Helena G. Karstensen vart ph.d. á Københavns Universitet

Fríggjadagin 18. september vardi Helena Gásdal Karstensen ph.d. ritgerð sína við Panum Instituttet í København. Ritgerðin kallast: “Towards the genetic basis of hereditary anosmia in the Faroe Islands”.

Ritgerðin snýr seg i høvðsheitum um kanningar av genetiskum orsøkum til viðføddan vantandi luktisans. Hetta brekið kallast Isolated congenital anosmia (ICA).

Helena G. Karstensen hevur í sinum arbeiði kanna fólk í Føroyum og fólk í norðurevropa við ICA, bæði fyri at fáa definera, hvat slag av ICA føroyingarnir hava og fyri at finna møguligar genetiskar samanhangir.

21 føroyingar við ICA hava verið við í kanningini umframt frískir kontrolpersónar. Kanningar í ættarbandsskránni hjá Ílegsavninum hava staðfest, at teir 21 føroyingarnir stava frá sama forfedri í 19. øld.

Gentisku kanningarnar hava staðfest strukturellar broytingar í heilanum og genetiskar mutatiónir í sambandi við ICA.

Vegleiðari hevur verið Niels Tommerup, professari dr.med. við Wilhelm Johannsen centret for funktionel genom forskning, afdeling for cellulær og molekylær medicin, , Københavns Universitet.

Í metingarnevndini vóru:
Lisbeth Tranebjærg, professari, Audiologisk Afdeling, Bispebjerg Hospital/Rigshospitalet
Torben Kruse, professari og greanskingarleiðari, Human Genetik og Klinisk Biokemi, Syddansk Universitet
Anette Drøhse Kjeldsen, kliniskur professari, Klinisk Institut, Oto-Rhino-Laryngology, Syddansk Universitet

Verjan gekst væl og metingarnevndin lat væl at arbeiðinum, sum tey mettu vera spennandi.

Verkætlanin hevur verið lutvíst fíggjað úr Granskingargrunninum. Les um verkætlanina í Stuðulsyvirlitinum 

Summary
Isolated congenital anosmia (ICA) is a rare condition with an estimated frequency of 1:2.000-1:10.000, where otherwise healthy individuals present with absence of smell since birth. Investigations in the congenital blind and deaf have provided insight to the function of the sensory systems of vision and hearing, respectively. In analogy ICA offers a unique opportunity to gain insight into the function of the olfactory system, of which we know relatively little. The genetic unraveling of ICA is still in its infancy as is the understanding of the underlying pathophysiological mechanisms. Congenital anosmia and hyposmia (decreased sense of smell) has been described in several syndromes, where Kallmann syndrome is the best defined. By magnetic resonance imaging (MRI), visualization of the olfactory system, including the olfactory bulbs and olfactory sulci is possible. Individuals with ICA have previously been characterized by diminished olfactory bulbs and flattened olfactory sulci along with structural abnormalities in pirifom and orbitofrontal cortex.

The subjects included in this study consisted of individuals with ICA or isolated congenital hyposmia (ICH) from the Faroe Islands (Cohort A) and individuals with ICA ofNorthern European descent (Cohort B). The aims of this thesis were twofold: i) to define the phenotype in Cohort A by a combination of functional tests and structural MRI, and ii) to search for genetic factors involved in the pathogenesis of ICA and ICH in Cohort A and B.

In cohort A, participants were recruited through local Faroese radio and newspapers, and a total of 21 individuals with ICA or ICH were included in the study along with healthy Faroese control subjects. Affected individuals were identified in four branches of the original family and four cases were apparently sporadic; however, using the Faroese genealogy database the four cases could be traced back to a common ancestor in the 19th century. Of the 21 affected individuals, 18 were examined using MRI to assess structural brain changes in olfactory bulb volume, olfactory sulcus size and cortical gray matter volume. Morphometric measurements showed that ICA and ICH was associated with diminished olfactory bulb volume and olfactory sulcus size. Furthermore, ICA was associated with increased grey matter volume in the right piriform cortex and decreased volume in the left orbitofrontal cortex. We further observed structural brain alterations in the medial orbital gyrus within the secondary olfactory cortex, in the middle frontal gyrus and in the medial occipital cortex.

Initial genetic testing included exclusion of structural variations by karyotyping and disease segregating copy number variants assessed by genome-wide array genotyping (Affymetrix SNP 6.0). No homozygous regions shared among affected individuals were identified. To narrow down a linkage region two branches of the family were selected for linkage analysis assuming dominant inheritance resulting in positive regions on chromosome 5, 10, 12, 13 and 21, totalling 63 Mb in one branch, and regions corresponding to 90,6 Mb on chromosome 2, 4, 5, 7, 9, 14, 16 and 20 in the other branch. However, no regions were overlapping between the two branches of the family, suggesting that more than one locus could be involved in the pathogenesis. To search for mutations within the protein coding part of the genome we carried out whole-exome sequencing in 11 selected individuals. Mutations in the known Kallmann syndrome genes were excluded, in support of a novel genetic mechanism involved in the disorder; however, no non-synonymous disease segregating variants were identified.

In Cohort B a total of 38 individuals with ICA and one with ICH were recruited, divided into five small nuclear families and 30 isolated cases. Array based screening for rare disease causing copy number variants (CNVs; Affymetrix SNP 6.0) identified a heterozygous deletion spanning the promoter region and first coding exon of RARB (chr3:25439824-25464322, hg19) in two siblings with ICA and their affected father. Furthermore, whole-exome sequencing was carried out in six selected individuals. In two brothers with ICA we identified a single nucleotide substitution (c.634C>T) resulting in a stop-codon (p.R212*) in CNGA2 on the X-chromosome. The variant was subsequently identified in the heterozygous state in the mother, who had a normal sense of smell.

In conclusion, we report a unique cohort with ICA and ICH from the Faroe Islands and describe structural brain changes and the ongoing efforts to unravel the genetic cause(s) of the disorder (Cohort A). Furthermore, we identified a likely disease causing deletion of RARB in a small family with ICA and ICH. The possible involvement of the retinoic acid receptor RARB in the pathophysiology of congenital olfactory deprivation is ongoing. Finally, we identified a stop mutation in CNGA2 in two brothers with ICA. An ICA knock out mouse model for Cnga2 exists and the gene has been classified as the highest ranking ICA candidate gene. Thus, the p.R212* variant identified in this study is likely the first mutation described in humans with ICA.