometer.
• Technique: MALDI-CASI-FTICR. Unlike standard MALDI which detects all ions, CASI allows for the pre-selection and accumulation of specific ions of interest before detection.
• Targeted Peptides: The method specifically targets five peptides:
  1. AMELX1 (m/z 829.4566)
  2. AMELYL (m/z 879.4393)
  3. AMELYH (Heavy standard, m/z 889.4491)
  4. AMELX2L (m/z 1079.5520)
  5. AMELX2H (Heavy standard, m/z 1089.5621)
• Acquisition Time: Less than 1 minute per sample.

C. Data Analysis & Sex Determination

• Quantification: Biological sex was determined by calculating the Log2 intensity of the native AMELX and AMELY peptides.
• Thresholds:
  • Male: Presence of AMELY signal (Log2 AMELY > 16–17) and high AMELX.
  • Female: Presence of AMELX signal only (Log2 AMELY < 16–17).
  • Indeterminate: Samples where the ratio of Heavy/Light peptides indicated signal degradation or insufficient intensity (e.g., Ratio 889/879 > 10 or 1089/1079 > 120).
• Validation: The method was first validated on 28 modern teeth (14 male, 14 female) with known sex.

3. Key Contributions

1. Novel High-Throughput Workflow: This is the first study to apply MALDI-CASI-FTICR for targeted paleoproteomic sex estimation. It eliminates the need for liquid chromatography (LC), drastically reducing analysis time.
2. Ultra-High Resolution: The FTICR instrument provides a resolution of 1,000,000 at m/z 200, allowing for precise distinction between target peptides and background noise/contaminants without extensive purification.
3. Internal Quantification Strategy: By using isotope-labeled internal standards (AQUA™), the method corrects for instrumental drift and matrix effects, offering higher reliability than external calibration methods used in previous studies.
4. Scalability: The authors estimate a throughput of up to 1,440 samples per day, making it feasible to analyze entire archaeological populations rather than just select specimens.

4. Results

• Modern Validation: The method achieved 100% specificity on the 28 modern individuals (14 males, 14 females). Linear regression showed strong correlations between AMELX and AMELY intensities in males.
• Archaeological Application: The method was applied to 130 individuals from two Early Medieval Great Moravian sites (Mikulčice and Rajhrad, 9th–10th centuries).
  • Success Rate: Sex was successfully estimated for 129 out of 130 individuals (99.2%).
  • Breakdown: 60 females, 69 males, and 1 indeterminate case.
• Correction of Morphological Errors:
  • Adults (Mikulčice): The proteomic results contradicted previous osteomorphological assessments for 3 out of 30 individuals (Grave numbers H392, H493, H1082), correctly reassigning their sex.
  • Juveniles (Rajhrad): The method successfully sexed 100 non-adult individuals (48 males, 51 females), a demographic where morphological methods are typically impossible. It confirmed the accuracy of 11 out of 13 metric-based sex assessments previously performed by Pospíšilová.

5. Significance and Implications

• Demographic Reconstruction: The ability to reliably sex juvenile remains allows for more accurate reconstructions of past population structures, mortality patterns, and sex-specific burial practices, which were previously inaccessible.
• Forensic and Anthropological Utility: The method offers a rapid, minimally destructive alternative for forensic identification and resolves ambiguities in skeletal remains where the pelvis or skull is missing or damaged.
• Cost-Benefit Trade-off: While the method is highly efficient and accurate, the authors note a limitation: the high cost of synthetic isotope-labeled internal standards (approx. €500/peptide) compared to external standards. However, this cost is offset by the massive reduction in analysis time and the increased confidence in results.
• Future Directions: The study highlights the need to expand cohorts to refine false-positive/negative rates and to account for biological variations (e.g., Y-chromosome deletions or Klinefelter syndrome) that could affect proteomic interpretation.

In conclusion, this paper presents a robust, high-throughput technological leap in paleoproteomics, enabling the rapid and accurate biological sex estimation of large archaeological assemblages, including subadults, thereby transforming the scale and precision of bioarchaeological research.