Genetics of diverse phenotypes in Hirschsprung disease - extension of aganglionosis, heredity and medullary thyroid carcinoma

The aim of this doctoral thesis was to identify the genetic variations of Hirschsprung disease (HD) among Finnish HD patients. How the different genotypes affect different HD phenotypes, such as the extension of aganglionosis, heredity, and HD-associated medullary thyroid cancer (MTC) was also explored. The entire patient cohort consisted of 112 HD patients. Of them, 91 adult HD patients participated the screening study for thyroid cancer. The participants went through a cross-sectional study where sequencing of the coding region of the RET gene was combined with blood tests and ultrasound of the thyroid gland. If necessary, a fine needle biopsy was also performed. An additional 21 familial HD patients participated in the second stage of the study where diverse genetic analyses were performed. In the second stage of the study, the material was analyzed comprehensively using four complementary methods. Of these, capillary sequencing of RET exons and targeted sequencing of all genes known to be associated with HD was targeted to those gene loci that have shown an association with HD in previous studies. Additionally, a genome wide association study (GWAS) and whole-exome sequencing were performed to examine the whole genome. In the screening study of thyroid cancer, two cases of MTC and one case of papillary thyroid cancer were observed. Both patients with MTC had RET variants, [p.Cys611Arg] and [p.Cys620Arg], which are known to be associated with MTC. In addition, MTC-associated RET variants were observed in four patients with no clinical signs of thyroid cancer; these patients were selected for follow-up. Based on this study, the risk of MTC among HD patients is over 300 times higher than in the normal population. The sequencing of all RET exons revealed 10 rare variants that affected gene function in 16 (14%) patients. An EDNRB frameshift variant was identified in two patients from the same family using whole-exome sequencing. GWAS confirmed the strong association of the RET gene with HD. About half of the cases in our entire sample may be statistically attributed to the common non-coding RET variants. Sequencing of all genes known to be associated with HD revealed 10 variants affecting gene function in nine patients. Overall, performing extensive gene analysis revealed a coding sequence variant affecting gene function in 31 (28%) patients from whole study cohort. Screening and gene testing are important to significantly improve the diagnostics, treatment planning, and patient counseling of HD. In particular, patients with genetic defects associated with an increased risk of thyroid cancer can be identified and treated in a timely manner.