Hereditary hair loss and the ancient signaling pathways that regulate ectodermal appendage formation.

All epidermal appendages, including hair, teeth, and nails, begin as a thickening of the ectoderm, called a placode. The placode arises from a primary induction signal that is sent from the underlying mesenchyme to the overlying epidermis. In mammals, the precise arrangement of hair follicles in the skin is due to the amount and distribution of signals that promote and inhibit hair placode formation. Continued development of a hair follicle after placode formation requires a complex cross-talk between the mesenchyme and epidermis. Here, I will review recent studies in humans and mice that have increased our understanding of the role of these signaling pathways in normal development and in hereditary hair loss syndromes. The study of normal hair development may suggest ways to restore or eliminate hair and might identify possible targets for the therapy of basal cell carcinoma, a cancer which strongly resembles embryonic hair follicles.

Disruption of an imprinted gene cluster by a targeted chromosomal translocation in mice.

Genomic imprinting is an epigenetic process in which the activity of a gene is determined by its parent of origin. Mechanisms governing genomic imprinting are just beginning to be understood. However, the tendency of imprinted genes to exist in chromosomal clusters suggests a sharing of regulatory elements. To better understand imprinted gene clustering, we disrupted a cluster of imprinted genes on mouse distal chromosome 7 using the Cre/loxP recombination system. In mice carrying a site-specific translocation separating Cdkn1c and Kcnq1, imprinting of the genes retained on chromosome 7, including Kcnq1, Kcnq1ot1, Ascl2, H19 and Igf2, is unaffected, demonstrating that these genes are not regulated by elements near or telomeric to Cdkn1c. In contrast, expression and imprinting of the translocated Cdkn1c, Slc22a1l and Tssc3 on chromosome 11 are affected, consistent with the hypothesis that elements regulating both expression and imprinting of these genes lie within or proximal to Kcnq1. These data support the proposal that chromosomal abnormalities, including translocations, within KCNQ1 that are associated with the human disease Beckwith-Wiedemann syndrome (BWS) may disrupt CDKN1C expression. These results underscore the importance of gene clustering for the proper regulation of imprinted genes.

Optimizing the detection of nascent transcripts by RNA fluorescence in situ hybridization.

An unusual feature of the mammalian genome is the number of genes exhibiting monoallelic expression. Recently random monoallelic expression of autosomal genes has been reported for olfactory and Ly-49 NK receptor genes, as well as for Il-2, Il-4 and Pax5. RNA fluorescence in situ hybridization (FISH) has been exploited to monitor allelic expression by visualizing the number of sites of transcription in individual nuclei. However, the sensitivity of this technique is difficult to determine for a given gene. We show that by combining DNA and RNA FISH it is possible to control for the hybridization efficiency and the accessibility and visibility of fluorescent probes within the nucleus.

Dosage requirement and allelic expression of PAX6 during lens placode formation.

Pax6 is a member of the mammalian Pax transcription factor family. Many of the Pax genes display semi-dominant loss-of-function heterozygous phenotypes, yet the underlying cause for this dosage requirement is not known. Mice heterozygous for Pax6 mutations exhibit small eyes (Sey) and in embryos the most obvious defect is a small lens. We have studied lens development in Pax6(Sey)(-1Neu)/+ embryos to understand the basis of the haploinsufficiency. The formation of the lens pre-placode appears to be unaffected in heterozygotes, as deduced from the number of cells, the mitotic index, the amount of apoptosis and the expression of SOX2 and Pax6 in the pre-placode. However, the formation of the lens placode is delayed. The cells at the edge of the lens cup fail to express N-cadherin and undergo apoptosis and the lens fails to detach completely from the surface ectoderm. After formation, the lens, which has 50% of the cells found in wild-type embryos, grows at a rate that is indistinguishable from wild type. We rule out the possibility that monoallelic expression of Pax6 at the time of lens placode formation accounts for the 50% reduction in cell number by showing that expression of Pax6 is biallelic in the lens placode and optic vesicle. We propose instead that a critical threshold of PAX6 protein is required for lens placode formation and that the time in development at which this level is reached is delayed in heterozygotes.