Mosaicism for a KITLG Mutation in Linear and Whorled Nevoid Hypermelanosis

A case of familial progressive hyperpigmentation with or without hypopigmentation presenting with hypopigmented striae along the lines of Blaschko

Familial progressive hyperpigmentation with or without hypopigmentation (FPHH) is an autosomal dominant disorder characterized by widespread skin hyperpigmentation, café-au-lait spots, and hypopigmented circular macules, resulting from KITLG variants. KITLG, expressed by keratinocytes, binds to KIT on melanocytes, stimulating melanogenesis. Disturbances in the KITLG-KIT interaction result in diffuse hyperpigmentation in FPHH. However, the mechanisms behind hypopigmented macule formation remain unclear. This report presents a unique FPHH case in a patient with a novel KITLG mutation (Ser78Leu). Notably, the patient showed multiple hypopigmented macules and striae along the lines of Blaschko. Digital polymerase chain reaction analysis of the DNA from skin and blood tissues indicated a copy-neutral loss of heterozygosity at the KITLG locus, only in the hypopigmented macule. These findings suggest that the hypopigmented macules might result from revertant mosaicism. Conversely, café-au-lait spots do not follow the lines of Blaschko and can superimpose on the hypopigmented striae, indicating a distinct pathogenesis. This case contributes to the understanding of the genetic mechanisms in FPHH.

Therapeutic modulation of KIT ligand in melanocytic disorders with implications for mast cell diseases

KIT ligand and its associated receptor KIT serve as a master regulatory system for both melanocytes and mast cells controlling survival, migration, proliferation and activation. Blockade of this pathway results in cell depletion, while overactivation leads to mastocytosis or melanoma. Expression defects are associated with pigmentary and mast cell disorders. KIT ligand regulation is complex but efficient targeting of this system would be of significant benefit to those suffering from melanocytic or mast cell disorders. Herein, we review the known associations of this pathway with cutaneous diseases and the regulators of this system both in skin and in the more well-studied germ cell system. Exogenous agents modulating this pathway will also be presented. Ultimately, we will review potential therapeutic opportunities to help our patients with melanocytic and mast cell disease processes potentially including vitiligo, hair greying, melasma, urticaria, mastocytosis and melanoma.

Reflectance confocal microscopy-assisted diagnosis of lichen planus pigmentosus distributed along multiple unilateral Blaschko's lines in a child: A case report

A boy with a 2-year history of asymptomatic, linear pigmented macules involving the right side of the trunk and right upper limb. RCM revealed the dermal papillary rings were destroyed, and numerous irregular particulate structures with high refractive values were distributed in the superficial dermis. The RCM features implied the possibility of interface dermatitis. RCM was a complementary diagnostic tool for linear pigmented macules.

Pigmentary mosaicism as a recurrent clinical manifestation in three new patients with mosaic trisomy 12 diagnosed postnatally: cases report and literature review

Background

To date, only twenty-one cases diagnosed postnatally with mosaic trisomy 12 have been reported. The most frequent phenotypic manifestations are developmental delay, dysmorphic facial features, congenital heart defects, digital alterations, and pigmentary disorders. In the present report, detailed clinical and genetic profiles of three unrelated new patients with mosaic trisomy 12 are described and compared with previously reported cases.

Case presentation

In the present report, we include the clinical, cytogenetic, and molecular description of three Mexican patients diagnosed postnatally with mosaic trisomy 12. At phenotypic level, the three patients present with developmental delay, dysmorphic facial features, congenital heart defects and skin pigmentary anomalies. Particularly, patient 1 showed unique eye alterations as bilateral distichiasis, triple rows of upper lashes, and digital abnormalities. In patient 2 redundant skin, severe hearing loss, and hypotonia were observed, and patient 3 presented with hypertelorism and telecanthus. Hyperpigmentation with disseminated pigmentary anomalies is a common trait in all of them. The cytogenetic study was carried out under the strict criteria of analysis, screening 50–100 metaphases from three different tissues, showing trisomy 12 mosaicism in at least one of the three different tissues analyzed. With SNParray, the presence of low-level mosaic copy number variants not previously detected by cytogenetics, and uniparental disomy of chromosome 12, was excluded. STR markers allowed to confirm the absence of uniparental disomy as well as to know the parental origin of supernumerary chromosome 12.

Conclusions

The detailed clinical, cytogenetic, and molecular description of these three new patients, contributes with relevant information to delineate more accurately a group of patients that show a heterogeneous phenotype, although sharing the same chromosomal alteration. The possibility of detecting mosaic trisomy 12 is directly associated with the sensitivity of the methodology applied to reveal the low-level chromosomal mosaicism, as well as with the possibility to perform the analysis in a suitable tissue.

Clinical spectrum of MTOR-related hypomelanosis of Ito with neurodevelopmental abnormalities

Purpose

Hypomelanosis of Ito (HI) is a skin marker of somatic mosaicism. Mosaic MTOR pathogenic variants have been reported in HI with brain overgrowth. We sought to delineate further the pigmentary skin phenotype and clinical spectrum of neurodevelopmental manifestations of MTOR-related HI.

Methods

From two cohorts totaling 71 patients with pigmentary mosaicism, we identified 14 patients with Blaschko-linear and one with flag-like pigmentation abnormalities, psychomotor impairment or seizures, and a postzygotic MTOR variant in skin. Patient records, including brain magnetic resonance image (MRI) were reviewed. Immunostaining (n = 3) for melanocyte markers and ultrastructural studies (n = 2) were performed on skin biopsies.

Results

MTOR variants were present in skin, but absent from blood in half of cases. In a patient (p.[Glu2419Lys] variant), phosphorylation of p70S6K was constitutively increased. In hypopigmented skin of two patients, we found a decrease in stage 4 melanosomes in melanocytes and keratinocytes. Most patients (80%) had macrocephaly or (hemi)megalencephaly on MRI.

Conclusion

MTOR-related HI is a recognizable neurocutaneous phenotype of patterned dyspigmentation, epilepsy, intellectual deficiency, and brain overgrowth, and a distinct subtype of hypomelanosis related to somatic mosaicism. Hypopigmentation may be due to a defect in melanogenesis, through mTORC1 activation, similar to hypochromic patches in tuberous sclerosis complex.

Graphical Abstract

Identification of a novel mutation in the KITLG gene in a Chinese family with familial progressive hyper- and hypopigmentation

BACKGROUND

Familial progressive hyper- and hypopigmentation (FPHH, MIM 145250) is a rare hereditary skin disorder that is predominantly characterized by progressive, diffuse, partly blotchy hyperpigmented lesions intermingled with scattered hypopigmented spots, lentigines and sometimes Cafe-au-lait spots (CALs). Heterozygous mutations of the KIT ligand (KITLG, MIM 184745) gene are responsible for FPHH. To date, only eight KITLG mutations have been reported to be associated with FPHH, and no clear genotype-phenotype correlations have been established. This study aimed to identify the causative mutations in the KITLG gene in two Chinese FPHH patients.

METHODS

Direct sequencing of the coding regions of KITLG was performed. Pathogenicity prediction was performed using bioinformatics tools, including SIFT, Polyphen2, and SWISS-MODEL, and the results were further evaluated according to the 2015 American College of Medical Genetics and Genomics (ACMG) guidelines.

RESULTS

The novel mutation c.104A > T (p.Asn35Ile) and the recurrent mutation c.101C > T (p.Thr34Ile) in KITLG were identified. As shown using SIFT and Polyphen-2 software, both mutations identified in this study were predicted to be detrimental variations. Three-dimensional protein structure modeling indicated that the mutant KITLG proteins might affect the affinity of KITLG for its receptor, c-KIT. According to the 2015 ACMG guidelines, the novel mutation c.104A > T was 'likely pathogenic'.

CONCLUSIONS

To date, most of the identified KITLG mutations have been clustered within the conserved VTNNV motif (amino acids 33-37) in exon 2. The known mutations are only involved in 33 V, 34 T, 36 N, and 37 V but not 35 N. We have now identified a novel mutation in KITLG, c.104A > T, that was first reported in FPHH within the conserved 35 N motif. These results strengthen our understanding of FPHH and expand the mutational spectrum of the KITLG gene.

Mosaicism in genodermatoses

Genodermatoses are inherited disorders presenting with cutaneous manifestations with or without the involvement of other systems. The majority of these disorders, particularly in cases that present with a cutaneous patterning, may be explained in the context of genetic mosaicism. Despite the barriers to the genetic analysis of mosaic disorders, next-generation sequencing has led to a substantial progress in understanding their pathogenesis, which has significant implications for the clinical management and genetic counseling. Advances in paired and deep sequencing technologies in particular have made the study of mosaic disorders more feasible. In this review, we provide an overview of genetic mosaicism as well as mosaic cutaneous disorders and the techniques required to study them.

Linear and whorled nevoid hypermelanosis and Joubert syndrome: a novel association: A case report and literature review

This report discusses a case of linear and whorled nevoid hypermelanosis associated with cerebellar atrophy, ocular and developmental anomalies compatible with Joubert syndrome. Linear and whorled nevoid hypermelanosis is a rare disorder of skin pigmentation characterized by swirls and whorls of hyperpigmented macules in a reticulate pattern along Blaschko’s lines. Neurologic, cardiac, skeletal and developmental anomalies have been reported. We present a case of linear and whorled nevoid hypermelanosis on an 18-year-old woman who also presented with cerebellar atrophy, jerk nystagmus, macrocephaly and developmental delay. Those symptoms were compatible with Joubert syndrome. A complete work-up failed to reveal other systemic or skeletal anomalies. No chromosomal alteration was found on karyotyping carried out on a skin specimen. Much remains to be known about linear and whorled nevoid hypermelanosis. It is generally a benign condition but association with various congenital anomalies have been reported. Proper work-up is advised in order to exclude congenital anomalies.

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.