Molecular genetics of rare growth and puberty disorders in Finland

Growth and pubertal development are complex and interconnected processes, disruption of which leads to abnormal development of the adult height or secondary sexual characteristics or both, and often causes notable distress and even adverse health effects for the individual. Growth and pubertal development are both dependent on hormones secreted from the pituitary gland. Growth hormone (GH), secreted from the pituitary somatotropes, is required for growth of the bones and cartilage and achievement of the adult height. Formation of GH-secreting pituitary tumors, somatotropinomas, leads to excessive GH secretion and acromegaly or gigantism, which are both characterized by exaggerated growth, either at peripheral body parts or at the long bones depending on the onset of GH excess. In a proportion of cases, a germline gene defect can predispose to somatotropinoma formation. The ability to reproduce is achieved in puberty once the sex organs and other sexual characteristics mature into the adult form. The onset of pubertal development occurs upon the reactivation of the hypothalamic-pituitary-gonadal axis after quiescency following the previous activation phase in infancy. In the pubertal reactivation, increased gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus triggers the secretion of gonadotropins from the pituitary. Premature activation of gonadotropin secretion leads to central precocious puberty (CPP), where the pubertal development begins before the age of eight in girls or the age of nine in boys. In turn, deficient gonadotropin secretion leads to congenital hypogonadotropic hypogonadism (CHH), characterized by delayed, absent, or partial puberty. If defective sense of smell co-occurs with CHH, the condition is named Kallmann syndrome (KS). Despite multiple genes are implicated in the disorders of pituitary hormone secretion, a great proportion of patients miss a molecular genetic diagnosis. The aim of this thesis was to discover defects in specific genes and evaluate their roles in disorders of growth and pubertal development, which originate from aberrant GH or gonadotropin secretion. The disorders were gigantism, acromegaly, CPP, and CHH. Variants in the potassium channel gene KCNQ1 have been previously implicated in growth hormone deficiency, and co-expression of the mutated KCNQ1 with the potassium channel subunit KCNE2 has decreased adrenocorticotropin secretion from a pituitary tumor cell line. KCNQ1 and KCNE2 were screened for germline variants in 49 Finnish patients and four patients of other ethnicities with acromegaly, who represented the phenotypic model opposite to growth hormone deficiency. In KCNQ1, deep intronic and common synonymous variants were identified, and one heterozygous variant with unknown significance in KCNE2 was found in three patients. The frequency of the KCNE2 variant was significantly higher among the patients compared to controls. Two Polish and one Finnish CPP patient as well as their family members were screened for variants in MKRN3, a maternally imprinted gene suggested to function as a pubertal brake. Novel, deleterious variants segregating with CPP in a paternally inherited manner were identified in both families. The first MKRN3 variant in Finnish CPP patients and the first long-term effects of a variant in this gene in a boy with CPP are described. Twenty-four Finnish patients with normosmic CHH or KS were screened for variants in the microRNA genes MIR7-3, MIR141, MIR429, and MIR200A-C, which were predicted to regulate CHH-related genes based on evidence from animal models, literature, or bioinformatic analyses or all. A common, heterozygous variant in MIR200A was detected in one patient. The genetic basis of KS in a Finnish patient with a de novo 2.38 Mb deletion in 9q31.2 and no likely pathogenic variants in a KS gene targeted sequencing panel was investigated with whole genome linked-read sequencing, whole exome sequencing, and RNA sequencing. In the whole genome linked-read sequencing, the deletion was found to encompass six protein-coding genes, including ZNF462, consistent with his Weiss-Kruszka syndrome. The deletion did not cover the nearby KS candidate gene PALM2AKAP2, expression of which was not suppressed by the deletion. The patient carried no rare variants in thirty-two known KS genes in the whole exome sequencing and displayed no abnormal splicing of fifteen KS genes expressed in peripheral blood leukocytes. He is the first reported patient with a 9q31.2 deletion, KS, and Weiss-Kruszka syndrome. Screening of sixteen other Finnish KS patients for variants in PALM2AKAP2 revealed no likely pathogenic defects in this gene. In conclusion, the thesis produced new information on the association of KCNQ1, KCNE2, and the selected microRNA gene variants with disorders of aberrant pituitary hormone secretion. The results demonstrate that germline variants in KCNQ1 or KCNE2 do not seem to account for somatotropinoma formation and that variants in the microRNA genes are unlikely causes of CHH. In turn, deletions in 9q31.2 appear to underlie KS, but based on the results, variants in the KS candidate gene PALM2AKAP2 do not seem to contribute to the condition in the investigated cohort. In addition, novel variants in MKRN3 were identified, and they were found to underlie CPP in Finnish patients for the first time. Finally, an interesting finding was that male carriers of MKRN3 variants may reach their target height without treatment.