The correlation of TRPM1 (Melastatin) mRNA expression with microphthalmia‐associated transcription factor (MITF) and other melanogenesis‐related proteins in normal and pathological skin, hair follicles and melanocytic nevi

Abstract

Background Melastatin (TRPM1), a.k.a. transient receptor potential cation channel, subfamily M, member 1 (TRPM-1) regulates melanocyte differentiation and proliferation. TRPM1 is transcriptionally regulated by the essential melanocyte transcription factor MITF (microphthalmia-associated transcription factor). For the most part, MITF expression is preserved during melanoma progression, while TRPM1 mRNA expression decreases or is completely lost. The loss of TRPM1 is associated with melanomas that are more aggressive. Objective To assess the relationship between TRPM1 mRNA expression and the expression of MITF and nine other markers of melanocytes and melanin-related proteins by immunohistochemistry in normal skin, scars, hair follicles and ordinary melanocytic nevi. Methods Samples of normal skin (n = 102; from tumor excisions and plastic procedures), scars (n = 5; from re-excision specimens) and compound melanocytic nevi (n = 4) were evaluated for the presence of TRPM1 mRNA transcripts as detected by chromogenic in situ hybridization (CISH). Immunohistochemical techniques were used to detect melanin-related proteins including: MITF, S100 protein, Mart-1, tyrosinase, Mel5, HMB45, tyrosinase-related protein-1 (TRP1), TRP2 and α-melanocyte stimulating hormone (αMSH). The labeling index (LI) was defined as the number of intraepidermal cells expressing mRNA or protein per one hundred basal keratinocytes. Results A wide range of LI was found for all markers (0–33 positive cells/100 keratinocytes). When these LI were compared, no significant differences in the expression of MITF, S100, Mart1, tyrosinase proteins and TRPM1 mRNA were identified. The LI for TRPM1 mRNA expression ranged from 74% of that for MITF to 86% for tyrosinase. The LI for TRP-1, TRP-2 and Mel5 was similar to that of TRPM1, while HMB-45 had a significantly lower LI than all other markers. TRPM1 mRNA correlated most tightly with MITF and tyrosinase expression (r = 0.81 and 0.68, respectively, both p = 0.0001). Likewise, the strongest correlation among all the melanin-related proteins existed between tyrosinase and MITF (r = 0.79, p = 0.0001). There was variable expression of melanin-related proteins when LI were analyzed by anatomic site, patient age, extent of sun-damage and proximity to a melanocytic tumor. Anogenital skin showed the highest and acral skin the lowest LI for TRPM1, MITF, S100 protein, Tyrosinase, Mel5 and HMB45. Advanced age (>60 years) was associated with decreased TRPM1 expression. Sun-damaged skin exhibited significantly increased LI as measured by MITF, S100 protein, Mart1, tyrosinase and HMB-45, but no differences for TRPM1. However, the MITF-TRPM1 differential (i.e. MITF LI-TRPM1 LI = MITF+TRPM1 – melanocytes) was significantly increased in site-matched skin (4.6 ± 4.4 vs. 1.5 ± 2.5, p = 0.01). There was a suggestion of reduced LI in normal skin in the proximity of melanoma (from melanoma re-excision specimens) for S100, HMB45 and TRPM1 mRNA. TRPM1 LI was significantly decreased in scars compared to normal skin (5.6 ± 1....

Colocalization of vitiligo and alopecia areata presenting as poliosis

“…The presence of root sheath cells expressing Melan‐A with absence of Melan‐A‐positive cells in the epidermis may be due to the fact that epidermal melanocytes are believed to be antigenically distinct from melanocytes within follicular epithelium . The loss of MiTF‐expressing cells in the follicular epithelium of our patient may represent loss of putative melanocyte stem cells, which have been identified as a population of MiTF‐positive, Melan‐A‐negative non‐pigment‐producing melanocytes located in this portion of the hair follicle …”

Sox10 is expressed in primary melanocytic neoplasms of various histologies but not in fibrohistiocytic proliferations and histiocytoses

“…The number of melanocytes highlighted by immunostaining in normal skin without significant sun exposure was compared with dermatoheliotic skin and epidermis immediately overlying scars approximately 6 weeks of age (ie, conditions demonstrating a regenerating melanocyte phenotype 16 were compared. The mean number of melanocytes highlighted was greater in both regenerating scars and dermatoheliotic skin when compared with normal skin (Table III); however, this difference was not statistically significant.…”

TRPM1 (melastatin) expression is an independent predictor of overall survival in clinical AJCC stage I and II melanoma patients

“…In addition to functioning as a tumor suppressor gene, several lines of evidence indicate that TRPM1 gene regulates melanin expression: decreased TRPM1 expression is found in nonpigmented skin in the Appaloosa horse; TRPM1 knockdown results in a decrease in intracellular melanin pigment; human melanocytes from light skin have an 80% reduction in the abundance of TRPM1 mRNA compared to melanocytes from dark skin; TRPM1 mRNA expression directly correlates with tyrosinase expression in normal melanocytes in vivo; and TRPM1 knockdown results in a decrease in tyrosinase activity and intracellular melanin pigment . Whether or not the relationship between melanin and TRPM1 expression exists in melanoma has not been examined and may have relevance to melanoma genotype.…”