Quantification of liver iron overload disease with laser ablation inductively coupled plasma mass spectrometry

Genetic iron storage diseases encompass a genetically heterogeneous group of disorders with strong environmental modifiers of disease expression [1]. Homozygozity for the c845G > A mutation within the HFE gene (OMIM: 613609) causing a substitution of the cysteine with a tyrosine residue in position 282 of the HFE gene product (p.Cys282Tyr) is, by far, the most frequent genetic form of iron overload. Autosomal recessive HFE-related iron overload is associated with typical clinical symptoms resulting from excess iron in several organs, mainly the liver representing the main storage site for iron. These include hepatomegaly, cirrhosis, hepatocellular carcinoma, diabetes mellitus, cardiomyopathy, hypogonadism, arthropathy, and increased skin pigmentation [2].

Genetic testing should be requested in patients with increased transferrin saturation, but results from population-based genetic screening studies have shown that the homozygosity for the p.Cys282Tyr polymorphism is associated with hemochromatosis in only 14% of individuals with this potentially disease-associated genotype. As hemochromatosis can also be caused by other mutation in the HFE gene or so called non-HFE hemochromatosis genes (HJV, HAMP, TFR2), homozygosity for p.Cys282Tyr is neither sufficient nor necessary for the diagnosis hemochromatosis. Accurate quantification of hepatic iron content by histological grading of stainable iron using Prussian blue or other dyes, direct chemical iron determination as well as magnetic resonance imaging (MRI) therefore remain essential diagnostic tools to detect and evaluate abnormal iron deposition in hereditary hemochromatosis (HH).

Genetic testing for common HFE variants associated with iron overload (C282Y, H63D) is a simple and economical procedure to confirm the diagnosis HH. A large variety of high-throughput methods for HFE genotyping are in daily routine use [3]. In addition, the repertoire of methods used for genetic diagnosis of HH contains other less commonly used methods such as PCR and reverse hybridization, direct sequencing, allele-specific PCR, PCR and high-resolution melting or single-stand conformational polymorphism, pyrosequencing and singe base extension [3]. Although classical liver biopsy with determination of hepatic iron concentration in fresh and paraffin-embedded tissue [4, 5] that has long been the gold standard for diagnosis of HH lost in importance, liver biopsy continues to have a very important diagnostic and prognostic implications in several hemochromatosis patients, especially in patients with suspected non-HFE hemochromatosis [6]. Such histochemical reactions are used to detect the presence of iron in biopsy specimens by forming insoluble complexes with appropriate dyes. Moreover, quantitative tissue iron determination and calculation of the hepatic iron index, defined as a quotient of micromoles or iron per gram of dry liver tissue and age of patient in years, was introduced to distinguish early hemochromatosis from alcoholic siderosis [7]. However, conventional tissue-ashing protocols, which rely on prolonged exposure to heat and caustic acids to achieve complete tissue decomposition are time-consuming, prone to sample loss and contamination, and potentially dangerous (e.g. release of hazardous fumes). Therefore, these procedures are for occupational medical reasons critical to adapt to routine clinical laboratories.

Most importantly, the subsequent determination of iron by atomic absorption spectrometry or chemically titration allows only the absolute quantitation of iron concentrations, but is not suitable to identify patterns, zonal gradients, or abnormal regional deposits or iron within the tissue. A histological hallmark of HH is that the accumulation and deposition or iron initially occurs in periportal hepatocytes (zone 1) in early phases of disease, while it extends to midzonal (zone 2) and centrilobular (zone 3) hepatocytes and biliary epithelium during progression of disease [8]. In contrast, transfusional iron overload, alcoholic siderosis or iron overload caused by SLC40A1 mutations are typically characterized by iron laden macrophages. Therefore, regional differences in hepatic iron concentrations are relevant providing information about the cause of iron overload, its stage and progression of the disease.

Recently, we established novel laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)-based protocols for hepatic metal bio-imaging. Here we demonstrate that these protocols are a significant asset in the diagnosis of iron overload allowing iron measurements with high sensitivity, spatial resolution, and quantification ability. These protocols will be valuable in estimating liver iron content in all forms iron imbalances resulting from genetic disorders (hemochromatosis, β-thalassemia, and aceruloplasminemia), iron malabsorption, internal chronic bleeding, excessive menstrual bleeding, and nutritional deficiencies resulting for example from strict vegetarian food consumption.

Previously, we have optimized laser ablation inductively coupled mass spectrometry (LA-ICP-MS)-based methods for trace metal imaging in liver sections [14]. These protocols have multi-element capability suitable to simultaneously measure and quantify a large variety of different metals and metalloids within the tissue with high spatial resolution. In brief, a focused laser beam ablates a small quantity of biopsy material, and the aerosol produced is transported in an inert carrier gas stream to an ICP-MS, where it is then atomized and ionized. Subsequently, the different ions are separated according to their mass-to-charge ratio and quantified [15].

As demonstrated in our study, the LA-ICP-MS technique allows precise measurement and visualization of iron concentrations in high resolution. This is a great advantage when compared to other standard techniques. Quantitative measurements by spectrophotometry or semi-quantitative iron determinations by histology suffer from two limitations. The within-organ standard deviation (SD) of hepatic iron concentrations (HIC) can vary widely. Moreover, HIC values determined in microtome samples and biopsy-sized samples can have large coefficients of variations (CVs) reaching values of up to 71% in patients with end-stage cirrhosis [18]. It is also reasonable to speculate that histological stains are only semi-quantitatively because these are artificially lowered by removal of iron from the liver tissue during the fixing, washing and staining steps in the histochemical procedure. Likewise, the chemical determination of total iron in percutaneously obtained liver biopsy from patients with suspected primary iron overload identified by colorimetric analysis, flame atomic absorption and flameless atomic absorption spectrophotometry by a graphite furnace method revealed CVs ranging from 11 to 19% resulting from sample variation due to inhomogeneous distribution of iron through the liver [19]. Moreover, quantification of liver iron with MRI, computed tomography (CT), magnetic resonance spectroscopy (MRS), liver susceptometry, and relaxometry are partially limited. Although they are rapid, non-invasive, and cost effective techniques limiting the use of liver biopsy in assessment of liver iron content, these methods have some analytical drawbacks [20, 21]. In particular, the occurrence of concomitant fat, inflammation and fibrosis within the liver corrupts the ability of gradient echo methods, often requiring the correlation with chemically determined liver iron concentration for establishment of empirical calibration curves. This provides a challenge in current MRI measurements of precise liver iron concentration in correcting for the transverse rate R2 (= 1/T₂) and the faster and more sensitive R2* (= 1/T₂*) cellular interference, including fibrosis, fat, inflammation, and other histologic changes in hepatic cellularity that are associated with tissue damage resulting from iron overload [22].

In addition, iron-overloaded livers can show iron heterogeneity over spatial scales spanning three orders of magnitude in regard to intracellular, intercellular and zonal compartments. These circumstances affect relaxation times during measurements, thereby introducing unavoidable errors [20].

In the last decade, several efforts were made to overcome these potential limitations in MRI and CT. Exemplarily, a new R2-MRI imaging technology termed “FerriScan” was introduced some years ago. This technology allows accurate measurement of liver iron concentration with high sensitivity and specificity and is unaffected by hepatic fat content, inflammation, fibrosis or cirrhosis [23, 24].

LA-ICP-MS imaging is a sophisticated tool for investigating the regional and spatial distribution of metals with high sensitivity, capability, and relatively good lateral resolution at micrometer resolution [25]. Therefore, this technology is becoming an essential tool in diverse biological research fields, and of course in clinical applications. We here have demonstrated that LA-ICP-MS is highly suitable to measure and localize iron concentrations of deposits in liver samples. Therefore, this technique might be relevant in the more precise determination of hepatic iron status, localization of iron deposits, or in HIC monitoring during HH therapy.

Presently, therapeutic phlebotomy is the standard clinical practice in the therapy of HH [26]. However, there is no evidence base on which to direct the optimal endpoint of this therapy. Actually, most clinicians attempt to achieve a target of serum ferritin lesser than 50 μg/L. However, this value does not necessarily correlate with hepatic iron content. Possibly, the direct measurement of iron concentration and distribution in liver biopsy with LA-ICP-MS will be more suitable to mark a therapeutic endpoint in the treatment of HH patients.

The LA-ICP-MS protocols used in our study will be potentially relevant not only in estimating the degree of iron overload in hemochromatosis patient. There is a large set of other genetic or acquired disorders that lead to strong imbalances in iron homeostasis. Beside hemochromatosis, system iron overload syndromes can have many other genetic or acquired origins including hereditary aceruloplasminemia, dyserythropoiesis, different forms of β-thalassemia, and several other conditions requiring multiple transfusions including myelodysplasia or hematopoietic stem cell transplantation resulting in iatrogenic iron overload [27–29]. Likewise, monosyndromic or polysyndromic sideroblastic anemias are known to develop both compartimental iron excess and systemic iron overload [27]. On the other side, iron malabsorption, internal chronic gastrointestinal bleeding, excessive menstrual bleeding, pregnancy, parasitic infection, and nutritional deficiencies resulting for example from strict vegetarian food consumption can provoke iron shortcomings in body’s iron homeostasis [30, 31] that might be also reflected in lowered liver iron content.