Melanoma Stem Cells May Hold Treatment Key

BUENOS AIRES — The successful treatment of melanoma in the future is likely to be a two-step process involving combination therapy, Meenhard Herlyn, D.Sc., said in a keynote address at the 21st World Congress of Dermatology.

The aim of the initial treatment will be to debulk the tumor from the major transit-amplifying cells. The second stage will then be to focus on eradication of the cancer stem cells.

"If you do not eliminate these tumor stem cells with therapy, the tumor will come back over and over again," said Dr. Herlyn, professor and program leader in the molecular and cellular oncogenesis program at the Wistar Institute, Philadelphia.

Melanoma arises from malignant transformation of melanocytes or nevus cells, which are formed by overproliferation of melanocytes. Progression from nevi occurs in approximately 50% of melanomas. In normal skin, keratinocytes control the growth of melanocytes in a sort of master-slave relationship.

The decoupling of melanocytes from keratinocytes is the first step in the development of nevi. Overproduction of growth factors from fibroblasts then drives melanocyte proliferation, Dr. Herlyn explained.

For development of melanoma, the most important pathway is the mitogen-activated protein (MAP) kinase pathway, he said.

A genomewide screen performed at the Wellcome Trust Sanger Institute showed that approximately two-thirds of melanomas harbor an activating mutation in BRAF, one of the Raf isoforms in the MAP kinase pathway (Nature 2002;417:949–54).

In 98% of these cases, the BRAF mutation is V600E. A single mutation, however, is not sufficient for the development of melanoma.

Mutations in other signaling pathways, such as the P13-kinase/Akt pathways or cell cycle regulatory pathways, have also been implicated in the development of melanoma, said Dr. Herlyn, who reported no conflicts of interest.

Earlier models of cancer development assumed that melanoma arose as the accumulation of sequential mutations in a mature, differentiated melanocyte eventually leading to malignant transformation of the cell. Identification of another type of cell in melanomas, however, suggests that this model may be too simplistic.

"There are rare cells in tumors that act like stem cells and always replenish the tumor with new cells," said Dr. Herlyn.

The cell that undergoes malignant transformation to produce melanoma might not be a terminally differentiated cell, but instead might be a type of stem cell.

When they are grown under conditions suitable for embryonic stem cells, melanoma cells form spheres that look similar to embryoid bodies formed by human stem cells (Cancer Res. 2005;65:9328–37).

These melanoma spheres meet the definition of tumor stem cells. Melanoma spheres are capable of self-renewal. The cells are highly tumorigenic and quickly form lethal melanomas when injected into nude mice.

They also are multipotent—under laboratory conditions, melanoma spheres can differentiate into melanocytes, adipocytes, chondrocytes, and osteoblasts.

"There is an incredible plasticity in melanoma cells that we have not known before," said Dr. Herlyn. "The question is: What is the true melanoma stem cell?"

Melanoma spheres consist of a mixed cell population. Among the population are cells expressing the hematopoietic marker CD20 and cells expressing the embryonic stem cell marker CD133. Melanoma spheres include side population cells that extrude Hoechst dye via active efflux.

Particularly intriguing are the label-retaining melanoma cells, which proliferate slowly or not at all. When removed from the surrounding cells, however, the label-retaining cells begin to divide rapidly. "If you isolate them, they literally explode," he said.

The label-retaining cells may be the melanoma stem cells, responsible for tumor dormancy and relapse. In this model, the CD20 cells, the CD133 cells, and the side population cells function as progenitor cells.

The label-retaining cells and the progenitor cells could give rise to transit-amplifying cells, Dr. Herlyn suggested at the meeting.

Melanomas are phenotypically heterogeneous tumors. "Tumor heterogeneity is not random," he noted. "Tumor heterogeneity represents a clonal hierarchy in which the different populations compete with each other."

In the center of the hierarchy is the melanoma stem cell. Although melanomas may differ from one patient to the next, they all contain melanoma stem cells. Targeting the stem cell is critical for treatment outcome. "This one cell can give rise to all the others," said Dr. Herlyn. "If we let the cancer stem cell escape, we will always get the cancer back."

Under suitable conditions, melanoma cells form spheres (shown here at 40X magnification) that are capable of self-renewal and are highly tumorigenic. Courtesy Meenhard Herlyn, D.Sc.

'This one cell can give rise to all the others. If we let the cancer stem cell escape, we will always get the cancer back.' DR. HERLYN

BUENOS AIRES — The successful treatment of melanoma in the future is likely to be a two-step process involving combination therapy, Meenhard Herlyn, D.Sc., said in a keynote address at the 21st World Congress of Dermatology.

The aim of the initial treatment will be to debulk the tumor from the major transit-amplifying cells. The second stage will then be to focus on eradication of the cancer stem cells.

"If you do not eliminate these tumor stem cells with therapy, the tumor will come back over and over again," said Dr. Herlyn, professor and program leader in the molecular and cellular oncogenesis program at the Wistar Institute, Philadelphia.

Melanoma arises from malignant transformation of melanocytes or nevus cells, which are formed by overproliferation of melanocytes. Progression from nevi occurs in approximately 50% of melanomas. In normal skin, keratinocytes control the growth of melanocytes in a sort of master-slave relationship.

The decoupling of melanocytes from keratinocytes is the first step in the development of nevi. Overproduction of growth factors from fibroblasts then drives melanocyte proliferation, Dr. Herlyn explained.

For development of melanoma, the most important pathway is the mitogen-activated protein (MAP) kinase pathway, he said.

A genomewide screen performed at the Wellcome Trust Sanger Institute showed that approximately two-thirds of melanomas harbor an activating mutation in BRAF, one of the Raf isoforms in the MAP kinase pathway (Nature 2002;417:949–54).

In 98% of these cases, the BRAF mutation is V600E. A single mutation, however, is not sufficient for the development of melanoma.

Mutations in other signaling pathways, such as the P13-kinase/Akt pathways or cell cycle regulatory pathways, have also been implicated in the development of melanoma, said Dr. Herlyn, who reported no conflicts of interest.

Earlier models of cancer development assumed that melanoma arose as the accumulation of sequential mutations in a mature, differentiated melanocyte eventually leading to malignant transformation of the cell. Identification of another type of cell in melanomas, however, suggests that this model may be too simplistic.

"There are rare cells in tumors that act like stem cells and always replenish the tumor with new cells," said Dr. Herlyn.

The cell that undergoes malignant transformation to produce melanoma might not be a terminally differentiated cell, but instead might be a type of stem cell.

When they are grown under conditions suitable for embryonic stem cells, melanoma cells form spheres that look similar to embryoid bodies formed by human stem cells (Cancer Res. 2005;65:9328–37).

These melanoma spheres meet the definition of tumor stem cells. Melanoma spheres are capable of self-renewal. The cells are highly tumorigenic and quickly form lethal melanomas when injected into nude mice.

They also are multipotent—under laboratory conditions, melanoma spheres can differentiate into melanocytes, adipocytes, chondrocytes, and osteoblasts.

"There is an incredible plasticity in melanoma cells that we have not known before," said Dr. Herlyn. "The question is: What is the true melanoma stem cell?"

Melanoma spheres consist of a mixed cell population. Among the population are cells expressing the hematopoietic marker CD20 and cells expressing the embryonic stem cell marker CD133. Melanoma spheres include side population cells that extrude Hoechst dye via active efflux.

Particularly intriguing are the label-retaining melanoma cells, which proliferate slowly or not at all. When removed from the surrounding cells, however, the label-retaining cells begin to divide rapidly. "If you isolate them, they literally explode," he said.

The label-retaining cells may be the melanoma stem cells, responsible for tumor dormancy and relapse. In this model, the CD20 cells, the CD133 cells, and the side population cells function as progenitor cells.

The label-retaining cells and the progenitor cells could give rise to transit-amplifying cells, Dr. Herlyn suggested at the meeting.

Melanomas are phenotypically heterogeneous tumors. "Tumor heterogeneity is not random," he noted. "Tumor heterogeneity represents a clonal hierarchy in which the different populations compete with each other."

In the center of the hierarchy is the melanoma stem cell. Although melanomas may differ from one patient to the next, they all contain melanoma stem cells. Targeting the stem cell is critical for treatment outcome. "This one cell can give rise to all the others," said Dr. Herlyn. "If we let the cancer stem cell escape, we will always get the cancer back."

Under suitable conditions, melanoma cells form spheres (shown here at 40X magnification) that are capable of self-renewal and are highly tumorigenic. Courtesy Meenhard Herlyn, D.Sc.

'This one cell can give rise to all the others. If we let the cancer stem cell escape, we will always get the cancer back.' DR. HERLYN

BUENOS AIRES — The successful treatment of melanoma in the future is likely to be a two-step process involving combination therapy, Meenhard Herlyn, D.Sc., said in a keynote address at the 21st World Congress of Dermatology.

The aim of the initial treatment will be to debulk the tumor from the major transit-amplifying cells. The second stage will then be to focus on eradication of the cancer stem cells.

"If you do not eliminate these tumor stem cells with therapy, the tumor will come back over and over again," said Dr. Herlyn, professor and program leader in the molecular and cellular oncogenesis program at the Wistar Institute, Philadelphia.

Melanoma arises from malignant transformation of melanocytes or nevus cells, which are formed by overproliferation of melanocytes. Progression from nevi occurs in approximately 50% of melanomas. In normal skin, keratinocytes control the growth of melanocytes in a sort of master-slave relationship.

The decoupling of melanocytes from keratinocytes is the first step in the development of nevi. Overproduction of growth factors from fibroblasts then drives melanocyte proliferation, Dr. Herlyn explained.

For development of melanoma, the most important pathway is the mitogen-activated protein (MAP) kinase pathway, he said.

A genomewide screen performed at the Wellcome Trust Sanger Institute showed that approximately two-thirds of melanomas harbor an activating mutation in BRAF, one of the Raf isoforms in the MAP kinase pathway (Nature 2002;417:949–54).

In 98% of these cases, the BRAF mutation is V600E. A single mutation, however, is not sufficient for the development of melanoma.

Mutations in other signaling pathways, such as the P13-kinase/Akt pathways or cell cycle regulatory pathways, have also been implicated in the development of melanoma, said Dr. Herlyn, who reported no conflicts of interest.

Earlier models of cancer development assumed that melanoma arose as the accumulation of sequential mutations in a mature, differentiated melanocyte eventually leading to malignant transformation of the cell. Identification of another type of cell in melanomas, however, suggests that this model may be too simplistic.

"There are rare cells in tumors that act like stem cells and always replenish the tumor with new cells," said Dr. Herlyn.

The cell that undergoes malignant transformation to produce melanoma might not be a terminally differentiated cell, but instead might be a type of stem cell.

When they are grown under conditions suitable for embryonic stem cells, melanoma cells form spheres that look similar to embryoid bodies formed by human stem cells (Cancer Res. 2005;65:9328–37).

These melanoma spheres meet the definition of tumor stem cells. Melanoma spheres are capable of self-renewal. The cells are highly tumorigenic and quickly form lethal melanomas when injected into nude mice.

They also are multipotent—under laboratory conditions, melanoma spheres can differentiate into melanocytes, adipocytes, chondrocytes, and osteoblasts.

"There is an incredible plasticity in melanoma cells that we have not known before," said Dr. Herlyn. "The question is: What is the true melanoma stem cell?"

Melanoma spheres consist of a mixed cell population. Among the population are cells expressing the hematopoietic marker CD20 and cells expressing the embryonic stem cell marker CD133. Melanoma spheres include side population cells that extrude Hoechst dye via active efflux.

Particularly intriguing are the label-retaining melanoma cells, which proliferate slowly or not at all. When removed from the surrounding cells, however, the label-retaining cells begin to divide rapidly. "If you isolate them, they literally explode," he said.

The label-retaining cells may be the melanoma stem cells, responsible for tumor dormancy and relapse. In this model, the CD20 cells, the CD133 cells, and the side population cells function as progenitor cells.

The label-retaining cells and the progenitor cells could give rise to transit-amplifying cells, Dr. Herlyn suggested at the meeting.

Melanomas are phenotypically heterogeneous tumors. "Tumor heterogeneity is not random," he noted. "Tumor heterogeneity represents a clonal hierarchy in which the different populations compete with each other."

In the center of the hierarchy is the melanoma stem cell. Although melanomas may differ from one patient to the next, they all contain melanoma stem cells. Targeting the stem cell is critical for treatment outcome. "This one cell can give rise to all the others," said Dr. Herlyn. "If we let the cancer stem cell escape, we will always get the cancer back."

Under suitable conditions, melanoma cells form spheres (shown here at 40X magnification) that are capable of self-renewal and are highly tumorigenic. Courtesy Meenhard Herlyn, D.Sc.

'This one cell can give rise to all the others. If we let the cancer stem cell escape, we will always get the cancer back.' DR. HERLYN