Abstract:
The ectoderm is the outermost layer of the developing embryo that gives rise to epidermis
and its appendages. Ectodermal appendages share a common developmental program that
relies on extensive coalition amongst epithelia and nearby mesenchyme. The genetic
pathways involved in the regulation of appendage development are widely conserved
between species and different appendages.
In the present investigation nine consanguineous, multigeneration families (A-I)
segregating autosomal recessive hereditary disorders of ectodermal appendages were
ascertained from different zones of Pakistan. Homozygosity mapping via polymorphic
microsatellite markers was used to elucidate the gene defect in eight families while for one
family microarray technology was used.
In family A, affected individuals showed characteristic clinical features of hereditary
hypotrichosis including sparse hair on the scalp and rest of the body. Interestingly,
eyebrows and eyelashes were not affected in any of the affected individuals. Human
genome scan using 400 highly polymorphic microsatellite markers mapped the disease
locus to a large region on chromosome 10. This novel locus maps to 29.18 cM (28.50 Mb)
region, flanked by markers D10S538 and D10S2327 on chromosome 10q11.23–q22.3. A
maximum multipoint LOD score of 3.26 was obtained at recombination fraction zero with
several markers in this region. Exons and splice-junction sites of five putative candidate
genes (DKK1, MYST4, ZMYND17, P4HA1, ZNF365), located in the linkage interval, were
sequenced but were negative for functional sequence variants and therefore their
involvement in causing hypotrichosis phenotype at the present novel locus is not
supported.
In family B, affected individuals showed clinical features of autosomal recessive
hypotrichosis (LAH2) including thin fragile hair on scalp, sparse to absent eyebrows and
eyelashes, and sparse hair on rest of the body. Morphological inspection of hairs by light
microscopy did not reveal any nodes or constriction. Histopathological examination of the
biopsy showed complete absence of normal hair follicle structures. Genotyping data
showed linkage of the family to LIPH gene on chromosome 3q27.3. Sequence analysis of
LIPH gene revealed a novel homozygous missense mutation (c.2T>C, p.M1T) in all the
affected individuals.Three families (C, D, E) showed archetypal clinical features of autosomal recessive
hypotrichosis (LAH3). The patients showed sparse to absent twisted scalp hair, sparse
eyebrows and eyelashes, sparse axillary and body hair. Genotyping data showed linkage of
the families to LPAR6 gene on chromosome 13q14.2. Sequence analysis of the LPAR6
gene revealed one novel homozygous missense mutation (c.8G>C, p.S3T) in family C,
and
two
previously
reported
mutations
(c.436G>A,
p.G146R;
c.69insCATG,
p.24insHfsX29) in the other two families (D and E).
In family F, affected individuals showed clinical phenotypes of autosomal recessive
hypotrichosis (LAH1). Out of 12 individuals over four generations, only two females were
found to be affected. They showed complete loss of scalp hairs and eyebrows, however
very thin eyelashes were present. Genotyping data showed linkage of the family to
desmoglein and desmocollin gene cluster on chromosome 18q12.1. Sequence analysis of
six candidate genes (DSG1-4, DSC1, DSC3) failed to detect any functional sequence
aberration, that could be responsible for the disease phenotype in this family.
Family G showed clinical features of atrichia with papular lesions. Patients showed
complete hair loss of scalp, absent eyebrows and eyelashes, loss of axillary and pubic hair.
Genotyping results showed linkage of the family to human hairless gene (HR) located on
chromosome 8p21.3. Sequence analysis of the coding exons and splice junction sites of
the HR and its upstream region (U2HR) failed to detect any functional sequence variant.
In family H, affected individuals showed isolated nail dysplasia phenotype; thickening and
hyperpigmentation of all finger- and toenails that become claw like around teenage.
Genome scan using Illumina‟s Human 660W-quad chip was performed that showed a
single homozygous region on chromosome 8q22.3. In parallel sequence analysis of five
selected candidate genes (ATP6V1C1, BAALC, CTHRC1, KLF10, FZD6) in the linked
region revealed the disease causing homozygous nonsense mutation (c.1750G>T,
p.E584X) in the frizzled 6 (FZD6) gene, a member of Wnt signaling pathway. Expression
analyses in nail sections showed a strong expression of FZD6 in the ventral nail matrix
and to lesser extent, the nail bed. Manuscript describing “FZD6 encoding the Wnt receptor
frizzled-6 is mutated in autosomal recessive nail dysplasia” was submitted for publication
to American Journal of Human Genetics. However, before the manuscript could be sent
for review, another group published FZD6 mutation paper in the same journal. Now the
present manuscript has been submitted for publication to British Journal of Dermatology.In family I, affected individuals showed clinical features of ectodermal dysplasia (ED) of
hair and nail type; characterized by total or partial hair loss and dystrophy of finger- and
toenails since birth. Genotyping data showed linkage of the family to type II keratin gene
cluster on chromosome 12p11.22–12q14.1. This region corresponds to 30.92 Mb (22.34
cM) according to the sequenced based physical map (Build 36.2) of the human genome
and flanked by markers D12S1631 and D12S298. A maximum two-point LOD score of
3.59 was obtained with three markers while maximum multipoint LOD score exceeding
4.0 was obtained with nine markers along the disease interval at recombination fraction
zero. KRT85 has previously been reported as a causative gene for ED of hair and nail type
at chromosome 12q13.13. KRT85 along with twelve other genes (KRT81, KRT82, KRT83,
KRT84, KRT86, KRT6C, KRT71, KRT72, KRT74, KRT75, KRT78, WNT10B) were
sequenced. However sequence analysis failed to detect any functional sequence variants.