Homozygous Mutations in Thyroid Peroxidase (TPO) in Hypothyroidism with Intellectual Disability, Developmental Delay, and Hearing and Ocular Anomalies in Two Families: Severe Manifestation of Untreated TPO-deficiency Poses a Diagnostic Dilemma

Abstract

Intellectual disability (ID) involves compromised intellectual, learning and cognitive skills, and behavioral capabilities with reduced psychomotor skills. One of the preventable causes of ID is congenital hypothyroidism (CH), which may be due to biallelic mutations in thyroid peroxidase (TPO). In low- and middle-income countries with no newborn screening programs, CH poses a great risk of ID and long-term morbidity. We report two large Pakistani families with a total of 16 patients afflicted with CH. Detailed clinical and behavioral assessments, SNP-based homozygosity mapping, linkage analysis, and exome sequencing were performed. Initially, affected individuals were referred as suffering ID (in 11 of 16 patients) and developmental delay (in 14). Secondary/associated features were verbal apraxia (in 13), goiter (in 12), short stature (in 11), limb hypotonia (in 14), no pubertal onset (five of 10 of age ≥14 years), high myopia (in eight), muscle cramps (in six), and in some, variable microcephaly and enuresis/encopresis, fits, chronic fatigue, and other behavioral symptoms, which are not characteristics of CH. Molecular genetic analyses led to the discovery of homozygous variants in TPO: novel missense variant c.719A>G (p.Asp240Gly) in family 1 and rare c.2315A>G (p.Tyr772Cys) in family 2. In low-resource countries where neonatal screening programs do not include a CH test, the burden of neurodevelopmental disorders is likely to be increased due to untreated CH. Secondly, in the background of the high prevalence of recessive disorders due to high parental consanguinity, the severe manifestation of TPO-deficiency mimics a wide range of neurological and other presentations posing a diagnostic dilemma.