A complete listing of laboratories offering HB genetic testing that have registered with GeneTests can be found on that Web site.6 In general, testing can be divided into direct and indirect testing, and selected laboratories offer prenatal testing.
Generally, most patients with HB have mild disease. Thus, given that up to 25% of white patients with mild HB have one of three founder mutations (Gly60Ser, IIe397Thr, or Thr296Met),13 a logical first step is to perform limited testing for these founder mutations. For those HB patients in whom a founder mutation is not identified and for severe HB patients and at-risk carriers, the logical next step is screening or sequencing regions of functional significance in the F9 gene. Although the majority of HB patients have one mutation, about 1% of HB patients are compound heterozygous for two mutations.
For probands with deletion of part of their F9 gene, carrier testing by PCR amplification of individual exons will likely not be useful. Thus, either linkage analysis or Southern blot analysis is required for deletion diagnosis.
Indirect DNA Analysis (Linkage Analysis)
As discussed above for HA, indirect DNA analysis in the proband does not provide any information on the proband's genotype, but does define a haplotype that identifies the abnormal F9 gene, which is useful for carrier testing of family members. Given the relatively small size of the F9 gene, direct sequencing also is feasible for carrier testing when the HB mutation is not a large F9 gene deletion.
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