Endocrine Abstracts

X-linked recessive hypoparathyroidism (XLHPT), due to congenital parathyroid agenesis, has been reported in two related kindreds from Missouri, USA. Affected individuals, who are males, suffer from epilepsy due to hypocalcaemia during infancy, whilst the females are normocalcaemic. Studies have mapped XLHPT to chromosome Xq27 and defined a 1.5 Mbp interval flanked centromerically by Factor IX and telomerically by DXS984. DNA sequence analysis of 4 candidate genes (proto-dbl, AS6, U7snRNA and SOX3) has not revealed abnormalities, and the occurrence of deletions of this entire critical region in some Haemophilia B patients who do not have XLHPT, indicated that other mechanisms may cause XLHPT. We therefore embarked on characterising this region further by single nucleotide polymorphism analysis, pulsed field gel electrophoresis, metaphase and DNA fibre-fluorescence in situ hybridisation (FISH) analysis. These studies revealed the combined occurrence of a <30Kb deletion with a >75Kb insertion. This deletional-insertion co-segregated with XLHPT (peak LOD score = 7.5 at 0% recombination). Combined use of a flow-sorted X chromosome specific library generated from an affected male, a modified vectorette PCR protocol, and DNA sequence analysis enabled both breakpoints of this deletional-insertion to be characterised, and additional use of a mono-chromosomal somatic cell hybrid DNA panel, and metaphase-FISH revealed the insertion to originate from chromosome 2p25. The use of YACs derived from 2p25 in DNA fibre-FISH analysis indicated that the insertion was 500-700Kb in size. Thus, XLHPT is caused by a molecular deletional-insertion involving chromosomes Xq27 and 2p25. This represents a novel genetic abnormality causing hypoparathyroidism, which will advance our understanding of the molecular basis of parathyroid development.