The patterns of birthmarks suggest a novel population of melanocyte precursors arising around the time of gastrulation

Anti-BCL2 therapy eliminates giant congenital melanocytic nevus by senolytic and immune induction

Giant congenital melanocytic nevus (GCMN) is a RAS/RAF mutation-driven syndrome characterized by extensive melanocytic lesions, posing psychological challenges and a lifelong risk of malignancy. Existing treatments like surgical resection and laser therapy fail to fully remove lesions, and MAPK inhibitors show limited efficacy. This study identified a predominant population of senescent cells and a minority of proliferative cells in GCMN, necessitating dual-targeted strategies. We found that the anti-apoptotic protein BCL2 is expressed in both senescent and proliferative cells from GCMN patients with various gene mutations. Coexpression of P16 and BCL2 indicated a phenotype of growth arrest and cell survival. BCL2 inhibitors (BCL2i) showed significant cytotoxicity to GCMN cells in vitro. Hypopigmentation and GCMN cell clearance were observed in patient-derived xenograft models and in NrasQ61K-mutated and BrafV600E-mutated transgenic models following BCL2i treatment. Histology of regressed GCMN indicated extensive immune cell infiltration, suggesting immune involvement. Single-cell sequencing and immunostaining revealed that activated neutrophils formed extracellular traps, synergizing with BCL2i to treat GCMN. Neutrophil depletion and immunosuppression reduce treatment efficacy, highlighting the crucial role of the immune response post-BCL2i treatment. Long-term follow-up showed no recurrence, with neutrophils and T cells residing in the dermis, indicating memory immune reactions. These findings present a promising therapeutic strategy and underscore the translational potential of BCL2i in treating GCMN.

Melanocyte lineage dynamics in development, growth and disease

Melanocytes evolved to produce the melanin that gives colour to our hair, eyes and skin. The melanocyte lineage also gives rise to melanoma, the most lethal form of skin cancer. The melanocyte lineage differentiates from neural crest cells during development, and most melanocytes reside in the skin and hair, where they are replenished by melanocyte stem cells. Because the molecular mechanisms necessary for melanocyte specification, migration, proliferation and differentiation are co-opted during melanoma initiation and progression, studying melanocyte development is directly relevant to human disease. Here, through the lens of advances in cellular omic and genomic technologies, we review the latest findings in melanocyte development and differentiation, and how these developmental pathways become dysregulated in disease.

Connecting the dots: Melanoma cell of origin, tumor cell plasticity, trans-differentiation, and drug resistance

Melanoma, a lethal malignancy that arises from melanocytes, exhibits a multiplicity of clinico-pathologically distinct subtypes in sun-exposed and non-sun-exposed areas. Melanocytes are derived from multipotent neural crest cells and are present in diverse anatomical locations, including skin, eyes, and various mucosal membranes. Tissue-resident melanocyte stem cells and melanocyte precursors contribute to melanocyte renewal. Elegant studies using mouse genetic models have shown that melanoma can arise from either melanocyte stem cells or differentiated pigment-producing melanocytes depending on a combination of tissue and anatomical site of origin and activation of oncogenic mutations (or overexpression) and/or the repression in expression or inactivating mutations in tumor suppressors. This variation raises the possibility that different subtypes of human melanomas (even subsets within each subtype) may also be a manifestation of malignancies of distinct cells of origin. Melanoma is known to exhibit phenotypic plasticity and trans-differentiation (defined as a tendency to differentiate into cell lineages other than the original lineage from which the tumor arose) along vascular and neural lineages. Additionally, stem cell-like properties such as pseudo-epithelial-to-mesenchymal (EMT-like) transition and expression of stem cell-related genes have also been associated with the development of melanoma drug resistance. Recent studies that employed reprogramming melanoma cells to induced pluripotent stem cells have uncovered potential relationships between melanoma plasticity, trans-differentiation, and drug resistance and implications for cell or origin of human cutaneous melanoma. This review provides a comprehensive summary of the current state of knowledge on melanoma cell of origin and the relationship between tumor cell plasticity and drug resistance.

The journey from melanocytes to melanoma

Over the past decade, melanoma has led the field in new cancer treatments, with impressive gains in on-treatment survival but more modest improvements in overall survival. Melanoma presents heterogeneity and transcriptional plasticity that recapitulates distinct melanocyte developmental states and phenotypes, allowing it to adapt to and eventually escape even the most advanced treatments. Despite remarkable advances in our understanding of melanoma biology and genetics, the melanoma cell of origin is still fiercely debated because both melanocyte stem cells and mature melanocytes can be transformed. Animal models and high-throughput single-cell sequencing approaches have opened new opportunities to address this question. Here, we discuss the melanocytic journey from the neural crest, where they emerge as melanoblasts, to the fully mature pigmented melanocytes resident in several tissues. We describe a new understanding of melanocyte biology and the different melanocyte subpopulations and microenvironments they inhabit, and how this provides unique insights into melanoma initiation and progression. We highlight recent findings on melanoma heterogeneity and transcriptional plasticity and their implications for exciting new research areas and treatment opportunities. The lessons from melanocyte biology reveal how cells that are present to protect us from the damaging effects of ultraviolet radiation reach back to their origins to become a potentially deadly cancer.

Risk of melanoma in congenital melanocytic nevi of all sizes: A systematic review

Congenital melanocytic nevi (CMN) are commonly encountered benign skin lesions in newborns. Larger CMN is associated with a higher lifetime risk of developing melanoma. However, the level of risk is unclear when CMN are small or medium-sized. Our objective was to assess melanoma risk in patients with CMN of all size categories. A literature review with meta-analysis was performed. Prevalence and incidence densities of melanoma at onset were calculated in the entire study population and according to CMN size, type of treatment and location of the CMN. A total of 91 melanomas were reported in 7915 patients (1.15%, 95% CI, 0.93-1.41). The overall incidence density was 0.057% person-years (95% CI_, 0.044-0.071). The risk ratio of melanoma incidence densities was 21.9 (95% CI, 8.55-56.3) in large to giant CMN compared with small to medium CMN at 15 years of age. The incidence density was higher in CMN located on the trunk and as well as in those which were untreated or partially treated versus complete excision. Our review suggests patients with CMN of medium, large and giant size are at risk of melanoma, whereas the risk remains unknown for small CMN.

Alpha-Synuclein and Its Role in Melanocytes

Pigmentation is an important process in skin physiology and skin diseases and presumably also plays a role in Parkinson’s disease (PD). In PD, alpha-Synuclein (aSyn) has been shown to be involved in the pigmentation of neurons. The presynaptic protein is intensively investigated for its pathological role in PD, but its physiological function remains unknown. We hypothesized that aSyn is both involved in melanocytic differentiation and melanosome trafficking processes. We detected a strong expression of aSyn in human epidermal melanocytes (NHEMs) and observed its regulation in melanocytic differentiation via the microphthalmia-associated transcription factor (MITF), a central regulator of differentiation. Moreover, we investigated its role in pigmentation by performing siRNA experiments but found no effect on the total melanin content. We discovered a localization of aSyn to melanosomes, and further analysis of aSyn knockdown revealed an important role in melanocytic morphology and a reduction in melanosome release. Additionally, we found a reduction of transferred melanosomes in co-culture experiments of melanocytes and keratinocytes but no complete inhibition of melanosome transmission. In summary, this study highlights a novel physiological role of aSyn in melanocytic morphology and its so far unknown function in the pigment secretion in melanocytes.

Tfap2b specifies an embryonic melanocyte stem cell that retains adult multifate potential

Melanocytes, the pigment-producing cells, are replenished from multiple stem cell niches in adult tissue. Although pigmentation traits are known risk factors for melanoma, we know little about melanocyte stem cell (McSC) populations other than hair follicle McSCs and lack key lineage markers with which to identify McSCs and study their function. Here we find that Tfap2b and a select set of target genes specify an McSC population at the dorsal root ganglia in zebrafish. Functionally, Tfap2b is required for only a few late-stage embryonic melanocytes, and is essential for McSC-dependent melanocyte regeneration. Fate mapping data reveal that tfap2b⁺ McSCs have multifate potential, and are the cells of origin for large patches of adult melanocytes, two other pigment cell types (iridophores and xanthophores), and nerve-associated cells. Hence, Tfap2b confers McSC identity in early development, distinguishing McSCs from other neural crest and pigment cell lineages, and retains multifate potential in the adult zebrafish.

Stabilization of β-catenin promotes melanocyte specification at the expense of the Schwann cell lineage

The canonical Wnt/β-catenin pathway governs a multitude of developmental processes in various cell lineages, including the melanocyte lineage. Indeed, β-catenin regulates transcription of Mitf-M, the master regulator of this lineage. The first wave of melanocytes to colonize the skin is directly derived from neural crest cells, whereas the second wave of melanocytes is derived from Schwann cell precursors (SCPs). We investigated the influence of β-catenin in the development of melanocytes of the first and second waves by generating mice expressing a constitutively active form of β-catenin in cells expressing tyrosinase. Constitutive activation of β-catenin did not affect the development of truncal melanoblasts but led to marked hyperpigmentation of the paws. By activating β-catenin at various stages of development (E8.5-E11.5), we showed that the activation of β-catenin in bipotent SCPs favored melanoblast specification at the expense of Schwann cells in the limbs within a specific temporal window. Furthermore, in vitro hyperactivation of the Wnt/β-catenin pathway, which is required for melanocyte development, induces activation of Mitf-M, in turn repressing FoxD3 expression. In conclusion, β-catenin overexpression promotes SCP cell fate decisions towards the melanocyte lineage.

Summary: Activation of β-catenin in bipotent Schwann cell precursors during a specific developmental window induces Mitf and represses FoxD3 to promote melanoblast cell fate at the expense of Schwann cells in limbs.

Congenital Pigmentary Anomalies in the Newborn

Congenital pigmentary anomalies may be evident at birth or soon after, with some birthmarks becoming apparent later in infancy or early childhood. It is important to recognize various pigmentary anomalies in the neonate, most of which are benign but a subset of which are associated with cutaneous morbidity or systemic ramifications and require further evaluation. This review will focus on pigmentary mosaicism, congenital melanocytic nevi, nevus spilus, dermal melanocytosis, and pigmentary anomalies associated with neurofibromatosis type 1 (café au lait spots, freckling, plexiform neurofibromas, nevus anemicus), tuberous sclerosis (hypomelanotic macules), and incontinentia pigmenti.

Partial unilateral lentiginosis: a clinicopathological analysis of 32 cases on the head and neck area in Korea

BACKGROUND

Partial unilateral lentiginosis (PUL) is a rare acquired circumscribed hyperpigmentation characterized by multiple simple lentigines involving half of the body. Since the previous studies of PUL were mostly based on case reports and the current literature lacks well-designed retrospective studies that involve a large number of cases, PUL is not a well-defined entity, and differential diagnosis with nevus spilus is still difficult. This study aims to evaluate clinical and histopathological characteristics and treatment outcomes of PUL on head and neck area of Koreans.

METHODS

Thirty-two patients with PUL on head and neck area were diagnosed clinicohistopathologically at the Asan Medical Center from 2004 to 2017. Their medical records, photographs, and biopsy specimens were reviewed, and immunohistochemical staining for protein kinase C (PKC)-βΙΙ was evaluated for melanogenic activity.

RESULTS

Four patients (12.5%) of PUL had congenital lesions, and 24 (75.0%) had age of onset younger than 10 years. Confluency of lentiginous lesions (100%) and mild to moderate background interlesional hyperpigmentation (90.6%) were observed. The lentiginous lesions showed increased melanocytes, melanophages, basal melanins, lentiginous hyperplasia, and perivascular inflammatory cells compared with background interlesional hyperpigmentation, and PKC-βΙΙ was focally positive in 7 of 12 stained PUL lesions. Among the 16 patients who received laser treatments, 10 (62.5%) showed more than 50% of improvement.

CONCLUSIONS

The findings of this study will allow for improved diagnosis of PUL and understanding of its features, which may facilitate proper management in the future.

Bilateral diffuse uveal melanocytic proliferation with multifocal diffuse integumentary melanocytic proliferation paraneoplastic syndrome: A case report

Bilateral diffuse uveal melanocytic proliferation (B-DUMP) is a rare paraneoplastic syndrome typically presenting with bilateral visual loss. B-DUMP is associated with extraocular systemic malignancies with the most common being lung cancer in males and uro-gynaecological cancer in females (mainly ovarian cancer). Cutaneous and/or mucosal involvement in patients with B-DUMP has been reported but it is not well characterised. Herein, we present a female in her 70s with diagnosis of stage IV vaginal clear-cell carcinoma and metastatic melanoma of unknown primary that developed progressive bilateral loss of visual acuity compatible with 'B-DUMP'. Simultaneously, she developed multifocal bilateral bluish-greyish patches on the skin that were shown to have a proliferation of dermal melanocytes. We propose that the clinical and histopathologic cutaneous findings seen in patients with B-DUMP be termed 'diffuse integumentary melanocytic proliferation (DIMP)'.

Congenital melanocytic naevi: An up-to-date overview

Congenital melanocytic naevi are hamartomas of the neuroectoderm caused by genetic mosaicism. Congenital melanocytic naevi are seen in 1-6% of all live births and commonly classified based on the projected size in adults. Congenital melanocytic naevi appear in different colours, shapes, and sizes, and occasionally present with complications. In this review, we sought to evaluate congenital melanocytic naevi, their clinical, dermatoscopic, and reflectance confocal microscopic features, behavioural pattern over time, new trends in classification, underlying genetic factors and their influence on clinical manifestations and management, associated risks, complications, magnetic resonance imaging findings and their management in the light of recent literature.

Genetic and clinical characterization of 73 Pigmentary Mosaicism patients: revealing the genetic basis of clinical manifestations

BACKGROUND

Pigmentary mosaicism constitutes a heterogeneous group of skin pigmentation alterations associated with multisystem involvement. The aim of this study was to establish a complete cytogenetic and molecular characterization of PM patients, emphasizing on searching for possible low chromosomal mosaicism and on establishing an accurate genotype-phenotype correlation.

RESULTS

A total of 73 patients were included (3 months to 18 years of age), 52% male and 48% female. Observed in 69 (95%) patients, the most frequent pattern of pigmentation was fine and whorled BL, which was associated with disseminated skin extent in 41 (59%) patients. Central nervous system (84%) alterations were the most frequent observed in the group of patients, followed by the musculoskeletal (53%) and ophthalmologic (27%) alterations. Considering the pattern of pigmentation, no significant differences in association with skin extent or extracutaneous manifestations were detected. Following a strict cytogenetic analysis strategy, screening metaphases from three different tissues (peripheral blood, hyperpigmented and hypopigmented skin) we found that 23/73 patients had chromosomal abnormalities classified as follows: 1) Mosaic with 2 or more different cell lines with structural alterations n = 19; 2) Polyploidy (mosaic) n = 1 and 3) Alterations in all cells in three different tissues n = 3. SNP array, array CGH and FISH were useful for the complete characterization of the chromosomal aberrations, for the detection of microdeletions in patients with normal karyotype but with strong clinical suspicious of chromosomal alteration, and for a better establishment of genotype-phenotype correlation. In 2 patients we found genes associated with some of the extracutaneous manifestations (SHH, MNX1, PPP2R2C).

CONCLUSIONS

This group of 73 patients finely described is the largest series of patients with pigmentary mosaicism reported worldwide. As we showed in this study, the followed analysis strategy allowed the detection of cytogenetic and molecular abnormalities, and made possible the establishment of genotype-phenotype associations in some patients. An important limitation of our study was the analysis of fibroblasts cultures instead of melanocytes and keratinocytes. In some cases the direct molecular DNA analysis of skin biopsy could be another choice.

The biker-glove pattern of congenital melanocytic nevi

Congenital melanocytic nevi (CMN) are common birthmarks with 20% occurring on the limbs. We describe 4 patients with acral CMN with a "biker-glove" distribution with sparing of the distal digits, as has previously been described in acral infantile hemangiomas (IH). The existence of the biker-glove pattern suggests that CMN arise from early mutations in melanocyte precursors and supports the recently described Kinsler-Larue hypothesis of mesenchymal distribution of melanocyte migration occurring in a circular field from a central point. Developmental errors in mesenchymal precursors with similar migration patterns may explain this shared pattern among CMN and IH.

Patterned cutaneous hypopigmentation phenotype characterization: A retrospective study in 106 children

BACKGROUND

Cutaneous patterned hypopigmentation's phenotype is highly variable and may be associated with extracutaneous anomalies.

OBJECTIVE

We evaluated the phenotypic and clinical characteristics of patients with cutaneous patterned hypopigmentation to determine whether certain patterns were more likely to be associated with underlying anomalies.

METHODS

The charts of 106 children with cutaneous patterned hypopigmentation were reviewed retrospectively (2007-2018) at Sainte-Justine University Hospital Centre, in Montreal, Canada. Retrieved information included sex, age at diagnosis, phototype, pattern, and distribution of the cutaneous lesions and the presence of extracutaneous findings. Data were recorded on a software tool which collects and analyzes phenotypic information.

RESULTS

The predominant types of cutaneous patterned hypopigmentation were along Blaschko's lines in narrow (38.7%) and broad bands (53.8%). Mixed patterns were observed in 22.5% of children. The anterior trunk and posterior trunk were most frequently affected (69% and 56%, respectively). Extracutaneous involvement, especially neurological and developmental, was present in 28.3% of patients and was significantly associated with ≥ 4 involved body sites.

CONCLUSION

Distribution and types of cutaneous patterned hypopigmentation were not predictive of extracutaneous findings, with the exception of multiple sites involvement and possibly centrofacial location and blocklike lesions. Follow-up until school entry should help identify subtler associated extracutaneous anomalies.

Phenotypic expansion of POFUT1 loss of function mutations in a disorder featuring segmental dyspigmentation with eczematous and folliculo-centric lesions

Appearance of mosaic disorders in thin Blaschko lines suggests that somatic mutations in keratinocyte precursors underlie their pathogenesis. Germline heterozygous mutations in POFUT1 gene cause Dowling-Degos disease (DDD), a skin disease that features flexural reticulated hyperpigmentation and follicular-based lesions. POFUT1 mosaicism has not been described to date. Here, we describe a 9-year-old female with segmental hyper- and hypopigmented patches with overlying eczematous plaques and follicular papules. Employing paired whole exome sequencing of saliva and keratinocytes isolated from affected skin, we found a novel germline heterozygous POFUT1 deletion causing frameshift and premature codon termination and somatic copy-neutral loss of heterozygosity on chromosome 20 encompassing POFUT1. Expression levels of POFUT1 as well as other key regulators of the notch signaling pathway-NOTCH1, NOTCH2, and HES1-were reduced in affected keratinocytes compared with normal keratinocytes. Our findings provide the first evidence of POFUT1 postzygotic mutation and a phenotypic expansion of POFUT1 loss of function mutations. We show that a recessive loss of function mutation in POFUT1 produces a distinct clinical presentation with features (e.g., dermatitis) that are absent in the generalized form of DDD. This study demonstrates how analysis of mosaic disorders can reveal unexpected phenotypes for known genes.

Does the gene matter? Genotype-phenotype and genotype-outcome associations in congenital melanocytic naevi

Background

Genotype–phenotype studies can identify subgroups of patients with specific clinical features or differing outcomes, which can help shape management.

Objectives

To characterize the frequency of different causative genotypes in congenital melanocytic naevi (CMN), and to investigate genotype–phenotype and genotype–outcome associations.

Methods

We conducted a large cohort study in which we undertook MC1R genotyping from blood, and high‐sensitivity genotyping of NRAS and BRAF hotspots in 156 naevus biopsies from 134 patients with CMN [male 40%; multiple CMN 76%; projected adult size (PAS) > 20 cm, 59%].

Results

Mosaic NRAS mutations were detected in 68%, mutually exclusive with BRAF mutations in 7%, with double wild‐type in 25%. Two separate naevi were sequenced in five of seven patients with BRAF mutations, confirming clonality. Five of seven patients with BRAF mutations had a dramatic multinodular phenotype, with characteristic histology distinct from classical proliferative nodules. NRAS mutation was the commonest in all sizes of CMN, but was particularly common in naevi with PAS > 60 cm, implying more tolerance to that mutation early in embryogenesis. Facial features were less common in double wild‐type patients. Importantly, the incidence of congenital neurological disease, and apparently of melanoma, was not altered by genotype; no cases of melanoma were seen in BRAF‐mutant multiple CMN, however, this genotype is rare.

Conclusions

CMN of all sizes are most commonly caused by mutations in NRAS. BRAF is confirmed as a much rarer cause of multiple CMN, and appears to be commonly associated with a multinodular phenotype. Genotype in this cohort was not associated with differences in incidence of neurological disease in childhood. However, genotyping should be undertaken in suspected melanoma, for guidance of treatment.

What's already known about this topic?

Multiple congenital melanocytic naevi (CMN) have been shown to be caused by NRAS mosaic mutations in 70–80% of cases, by BRAF mosaicism in one case report and by inference in some previous cases.

There has been debate about genotypic association with different sizes of CMN, and no data on genotype–outcome.

What does this study add?

NRAS mosaicism was found in 68%, BRAF in 7% and double wild‐type in 25% of cases of CMN.

NRAS was the commonest mutation in all sizes of CMN, but was nearly universal in projected adult size > 60 cm.

BRAF is often associated with a distinct multinodular clinical/histological phenotype.

Adverse outcomes did not differ between genotypes on current numbers.

https://doi.org/10.1111/bjd.18747 available online

Mosaic abnormalities of the skin: review and guidelines from the European Reference Network for rare skin diseases

BACKGROUND

Cutaneous mosaicism is an area of dermatology in which there has been an explosion of knowledge within the current decade. This has led to fundamental changes in the understanding of the conditions in this field, and to an ongoing paradigm shift in the approach to management of mosaic skin disorders.

OBJECTIVES

To lay out the general principles of mosaicism as they are currently understood, summarize the known cutaneous mosaic abnormalities of the skin with associated phenotypic and genotypic information, review the latest trials on targeted therapies and propose guidelines for the general approach to a patient with suspected mosaicism.

METHODS

This was a consensus expert review as part of the European Reference Network project (ERN-Skin).

CONCLUSIONS

This study provides clinicians with a practical approach to the patient with suspected mosaicism, redefines mosaicism for the modern genetic era, and proposes a new classification system based on genetic mechanism. What's already known about this topic? Cutaneous mosaicism is a complex field of dermatology that encompasses most birthmarks, and many rare syndromes. Some cutaneous patterns are known to be seen in mosaicism. Very few treatment options are available for most mosaic abnormalities of the skin. Recent high-sensitivity genetic techniques have led to an explosion of knowledge about genotype and phenotype in the literature. What does this study add? Expert consensus from the European Reference Network project. Review of knowledge of confirmed mosaic abnormalities of the skin, including cutaneous phenotype, extracutaneous associated features and genotype. Proposed new classification of mosaic abnormalities of the skin by genetic mechanism and therefore inheritance potential. Practical tips on correct sample collection and genetic investigation. Review of trials of targeted therapies. Guidelines for a practical clinical approach to the patient with suspected mosaicism.

Striking contiguous depigmentation across the lower limbs in piebaldism and its implications for understanding melanocytic migration and development

Piebaldism is a rare autosomal dominant disorder of pigmentation that is characterized by variable patches of depigmentation on the face, chest, abdomen, and extremities. We describe two cases of piebaldism, in whom the remarkable asymmetric distribution of the depigmented patches in a connected, contiguous pattern across the legs provides embryologic insights. This finding is not explained by the traditional theory that melanocytic migration only originates in the neural crest and progresses unilaterally down each leg. We propose that our cases, and other similar cases, can be explained by a recent theory of mesodermal melanocyte migration.

Biology of human melanocyte development, Piebaldism, and Waardenburg syndrome

Melanocyte development is orchestrated by a complex interconnecting regulatory network of genes and synergistic interactions. Piebaldism and Waardenburg syndrome are neurocristopathies that arise from mutations in genes involved in this complex network. Our understanding of melanocyte development, Piebaldism, and Waardenburg syndrome has improved dramatically over the past decade. The diagnosis and classification of Waardenburg syndrome, first proposed in 1992 and based on phenotype, have expanded over the past three decades to include genotype. This review focuses on the current understanding of human melanocyte development and the evaluation and management of Piebaldism and Waardenburg syndrome. Management is often challenging and requires a multidisciplinary approach.

[Cutaneous melanoma: from rationalized models to patients care]

Cutaneous melanoma derives from the tumoral transformation of melanocytes. These pigmented cells produce melanin prior transmitting it to the surrounding keratinocytes of the skin, hair and mane. The main function of melanin is to protect cells and their DNA from damage caused by ultraviolet light. Melanoma is the most aggressive skin cancer whose incidence has increased steadily in recent decades. Advances in basic research have resulted in a better understanding of the molecular and cellular events responsible for the initiation and progression of melanomas. In this review, we present an overview of the knowledge gained in recent years and show how recent advances lead to new targeted and more efficient therapeutic approaches.

The patterns of birthmarks suggest a novel population of melanocyte precursors arising around the time of gastrulation

Systematic work in the mouse and chicken has mapped out two neural crest-derived pathways of melanocyte precursor migration. With these in mind, this study reappraises the patterns of congenital pigmentary disorders in humans and identifies three recurrent patterns consistent across genetically different diseases. Only two of these are seen in diseases known to be melanocyte cell-autonomous. The segmental pattern correlates well with the classical dorsolateral population from animal studies, demonstrating respect of the midline, cranio-caudal axial mixing, unilateral migration and involvement of key epidermally derived structures. Importantly however, the melanocyte precursors responsible for the non-segmental pattern, which demonstrates circular, bilateral migration centred on the midline, and not involving key epidermally derived structures, have not been identified previously. We propose that this population originates around the time of gastrulation, most likely within the mesoderm, and ultimately resides within the dermis. Whether it contributes to mature melanocytes in non-disease states is not known; however, parallels with the patterns of acquired vitiligo would suggest that it does. The third pattern, hypo- or hyperpigmented fine and whorled Blaschko's lines, is proposed to be non-cell-autonomous.