Is hair loss a matter of heredity?

To date, various genetic forms of hair loss have been identified. The genes responsible for most of these disorders have been pinpointed. Hereditary abnormalities in hair growth manifest either as hypertrichosis (excess hair) or hypotrichosis (lack of hair). We have chosen to focus primarily on hypotrichosis, which results in hair loss, and to present a few forms of it in this article.

Androgenetic Alopecia.

Androgenetic alopecia is the most common form of hair loss in humans, typically resulting in baldness. It is estimated to affect a third of men by the age of 30 and nearly half by the age of 50, although women can also be affected.

HAMILTON was the first to identify the two causative factors involved in the development of androgenetic alopecia: a genetic predisposition (transmissible genetic mutations) and a hormonal dysfunction that accelerates the rate of hair renewal.

Studies have shown that changes in the sequence of the AR gene, a gene that codes for the androgen receptor, are found in patients suffering from androgenetic alopecia. Two other genes have also been identified to induce androgenetic alopecia: the EDA2R gene involved in hair maintenance and the OPHN1 gene, both located on the X chromosome.

Type 1 Simplex Hypotrichosis.

This refers to a rare autosomal dominant form of hereditary hair loss disorder. Progressing with age, those affected undergo hair loss and thinning of the hair shaft. It can be divided into 2 forms: the form limited to the scalp and the generalised form.

The generalised form is caused by genetic modifications of the APCDD1 gene. This gene comprises five exons and spans a region of 40 kb on chromosome 18p11.32-p11.23. Functional studies have revealed that APCCD1 operates upstream of β-catenin and exhibits inhibitory effects on Wnt signalling.

This signalling pathway is crucial for the fate of epithelial stem cells, as keratinocytes adopt an epidermal destiny in the absence of β-catenin. Thus, by inhibiting this pathway, the number of hair follicles decreases in the anagen phase.

Type 2 Simple Hypotrichosis of the Scalp.

This scalp condition is characterised by a cessation of hair growth. Individuals suffering from this pathology exhibit thinning hair that appears at birth or in early childhood and continues into adulthood. It only affects the hair and does not impact other body hair. It is a rare autosomal dominant form of hereditary hair loss disorder.

Mutations on the CDSN gene located on chromosome 6p21.3, which codes for corneodesmosin, appear to be responsible. Corneodesmosin is a late-differentiating epidermal protein, acting as an adhesion molecule for keratinocytes. This gene is essential in preserving the skin barrier by maintaining desmosomal integrity. Therefore, it seems to play a role in maintaining capillary architecture and thus limiting hair loss.

Atrichia with papular lesions.

This refers to a rare autosomal recessive inherited form of alopecia characterised by irreversible hair loss in all hair-bearing areas of the body during the neonatal period. The HR gene, also known as the "hairless human gene", located on chromosome 8p21, is the cause of this condition.

The HR mRNA is present throughout the hair life cycle, but the follicles involved in active hair growth do not contain detectable HR proteins. It has therefore been deduced that this gene plays a role in the cellular transition to the first adult hair cycle, and in its absence, the hair follicles disintegrate and new hair is never induced.

Other forms of hereditary hair loss disorders exist. Generally, they are linked to mutations on genes that affect hair follicles, which are passed on to offspring, promoting the onset of a form of alopecia.