Atypical fibroxanthoma (AFX) is a nodular ulcerative lesion arising from the sun-exposed skin of the head and neck, typically in the elderly 12. Solar elastosis associated with UV-radiation has been commonly observed in AFX cases 3. Association between AFX and ultraviolet (UV) radiation has been suspected. On the other hand, in its less common forms with weakened association with UV, AFX occurs on the extremities and the trunk 12. AFX is composed of spindle, plump, epithelioid and bizarre cells, in various proportions, arranged in haphazard, vaguely fascicular or storiform patterns. These histological features of AFX mimic those of high-grade sarcoma, such as malignant fibrous histiocytoma or leiomyosarcoma 24, which occurs deep within soft tissue.

The tumor-suppressor protein p53 is a transcriptional activator which is involved in cell-cycle control, DNA repair, apoptosis and chromosome/genome instability. UV-induced p53 gene mutations occurring at dipyrimidine sites have been demonstrated in AFX, suggesting a central role for UV radiation in the pathogenesis of AFX 5. These p53 functions are mediated through its transcriptional target (effector) proteins, such as Bax and Gadd45 (growth arrest and DNA damage inducible) 6.

Bax is a pro-PCD (programmed cell death) protein and belongs to the Bcl-2 family 7. Furthermore, Bax is important for p53-dependent PCD 89. Gadd45 is a DNA damage-responsive gene that is rapidly induced by agents that cause DNA damage, including UV radiation. Gadd45 is a multifunctional protein which has roles to play in cell-cycle arrest, genomic stability, nucleotide excision repair, chromatin accessibility and apoptosis 101112. Although mutation of the Gadd45 gene is not common in tumors 1314, some research has been reported showing that point mutations of Gadd45 gene at exon 4 were found in 13.6% of pancreatic cancer cases, suggesting the possibility that Gadd45 is dysfunctional in this tumor type 15. We have previously reported the expression of Gadd45 in a series of AFX without mutation analysis 16.

In the current study, we evaluate the immunoexpression pattern of Bax in AFX. The existence of a mutation at exon 4 of the Gadd45 gene is also examined in order to refute the possibility of any dysfunction of Gadd45, regardless of its expression. We also discuss the correlation of immunoexpression among Bax, Gadd45 and p53 in the correspondent cases. We use benign fibrous histiocytoma (BHF), or dermatofibroma, as the comparison. BHF is a benign fibrohistiocytic tumor composed of a mixture of fibroblastic and histiocytic cells, most commonly seen in the dermis and superficial subcutis 2.

AFX typically occurs on the head and neck of sun-exposed skin. This fact has long suggested a role for sun exposure in the tumorigenesis of AFX. UV radiation from sunlight is an important risk factor for skin cancer 20. UV-induced p53 gene mutations occurring at dipyrimidine sites have been demonstrated in AFX, suggesting a central role for UV radiation in the pathogenesis of AFX 517. p53 protein is a key regulator for cell-cycle arrest and apoptosis induced by DNA damage 21. Therefore Bax and Gadd45 are the central downstream effectors of p53 6. It has been reported that low doses of UV-radiation induced a rapid p53 accumulation followed by a decrease which was correlated with a transient cell-cycle withdrawal and presumably also DNA repair 22. The association between decreased DNA repair ability and AFX may be supported by a report showing that patients with xeroderma pigmentosum had AFX, and it is known that xeroderma pigmentosum is related to DNA repair defects 2324. On the other hand, high and sustained levels of p53 were observed after a high dose of UV-radiation in cells undergoing apoptosis 22. Therefore, regulation of p53 target genes was highly dependent upon the radiation dose used, with high doses inducing only Bax and Gadd45 expression 22. In the current study, it seemed that there was a correlation between Bax and Gadd45 in these AFX cases. These associations of Bax and Gadd45 noted in AFX may be consistent with p53 effector upregulation against UV radiation, although there seemed to be no correlation between p53 expression, and the expression of Bax or Gadd45.

Bax expression has been reported to be associated with histological grade 25 and unfavorable prognosis 26 in ovarian tumors. In the current study, Bax was expressed in about 50% of the AFX cases. The frequency of Bax expression in AFX was significantly higher than that in BFH. This difference suggests the essential difference between the two lesions. Bax is a pro-PCD (programmed cell death) protein and belongs to the Bcl-2 family 7. As far as we are concerned, there have been no reports presenting a direct comparison between AFX and BFH in terms of apoptotic behavior. However, it has been reported that apoptotic behavior in AFX is as high as that in high-grade sarcoma of MFH 27. In addition, another research article has reported that there is a significant difference between MFH and BFH 28. Therefore, it is reasonable to assume that the apoptotic behavior of AFX is higher than that of BFH. Moreover, the expression of Bax in AFX may reflect apoptotic behavior.

We have previously reported the expression of Gadd45 in a series of AFX, in which mutation of the Gadd45 gene was not analyzed 16. Gadd45 gene mutation is uncommon in human tumors 1314. However, point mutations were found at exon 4 of the Gadd45 gene in 13.6% of pancreatic cancer cases, thereby suggesting that the possible dysfunction of Gadd45 plays a role in tumor development in certain types of tumors 15. In the current study, no mutation of the Gadd45 gene at exon 4 was seen in a series of AFX, regardless of the immunohistochemical expression of Gadd45. This result may refute any possibility of dysfunction of Gadd45 in terms of gene mutation, at least at exon 4 of the Gadd45 gene.

AFX untypically occurs outside the head and neck region, such as in the extremities. The site-specific difference of AFX between the head and neck region and other regions is not well known. In the current series, 4 out of 7 cases occured in the head and neck region, while 3 cases occured in the extremities of the fingers and the leg. The expression of Gadd45 in AFX was more frequent than that in BFH, which was also true for the expression of Bax; BFH rarely demonstrated Gadd45 or Bax expression. Bax expression was seen in 1 out of 4 AFX cases in the head and neck region (25%), but in 2 out of 3 AFX cases (67%) in the extremities. On the other hand, Gadd45 expression was seen in 2 out of 4 in the head and neck region (50%), but in 2 out of 3 cases in the extremities (67%). It might be possible that the expression of Bax or Gadd45 in AFX is more prominent in the extremities, rather than in the head and neck region. Namely, the expression of Bax or Gadd45 in AFX may differ in a site-specific manner, possibly reflecting its pathogenesis. Further study will be needed in order to characterize the site-specific difference of AFX.

It remains controversial whether AFX and MFH are actually the same entity, or not. It seems that AFX and MFH share histological and immunological features. A histiocytic/macrophage marker, CD68, is positive in more than half of all AFX cases 2930. CD10 has been expressed in almost all cases of MFH, as well as in cases of AFX 1631. However, the number of AFX cases with moderate/strong or diffuse immunoreactivity for CD99 is significantly larger than that of MFH cases 32. Reduced immunoexpression for CD74 in AFX may be characterized compared to that in MFH 33. From a molecular aspect, AFX is characterized by a diploid pattern, while the majority of chromosomal changes in MFH are of an aneuploid pattern 3435. H-, K-, and N-ras gene mutations are not present in AFX, whereas MFH has H- and K-ras gene mutations, although only a small number of cases was studied 36. In contrast to MFH, AFX is strongly associated with UV radiation in its pathogenesis. The formation of DNA photoproducts by UV radiation has been reported to be responsible for the development of skin cancer 37. DNA photoproducts can interfere with the binding of several important cell-cycle regulatory and DNA damage-responsive transcription factors 37. It has been reported that the accumulation of DNA photoproducts may play an important role in the pathogenesis of AFX 17. It is also possible that AFX and MFH share the same pathway which determines their morphology, but that AFX and MFH may have different pathways of biological behavior which determine their clinical behavior.

In the current study, we analyzed the immunohistochemical expressions of the p53 effector proteins of Bax and Gadd45 in AFX. Possible association between Bax and Gadd45 without gene mutation at exon 4, was observed. This result may refute any possibility of dysfunction of Gadd45 in terms of gene mutation, at least at exon 4 of the Gadd45 gene.

The authors declare that they have no competing interests.

AS drafted the manuscript. AS and SA carried out the mutation analysis. YO participated in the design of the study. YI and MT conceived of the study, and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.