A phenocopy of CAII deficiency: a novel genetic explanation for inherited infantile osteopetrosis with distal renal tubular acidosis
- K J Borthwick1,
- N Kandemir2,3,
- R Topaloglu2,4,
- U Kornak5,
- A Bakkaloglu2,3,
- N Yordam2,3,
- S Ozen2,4,
- H Mocan6,
- G N Shah7,
- W S Sly7,
- F E Karet1,8
- 1Department of Medical Genetics, Cambridge University, Cambridge, UK
- 2Faculty of Medicine, Hacettepe University, Ankara, Turkey
- 3Division of Paediatric Endocrinology and Rheumatology, Hacettepe University, Ankara, Turkey
- 4Division of Paediatric Nephrology, Hacettepe University, Ankara, Turkey
- 5Zentrum fur Molekulare Neurobiologie Hamburg, Hamburg University, Hamburg, Germany
- 6Department of Paediatrics, Metropolitan Florence Nightingale Hospital, Istanbul, Turkey
- 7E A Doisy Department of Biochemistry, St Louis University School of Medicine, St Louis, Missouri, USA
- 8Division of Nephrology, Cambridge University, Cambridge, UK
- Correspondence to: Dr F E Karet, Cambridge Institute for Medical Research, Box 139, Addenbrooke’s Hospital, Cambridge CB2 2XY, UK; fek1000{at}cam.ac.uk
- Accepted 10 October 2002
- Revised 9 October 2002
Abstract
The rare bone thickening disease osteopetrosis occurs in various forms, one of which is accompanied by renal tubular acidosis (RTA), and is known as Guibaud-Vainsel syndrome or marble brain disease. Clinical manifestations of this autosomal recessive syndrome comprise increased bone density, growth failure, intracerebral calcification, facial dysmorphism, mental retardation, and conductive hearing impairment. The most common cause is carbonic anhydrase II (CAII) deficiency. Several different loss of function mutations in CA2, the gene encoding CAII, have been described. To date, there have been no exceptions to the finding of CAII deficiency in patients with coexistent osteopetrosis and RTA. Most often, the RTA is of mixed proximal and distal type, but kindreds are reported in which either distal or proximal RTA predominates.
We report the molecular genetic investigation of two consanguineous kindreds where osteopetrosis and distal RTA (dRTA) were both manifest. One kindred harbours a novel homozygous frameshift alteration in CA2. In the other, CAII levels were normal despite a similar clinical picture, and we excluded defects in CA2. In this kindred, two separate recessive disorders are penetrant, each affecting a different, tissue specific subunit of the vacuolar proton pump (H+-ATPase), providing a highly unusual, novel genetic explanation for the coexistence of osteopetrosis and dRTA. The osteopetrosis is the result of a homozygous deletion in TCIRG1, which encodes an osteoclast specific isoform of subunit a of the H+-ATPase, while the dRTA is associated with a homozygous mutation in ATP6V1B1, encoding the kidney specific B1 subunit of H+-ATPase.
This kindred is exceptional firstly because the coinheritance of two rare recessive disorders has created a phenocopy of CAII deficiency, and secondly because these disorders affect two different subunits of the H+-ATPase that have opposite effects on bone density, but which have only recently been determined to possess tissue specific isoforms.
