Human alpha-1 antitrypsin (AAT), also known as alpha1 proteinase inhibitor (α1-Pi) and SERPINA1 (Serine Protease Inhibitor, group A, member 1), is a circulating glycoprotein whose main function is to inhibit neutrophil elastase and other serine proteases in blood and tissues. The AAT gene has two alleles, which are transmitted from parents to their children by autosomal co-dominant Mendelian inheritance. Normal alleles, present in 85-90% of individuals, are denominated Pi (protease inhibitor) M. Thus, a normal individual has a Pi*MM genotype. The most prevalent deficiency alleles are denominated S and Z, and their prevalence in Caucasian populations ranges from 5-10% and 1-3%, respectively. Consequently, the vast majority of genotypes result from combinations of Pi*M, Pi*S and Pi*Z. The normal genotype, Pi*MM, is present in about of 85-95% of people and fully expresses AAT; Pi*MS, Pi*SS, Pi*MZ, Pi*SZ and Pi*ZZ are deficiency genotypes that are present in the other 5-15%, expressing approximately 80, 60, 55, 40 and 15% of AAT, respectively [1].

Severe AAT deficiency, defined as an AAT serum level less than 35% of the mean expected value, 50 mg/dL (measured by nephelometry), 11 μM, or 80 mg/dL (measured by radial immunodifusion, although this is now an obsolete technique), is usually associated with Pi*ZZ genotypes, and less frequently with combinations of Z, S, and about 45 “rare” or null alleles. Both Pi*S and Pi*Z, and the rare deficiency alleles MMalton, MDuarte, and SIiyama produce misfolded proteins that are retained in polymer-forming hepatocytes. These can cause not only cell stress and liver damage, but also, as a result of polymerization and retention in hepatocytes, blood and tissue concentrations of AAT that are too low to provide sufficient protection for tissues against the action of proteinases [2].

AAT deficiency is a hereditary condition that typically predisposes to premature onset of chronic obstructive pulmonary disease (COPD), liver cirrhosis, relapsing panniculitis, systemic vasculitis, and possibly a range of inflammatory and neoplastic diseases [1, 3]. In addition, several clinical studies have shown that subjects with AAT deficiency have an increased risk of developing malignancies, including hepatocellular carcinomas [4, 5], lung cancer [6–9], neoplasms of the urinary bladder [10] and gallbladder [11], malignant lymphomas [12], and colon cancers [13, 14].

The aim of the present study was to investigate whether AAT deficiency was more common in patients with CRC than in healthy subjects from Asturias, a northern (Cantabrian) coastal region of Spain, with one of the highest prevalences of AAT deficiency in Europe [15, 16], and a high incidence of CRC [17].

The only statistically significant finding of the present study was the markedly higher AAT serum concentrations in CRC patients than in healthy controls, regardless of whether their Pi phenotype was normal (Pi*MM) or deficient (Pi*MS, Pi*MZ or Pi*SZ).

The presence of high serum levels of AAT in patients with CRC was reported more than 35 years ago, and has even been linked to distant metastases [21]. Subsequently, other authors have found that serum AAT levels are associated with the clinical stage of the disease [22, 23]. In these pioneering studies, the correlation of serum CEA and serum AAT with the stage of disease were of a very similar level of statistical significance (p = 0.004 and 0.003, respectively). Coinciding with these preliminary results, a more recent study confirmed that serum levels of AAT are higher in CRC subjects than in controls, and that these high levels of serum AAT are directly correlated with the stage of CRC, making it a useful marker for distinguishing between early and advanced stages of this malignancy [24]. However, given the necessarily strict criteria, we cannot yet be certain whether this biomarker is also altered in patients with other inflammatory or neoplastic diseases.

Apart from CRC, various authors have found significantly elevated AAT serum levels in subjects with a range of cancers, including lung [25–30], liver [31, 32], pancreas [33], prostate [34], cervix [35], ovary [36, 37], breast [38], Hodgkin’s lymphoma [39], larynx and other head and neck carcinomas [40, 41]. The data provided by these studies taken together suggest that the presence of elevated serum levels of AAT in patients with any of these types of carcinomas is related to an invasive growth of these tumors. However, the low statistical power of the analyses that is the consequence of the small sample sizes means that the true value of this biomarker in the diagnosis and staging of cancers remains to be established.

On the other hand, AAT has been detected in histological sections of paraffin-embedded biopsy specimens obtained by endoscopy or surgically resected CRC samples, with a markedly higher incidence in advanced than in early carcinomas. These findings suggest a local production of AAT by CRC cells that tends to be associated with a more aggressive tumor behavior, more intense local growth and an increased tendency to metastasize to distant organs [42]. However, AAT overexpression in cancer tissues is not an exclusive feature of CRC, since it has also been found in other types of cancers in different organs, including lung carcinomas [43], hepatocellular carcinomas [44], adenocarcinomas of the stomach [45, 46], myeloid leukemia cells [47, 48], brain tumors [49], carcinoid tumors, malignant melanomas, and schwannomas [50]. In vitro production of AAT by tumor cells themselves also occurs in a variety of adenocarcinoma, sarcoma, glioblastoma and chordoma cell lines [51, 52]. Based on the results of these studies, the presence of AAT in tumors has typically been ascribed to its production by the tumor cells themselves, and patients with AAT expression in their tumors have been thought to have a worse prognosis than those without AAT expression.

However, two recently published studies have provided results that call into question these previously accepted concepts. Firstly, a study of tissue expression of AAT in a 372-dot tissue array, and its concentrations in sera of patients with CRC, using a methylation isotope-labeling-assisted gel-enhanced liquid chromatography-mass spectrometry strategy, found that CRC specimens expressed less AAT in both tissues and serum than did normal counterparts [53]. This surprising result was supported by a subsequent study of gastric cancer tissues and adjacent normal tissues obtained from surgery, using two-dimensional differential gel electrophoresis, validating protein expression by western blot and IHC, which found AAT to be significantly downregulated in gastric cancer patients [54]. In vitro comparative analysis of the human tumors and normal tissues revealed an association between reduced local AAT expression and more aggressive tumor growth [55].

Nevertheless, the role that AAT may play in tumor invasiveness is currently unknown. It has been suggested that since neutrophil elastase is present in colon carcinoma tissues, and its level is very similar to the degree of tissue infiltration by neutrophils, it is possible that an excess of free elastase promotes a favorable host environment for carcinogenesis [56]. Other authors have linked carcinogenesis to AAT degradation by matrix metalloproteinases activated by neutrophil elastase, cathepsin G, and proteinase-3 [57], ultimately resulting in the production of COOH-terminal fragments, which boosts tumor growth in vivo[58].

In addition to the markedly elevated AAT serum levels found in CRC patients compared with controls, other results of our study merit discussion, even though the small sample size and the marked deviation from the mean of some values meant that these differences between cases and controls were not statistically significant. Briefly, these findings were as follows: (1) CRC cases in advanced stages (III and IV) had higher AAT serum concentrations than those in early stages (I and II); (2) the gene frequency of the severe deficiency Pi*Z allele, and the prevalence of the Pi*MZ, Pi*SZ and Pi*ZZ deficiency phenotypes were higher in CRC patients than in controls; and (3) CRC patients with the Pi*MZ genotype tended to develop more locally advanced tumors, had a greater need for postoperative chemotherapy, and had a greater rate of mortality from causes directly related to the CRC than did subjects with the MM genotype.

Nonetheless, our results cast some doubt on the accuracy of the present study, because it might be biased by the small size of the samples studied, as suggested by the low PFS displayed by the two cohorts (both 5.4 points). This low value would require both samples to be approximately doubled in size to improve it sufficiently.

There is wide-ranging evidence about the relationship between AAT deficiency and the development of various types of malignancy, including CRC. The level of evidence, in terms of evidence-based medicine, is high with respect to the risk of subjects with Pi*ZZ genotype developing hepatocellular carcinomas, which reaches the very high percentage of 28% [1, 4, 5]. Regarding lung cancer, two studies found Pi*MS and Pi*MZ heterozygote individuals to be at increased risk of developing bronchial carcinomas, particularly of the squamous and bronchoalveolar cell types, independent of smoking habit and presence of COPD [6, 7]. The mechanism involved in lung carcinogenesis would be an excess of neutrophil elastase that is not neutralized by AAT and that stimulates development, invasion and metastasis. This same mechanism would probably be shared by all other types of cancers, including CRC [8, 9]. There is also some evidence of a relationship between AAT deficiency and the development of neoplasms of the urinary bladder and gallbladder, and malignant lymphomas [10–12].

Colorectal cancer, a leading cause of cancer deaths worldwide, has also been associated with AAT deficiency [13, 14]. It is known that both normal and cancer intestinal cells secrete AAT [56, 59] to neutralize elastase, which is present in high concentrations in colon carcinoma cells, in an attempt to maintain the protease-antiprotease balance. This prevents the activation of procathepsin B and proprotein convertase, and reduces the production of TNF-α and IL-1a, which prevents liver metastases [60–62]. However, the only two clinico-epidemiological studies carried out to date produced conflicting results [13, 14]. The first study, of patients with CRC and a microsatellite instability genotype, found a significantly higher prevalence of AAT deficiency alleles in CRC subjects than in the general population (21.6% vs. 9.4%), and that smokers with AAT deficiency had a 20-times greater risk than expected of developing high microsatellite instability compared with smokers without AAT deficiency [13]. Conversely, a more recent case-control study confirmed the link between smoking history and the high degree of microsatellite instability, but no difference in AAT deficiency frequency between cases and controls, irrespective of their microsatellite unstable subtype [14].

Our study found that patients with CRC have much higher serum AAT concentrations than healthy controls, regardless of the genotypes of the subjects. This finding is consistent with most published classic studies, but is unlike others published recently. Its meaning is therefore uncertain, and its potential role in the diagnosis and staging of CRC remains to be established. Further studies are needed in other diseases and other gastrointestinal tumors to determine the sensitivity and specificity of this biomarker.

On the other hand, based on our findings, our initial hypothesis that AAT deficiency is involved in the development and progression of CRC could neither be confirmed nor ruled out, since a trend towards more severe AAT deficiency with more advanced tumor stage was observed. Not enough Z alleles were analyzed in our study for statistical significance to be reached for an effect size of the observed magnitude. Similar studies but of greater statistical power are therefore required to settle this matter.