In this study, we identified and characterized the first patient with complete deficiency of GPIHBP1 due to homozygosity for a mutation that deletes the entire GPIHBP1
gene. The patient presented at 2 months of age with extremely severe hypertriglyceridemia (>25,000 mg/dl), comparable with that found in the worst cases of LPL deficiency. Thus, in contrast to mice, in which the hypertriglyceridemia associated with GPIHBP1 deficiency is modest prior to weaning and becomes progressively more severe with age (Beigneux et al. 2007
), GPIHBP1 deficiency is fully manifest in human infants. Prolonged infusion of heparin has been shown to reduce plasma triglyceride levels in GPIHBP1-deficient individuals (Franssen et al. 2010
), but infusion of a bolus of heparin into the two GPIHBP1-deficient patients in our study confirmed that acute heparin administration does not significantly lower plasma triglyceride levels. Our results do not rule out the possibility that higher doses or more prolonged administration of heparin might cause a reduction in plasma triglyceride level, as has been reported previously (Franssen et al. 2010
), although chronic high-dose heparin treatment is accompanied by other significant risks.
Development has proceeded normally in patient 1 in our study, and sequelae unrelated to hypertriglyceridemia have not been noted in this patient or in other GPIHBP1-deficient patients. Similarly, abnormalities reported to date in mice lacking GPIHBP1 are all attributable to defects in LPL function (Beigneux et al. 2007
). Young and colleagues found that GPIHBP1 is readily identifiable in many mammalian species, including monotremes, but not in fish, amphibians, or birds (Beigneux et al. 2009a
). The recent appearance of GPIHBP1 in vertebrate evolution is also consistent with a highly specific function for the protein. These findings, together with the observation that distribution of GPIHBP1 expression across tissues in general parallels that of LPL (Beigneux et al. 2009a
) suggests that GPIHBP1 does not have significant biological activity independent of its role in LPL action.
Defining the extent of the physiological roles of GPIHBP1 has important therapeutic implications. In humans, the phenotype of GPIHBP1 deficiency appears indistinguishable from that of LPL deficiency, whereas in mice, the hypertriglyceridemia is milder at birth in the Gpihbp−/−
mice than in the Lpl−/−
mice (Beigneux et al. 2007
). If the protein functions solely to facilitate hydrolysis of circulating triglycerides in humans, then GPIHBP1 deficiency should be fully treatable by lowering plasma triglyceride levels.
The release of LPL into the circulation by heparin is accompanied by a dramatic reduction in plasma triglyceride levels (Korn 1955
). The finding that heparin administration also significantly reduces plasma triglyceride levels in Gpihbp1
-deficient mice raised the possibility that heparin could be a potential therapy for GPIHBP1-deficient patients (Weinstein et al. 2008
). A recent study found that plasma triglyceride levels could be markedly reduced by a 6-h heparin infusion (from 1,780 to 534 mg/dl) but that little reduction was seen at earlier time points (Franssen et al. 2010
). When heparin was given to GPIHBP1-deficient patients as a bolus, little LPL was released into the circulation (Franssen et al. 2010
; Olivecrona et al. 2010
) and data from our study confirm that a single bolus of heparin does not significantly lower plasma triglyceride levels in these patients.
The deletion in patient 1 involved two 7-bp sequences that flank GPIHBP1
and are identical in sequence. The upstream breakpoint is located in a mammalian interspersed repeat (MIR), whereas the 3′ breakpoint is located 294 bp downstream of another MIR repeat. MIR repeat elements contain a tRNA-like domain and a “core” domain, followed by a variable long interspersed element (LINE)-like region (Gilbert and Labuda 1999
). MIR repeats have been documented to be directly involved in, or adjacent to, breakpoints of deletions that cause a variety of other diseases, including polycystic kidney disease (Bergmann et al. 2005
), phenylketonuria (Kozak et al. 2006
), maturity-onset diabetes of the young (MODY) (Ellard et al. 2007
), and congenital afibrinogenemia (Spena et al. 2004
). It is possible that the deletion observed in patient 1 occurred as a result of MIR-mediated nonhomologous end joining (Shaw and Lupski 2005
; Toffolatti et al. 2002
The parents of patient 1 were not known to be related but were both from a small town within Gujarat state in northwestern India. The high level of endogamy within Indian subpopulations has resulted in distinct genetic diversity between ethnicities (Indian Genome Variation Consortium 2008
) and a high prevalence of founder mutations within particular communities (Colah et al. 2010
; Zaidi et al. 2009
). The region of homozygosity surrounding the GPIHBP1
deletion in patient 1 extended 6.1 Mb, which is greater than expected when compared with individuals of European ancestry. The presence of longer-than-expected homozygous genomic segments in patient 1 may reflect the local genetic substructure. The multilineal inheritance of the deletion in this family is consistent with this view. To more fully address this question would require analysis and comparison of additional individuals from this region of India.
We also identified an individual with chylomicronemia who was homozygous for a C68Y mutation in GPIHBP1
that was previously identified in a Spanish woman with chylomicronemia (Coca-Prieto et al. 2011
). This mutation is one of six in GPIHBP1
that have been identified in patients with chylomicronemia (Table ). A seventh mutation, G56R, is likely not to be a causative mutation for chylomicronemia, as reported (Gin et al. 2007
; Wang and Hegele 2007
). All mutations, excluding the gene deletion described in this study, are missense mutations involving residues in the Ly6 domain of the protein. The Ly6 domain contains ten highly conserved cysteine residues that are predicted to form a three-fingered structure, based on structural studies of other proteins with Ly6 domains (Huang et al. 2007
; Kjaergaard et al. 2008
). Substitution of alanine for any of these cysteines, including C68, does not interfere with trafficking of GPIHBP1 in cultured cells but abolishes LPL binding in a cell-free assay (Beigneux et al. 2009c
). Olivicrona et al. (2010
) identified three siblings who were compound heterozygotes for mutations in two different cysteine residues (C65S and C68G), and both of these mutations were also associated with failure to bind LPL.
The deletion of GPIHBP1
described in this study was identified using microarray technology that provides efficient high-resolution screening of copy number variation across the entire human genome. Historically, copy number variations have primarily been associated with sporadic congenital syndromic disorders, but submicroscopic variations play an important role in several Mendelian disorders. Some 5% of neurofibromatosis cases are caused by deletions of the entire NF1
gene (Williams et al. 2009
), whereas ~10–15% of familial hypercholesterolemia (Hobbs et al. 1992
) and 65% of Duchenne muscular dystrophy (Prior and Bridgeman 2005
) are due to gene deletions. Analysis of several fully sequenced human genomes indicates that the number of submicroscopic deletions is far greater than was previously recognized (Mills et al. 2011
). Thus, whereas the substantial majority of single gene disorders are attributable to single nucleotide mutations, the increasing resolution of copy number microarrays and their widespread adoption for routine cytogenetic analysis is likely to reveal a greater role for submicroscopic deletions in Mendelian conditions as well as in sporadic conditions and complex traits.