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A REVIEW OF THE ROLE OF CYTOCHROME P450 17-HYDROXYLASE/17,20 LYASE (CYP17) IN HYPOCORTISOLISM IN SOUTH AFRICAN ANGORA GOATS 

 

K. Storbeck1, M.A. Snyman2 & P. Swart1#

1Department of Biochemistry, University of Stellenbosch, Private Bag X1, Matieland, 7601

2Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900

#E-mail: P Swart

 

INTRODUCTION 

One of the largest problems still facing the South African mohair industry today is the continual loss of young, newly shorn Angora goats during cold spells. These losses have severe financial implications for the industry. Early investigations into these cold stress related deaths implicated a dysfunction in the adrenal cortex as the probable cause. In mammals, physiological stress stimulates the release of the glucocorticoid cortisol (a steroid hormone) from the adrenal cortex, which favours glucose production at the expense of glycolysis (Munch, 1971). Selection for mohair production is believed to have resulted in reduced adrenal function resulting in a condition of hypocortisolism (abnormally low levels of cortisol) (Van Rensburg, 1971). In addition to the production of the glucocorticoids (metabolism and stress), the adrenal cortex is also responsible for biosynthesis of the mineralocorticoids (regulation of water and salt balance) and the adrenal androgens (precursors to sex hormones). Through studying the Angora goat’s adrenal steroidogenic pathway, it was demonstrated that the cause of the observed hypocortisolism lies with a single steroidogenic enzyme, namely cytochrome P450 17α-hydroxylase/17,20 lyase (CYP17) (Storbeck et al., 2007; Storbeck et al., 2008; Storbeck et al., 2009).

 

Within adrenal steroidogenesis, CYP17 catalyses two distinct reactions, a 17α-hydroxylation and a 17,20 lyase reaction. The 17α-hydroxylation of the Δ5- and Δ4-steroids, pregnenolone (PREG) and progesterone (PROG), by CYP17 yields 17-hydroxypregnenolone (17-OHPREG) and 17-hydroxyprogesterone (17-OHPROG), respectively (Figure 1). The 17,20 lyase reaction of CYP17 catalyses the cleavage of the C17,20 of 17-OHPREG and 17-OHPROG to yield the androgens dehydroepiandrosterone (DHEA) and androstenedione (A4), respectively (Nakajin & Hall, 1981; Nakajin et al., 1981; Zuber et al., 1986). The enzyme 3β hydroxysteroid dehydrogenase (3βHSD) converts the 3β-hydroxy-Δ5-steroid precursors PREG, 17-OHPREG and DHEA to the corresponding Δ4 3-ketosteroids, PROG, 17-OHPROG and A4 (Thomas et al., 1989). PROG and 17-OHPROG are substrates for cytochrome P450 21-hydroxylase (CYP21), which commits PROG to aldosterone (mineralocorticoid) biosynthesis and 17-OHPROG to cortisol biosynthesis.

 

 

Figure 1. Schematic representation of the reactions catalysed by CYP17 and 3βHSD. In the goat, the 17,20 lyase reaction of CYP17 favours the cleavage of 17OH-PREG to DHEA, with only negligible cleavage of 17OH-PROG to A4 observed (Storbeck et al., 2009)

 

The dual activity of CYP17 places this enzyme at a key branch point in the synthesis of mineralocorticoids, glucocorticoids and adrenal androgens. Due to the competition between CYP17 and 3βHSD for the same substrates, the ratio and substrate specificities of these two enzymes play a critical role in determining the steroidogenic output of the adrenal cortex and may have profound physiological effects as is demonstrated in this review.

 

EARLY INVESTIGATIONS INTO COLD STESS DEATHS

Preliminary studies found that the primary cause of stock losses during cold spells was due to an energy deficiency resulting from a decrease in blood glucose levels, which causes a drop in body temperature and subnormal heart function (Wentzel et al., 1979). Fourie (1984) subsequently demonstrated that Angora goats could not cope with cold, wet and windy conditions as well as the more hardy Boer goats, even when supplementary feeding practices were employed. Fourie (1984) concluded that the Angora goat does not have the metabolic capacity to produce sufficient heat, a problem that is further compounded by the weak insulation of the short hair found in shorn goats. Cronje (1992) later demonstrated that Angora does have a lower blood glucose concentration and a slower response of glucose synthesis rate to dietary energy increments than Boer goat does, providing further evidence of the inability of the Angora goat to mobilise glucose precursors.

 

Van Rensburg (1971) identified a negative relationship between plasma cortisol levels and hair production and suggested that selection for high mohair production indirectly resulted in reduced adrenal function with resultant reduced cortisol levels. Herselman (1990) confirmed that the high hair production in Angora goats resulted in the total energy metabolism being less effective when compared to other goats, and suggested that hair production might be at the expense of other physiological functions. Herselman & Van Loggerenberg (1995) subsequently investigated cortisol production in a number of small ruminant breeds with varying potentials for fibre production. While intravenous insulin injection caused a drop in blood glucose concentration in all breeds with a resulting increase in the plasma cortisol concentration, the peak plasma cortisol concentration was three to five times lower in the Angora when compared to the other breeds. Similarly, the response in plasma cortisol levels to intravenous corticotropin releasing hormone (CRH) was three to four times lower in the Angora than in the other breeds. This study concluded that a form of hypocortisolism contributes significantly to the disorders in carbohydrate metabolism observed in the Angora.

 

IS THE ADRENAL CORTEX TO BLAME?

Following the study of Herselman & Van Loggerenberg (1995), which indicated that the South African Angora goat may suffer from a condition of hypocortisolism, the biochemistry laboratory at Stellenbosch University started investigating the Angora's adrenal steroidogenic pathway. In a comparative study, Engelbrecht et al. (2000) investigated the adrenal response of Angora goats, Boer goats and Merino sheep to insulin-induced stress, as well as adrenocorticotropic hormone (ACTH) stimulation. Insulin induced a hypoglycaemic condition in all three species, while plasma cortisol levels increased significantly in both the Boer goat and sheep, but not in the Angora (Figure 2A). This confirmed the reports of hypocortisolism by previous studies (Van Rensburg, 1971; Herselman & Pieterse, 1992; Herselman & Van Loggerenberg, 1995). ACTH stimulation resulted in an increased plasma cortisol concentration in the three species, indicating that the HPA axis was functional in all three species. The response was, however, strongest in Merino sheep and weakest in Angora goats, confirming that the adrenal gland of Angora goats may have a reduced ability to produce cortisol.

 

Engelbrecht & Swart (2000) subsequently used subcellular fractions (microsomes and mitochondria) to investigate and compare adrenal steroidogenesis in the same three species investigated above. In the microsomal preparations, PREG was used as a substrate and the production of glucocorticosteroid precursors (deoxycorticosterone and deoxycortisol) and androgens, (DHEA and A4) were compared. Significantly less glucocorticosteroid precursors (36%) were produced by the Angora goat than the Boer goat (79%) and Merino sheep (82%). In contrast, the Angora goat produced significantly more 17-OHPREG and DHEA (35%) than did the other two species (Boer goat 9%, Merino sheep 0%). Unfortunately, 17-OHPREG and DHEA was not quantified individually during this study. In the case of PROG metabolism, the Angora produced significantly more deoxycorticosterone and significantly less deoxycortisol than the other species, while A4 and 17-OHPROG production was less than 5% in all three species.

 

Figure 2. Comparative study of (A) cortisol production and (B) CYP17 activity in Angora goats, Boer goats and Merino sheep. (A: Engelbrecht et al., 2000; B: Engelbrecht & Swart, 2000).

 

The differences in steroid output by the microsomal preparations from the three species suggested that there was a difference in activity of one or more of the steroidogenic enzymes. The activity of specific enzymes in the adrenal steroidogenic pathway were subsequently studied by the selective addition of cofactors. Only a single enzyme, CYP17 demonstrated a significant difference in activity between the species. CYP17 in the Angora goat adrenal microsomes converted PREG to DHEA significantly faster than in the other two species (Figure 2B). Engelbrecht & Swart (2000) concluded that the preference exhibited by Angora CYP17 for the Δ5-steroid pathway during adrenal steroidogenesis would likely result in an increased production of adrenal androgens in vivo, resulting in a decrease in the production of glucocorticoids when compared to the other species. Further studies were therefore carried out to investigate CYP17 activity in the South African Angora goat.

 

THE ROLE OF CYP17

While pursuing CYP17 as the possible cause of hypocortisolism in the South African Angora goat, Slabbert (2003) identified two CYP17 isoforms in this species, which differed at four nucleotide positions, one of which resulted in a change in the recognition site for the restriction enzyme ACS I. Sequencing of genomic DNA confirmed the presence of two CYP17 alleles, which were named CYP17 ACS- and CYP17 ACS+. A restriction digest based genotyping method was subsequently developed and 83 goats genotyped. Twenty four goats were homozygous for CYP17 ACS- and the remaining 59 were heterozygous. No goats homozygous for CYP17 ACS + were detected (Slabbert, 2003).

 

Subsequent to this initial study the two CYP17 isoforms were successfully cloned.  The CYP17 ACS+ isoform (GenBank accession no. EF524064) was shown to be 100% homologous with Boer goat (Capra hircus) CYP17 cDNA (GenBank accession no. AF251387), while the cDNA for CYP17 ACS- (GenBank accession no. EF524063) differs by four nucleotides (Storbeck et al. 2007). The cDNA encoding Angora 3βHSD (GenBank accession no. EF524066) and cytochrome b5 (GenBank accession no. EF524066) were also successfully cloned (Storbeck et al., 2007).

 

The four nucleotide difference between CYP17 ACS- and ACS+ results in three differences in the predicted amino acid sequences of the two proteins. Two of the amino acid substitutions (G6A and I213V) are conservative and not expected to affect the three-dimensional structure of the enzyme. However, the non-conservative amino acid substitution P41L, lies in the highly conservative PR, critical for the correct folding of the cytochromes P450 (Yamazaki et al., 1993; Kusano et al., 2001a; Kusano et al., 2001b). The absence of a proline residue at this position in CYP17 ACS+ may therefore influence the folding of the enzyme, resulting in a change in the three dimensional structure and in the enzymatic activity of the protein.

 

Both CYP17 isoforms were therefore expressed in non-steroidogenic COS-1 cells and assayed for activity. The transfected COS-1 cells converted PREG to 17-OHPREG and DHEA, while PROG metabolism yielded primarily 17-OHPROG. The 17α-hydroxylase activity of both enzymes were similar, however, a marked difference was observed for the 17,20-lyase activity. The ACS- isoform demonstrated a significantly increased lyase activity towards 17OH-PREG in both the presence and absence of cytochrome b5, an allosteric activator of the lyase reaction (Miller, 2005), when compared to the ACS+ isoform. Site-directed mutagenesis revealed that the difference in 17,20-lyase activity was primarily due to the non-conservative P41L substitution (Storbeck et al., 2007).

 

It was speculated that the increased 17,20-lyase activity observed for CYP17 ACS- might be the cause of the observed hypocortisolism, as it would result in a greater flux of steroids through the Δ5-steroid pathway. This is in agreement with the results obtained earlier by Engelbrecht & Swart (2000) who showed that the Angora produced significantly less glucocorticoid precursors, but more androgens, than Boer goat and Merino sheep in microsomal preparations.

 

As CYP17 and 3βHSD compete for the same substrates within the adrenal gland, with the ratio and the substrate specificities of these two enzymes determining the steroidogenic output, the role of these enzymes on the steroid output of the Angora was investigated by coexpressing the enzymes in COS-1 cells. In comparison to ACS+, ACS- expressed in COS-1 cells, strongly favoured androgen production by the Δ5 steroid pathway as a result of its significantly enhanced 17,20-lyase activity (Figure 3). As expected, this resulted in a decrease in glucocorticoid precursor production. The inclusion of cytochrome b5 in the cotransfections resulted in an increased difference in the steroid profiles of PREG metabolism, with ACS- expressing COS-1 cells predominantly producing adrenal androgens (≈68%), while glucocorticoid precursor production was predominant in ACS+ expressing cells (≈71%). This difference in androgen production in both the presence and absence of cytochrome b5 was attributed to the greater 17,20-lyase activity of CYP17 ACS-, which as expected, resulted in the greater flux through the Δ5 pathway, with a concomitant decrease in glucocorticoid precursors (Storbeck et al., 2007).

 

Figure 3. Steroid profile of PREG (1µM) metabolism after 8 h by Angora goat CYP17 and 3βHSD coexpressed in COS-1 cells, (A) without cytochrome b5 (B) and in the presence of cytochrome b5. (Storbeck et al., 2009).

 

Interestingly Angora CYP17 ACS+, which produces significant levels of glucocorticoid precursors, shares 100% sequence identity with Boer goat CYP17, thus implicating ACS- as the primary cause of the observed hypocortisolism. However, in order to investigate this in live animals a reliable genotyping method was required.

 

GENOTYPING CYP17

Slabbert (2003) had previously developed a restriction digest based genotyping method, but was unable to detect any goats homozygous for CYP17 ACS+. At the time this was thought to be as a result of the limits of the genotyping method. For this reason a more accurate and efficient real-time PCR based genotyping method was developed. This method was subsequently used to genotype 576 Angora goats from two separate populations. Three genotypes were expected: homozygous -/-, heterozygous -/+ and homozygous +/+. While the homozygous +/+ genotype remained undetected, an interesting observation was made. Genotyping of heterozygous samples with hybridisation probes typically yields two melting peaks of similar peak area. This was the case in 42.9% of the heterozygous animals investigated in this study. However, 40.6% of the heterozygous animals consistently yielded melting profiles with unequal peak areas, where the peak representative of CYP17 ACS+ had a substantially smaller area than that of CYP17 ACS- (Figure 4). Furthermore, this pattern was consistently observed for the same samples even when tested using different DNA isolations and blood samples. As a control, 107 Boer goats were also genotyped using the same method. These animals were all heterozygous and showed no distortion in peak area. Similarly all the sheep that were genotyped as heterozygotes demonstrated no peak distortion.

 

Figure 4. Melting curves of CYP17 ACS- and ACS+. (A) Typical melting curves for the Ho and He genotypes, as well as heterozygous Merino sheep. (B) Typical peak distortion obtained for the Hu genotype shown with the He genotype for comparison.

 

Since the copy number of individual alleles has a direct influence on respective peak areas when genotyping with hybridisation probes (Lyon, 2001), it was concluded that the difference in peak areas observed may be the result of differences in CYP17 copy number (Storbeck et al., 2008). Based on the melting peak profiles, goats were subsequently divided into three genotypes, namely: homozygotes for ACS- (Ho), heterozygotes yielding unequal peak areas (Hu) and heterozygotes yielding equal peak sizes (He) (Table 1).

 

Table 1. CYP17 genotyping by real-time PCR using hybridisation probes. Goats were divided into three genotypes (Ho, Hu and He) based on the melting peak areas as shown in Figure. 4.

 

Ho

Hu

He

Total

Population 1

30 (12.88%)

93 (39.91%)

110 (47.21%)

233

Population 2

65 (18.95%)

141 (41.11%)

137 (39.94%)

343

Angora goat Totals

95 (16.49%)

234 (40.63%)

247 (42.88%)

576

F2 generation goats a

1 (1.41%)

21 (29.58%)

49 (69.01%)

71

Boer goats

0 (0%)

0 (0%)

107 (100%)

107

a F2 generation of the 75% Angora goat: 25% Boer goat line (Snyman, 2004)

 

It was suggested that the unequal peak area ratio of the Hu group might indicate a lower abundance of CYP17 ACS+ in these goats. This was therefore investigated using quantitative real-time PCR to determine the relative copy number of CYP17 for each of the three identified genotypes. The He genotype revealed a significant (P<0.05) increase in copy number of approximately 2-fold when compared to the Ho group (Figure 5). In addition, all Boer goats (all Boer goats genotyped were He, Table 1) demonstrated the same approximate 2-fold increase in copy number. The Hu genotype yielded a 1.5-fold increase when compared to Ho. This genotype was, however, not significantly different from either the Ho or He genotypes (Figure 5). Furthermore, all heterozygous sheep showed no significant change in copy number, as the two Ovine CYP17 isoforms are two alleles of the same gene (Figure. 5). This data reveals the novel finding that, in both the South African Angora goat and the Boer Goat, CYP17 ACS- and ACS+ are not two alleles of a single CYP17 gene, but instead two separate genes (Figure 6). The Ho genotype has only one CYP17 gene, namely ACS-, while the He genotype has both CYP17 genes (ACS+ and ACS-) at two different loci and therefore twice the copy number of Ho (Figure 5). Furthermore, ACS- is always present with ACS+, thus the homozygote for ACS+ is never detected. Crossing Ho and He goats would yield the intermediate genotype Hu. This genotype receives both ACS- and ACS+ from its He parent, but only ACS- from the Ho parent (Figure 6). Therefore in this genotype the ratio of ACS- : ACS+ is 2:1, which corresponds to the distortion in peak areas obtained during genotyping with hybridisation probes. This is further supported by the copy number determination, where Hu genotype yielded an approximate 1.5-fold increase when compared to Ho, but was not significantly different from either the Ho or He genotypes (Storbeck et al., 2008).

 

Figure 5. CYP17 copy number for the three Angora genotypes (Ho, Hu and He), Boer goat and heterozygous Merino sheep relative to a Ho calibrator (Storbeck et al., 2008). All Boer goats were He genotype. Only heterozygous Merino sheep were used for copy number determinations.

 

Figure 6. Schematic representation of a cross between the Ho and He genotypes yielding the Hu genotype. The difference in copy number shown in Figure 5 is clearly demonstrated in this schematic. Both ACS- and ACS+ are shown on the same chromosome in order to simplify the schematic, though the genes are yet to be mapped (Storbeck et al., 2008).

 

The observation that all Boer goats, but not all Angoras genotyped to date are He suggests that this genotype originated in the Boer goat and not the Angora. Although the origins of the Boer goat are vague, it is commonly accepted that this breed was developed by farmers in South Africa from indigenous African goats from as early as 1800, with the emergence of a distinct breed by the beginning of the 20th century (Casey & Van Niekerk, 1988). It is feasible that a gene duplication, resulting from non-homologous recombination, occurred during the development of this sub species (Ohno, 1970). The Angora goat was first imported into South Africa from Turkey in 1838. Many of the imported Angoras were crossed with the native goats during the early development of the mohair industry in South Africa. Some pure Angoras were, however, maintained for stud purposes (Hayes, 1882). The Boer goat fits the description of the native goats used in these breeding practices, albeit in the early stage of its development (Hayes, 1882). It is therefore suggested that it was these early breeding practices that led to the introduction of the second CYP17 gene (ACS+) into the South African Angora population.

 

Recently, a breeding program was carried out, where South African Angora goats were crossed with Boer goats in order to establish a more hardy mohair producing goat with a relatively high reproductive ability and good carcass characteristics (Snyman, 2004). Crossbred does (50% Angora goat : 50% Boer goat) were mated to Angora bucks in order to obtain 75% Angora goat : 25% Boer goat progeny. These were then mated to each other to establish a 75% Angora goat: 25% Boer goat line. A number of F2 generation goats of these line have subsequently been genotyped for this study and the results confirmed that crosses with Boer goats significantly increased the frequency of the He genotype in the Angora population, while decreasing the Ho and Hu genotypes as expected (Table 1).

 

 

IN VIVO STUDY


Subsequent to the identification of the three genotypes in the South African Angora goat population, the question arose whether the genotypes differed in their ability to cope with cold stress. The results obtained in the in vitro studies discussed earlier demonstrated that ACS- isoform had a significantly increased 17,20-lyase activity, which would likely favour androgen production via the Δ5 pathway at the expense of glucocorticoid synthesis. The in vivo ability of each CYP17 genotype to produce cortisol in response to intravenous insulin injection was therefore investigated. Insulin injection induces a state of hypoglycemia, which activates the HPA axis (Plotsky et al., 1985; Suda et al., 1992). No significant difference in the basal cortisol levels for the three genotypes were observed prior to injection and the three groups all demonstrated a similar decrease in plasma glucose concentration post injection. Similarly, the three groups all demonstrated an increase in plasma cortisol in response to the induced hypoglycemic state (Figure 7). However, the amplitude of the response was significantly greater in the He (P<0.05) group than in the Ho group.

 

Figure 7. Plasma cortisol levels in the three Angora genotypes (n=10 per group) following intravenous insulin injection (Storbeck et al., 2007).

 

After 120 minutes, the mean plasma cortisol concentration (155.5 ± 66.8 nmol/L) of the He group was 1.4-fold greater than that of the Ho group (114.6 ± 42.1 nmol/L). The cortisol response in the Hu group was not significantly different from either the Ho or the He group, with the mean plasma cortisol levels (134.6 nmol/L) lying between the values of the Ho and He group 120 minutes post injection (Storbeck et al., 2008).

 

This data correlates well with the in vitro studies, and confirms the greater capacity of the ACS+ gene to produce glucocorticoid precursors, as both genotypes containing this gene produced more cortisol than the Ho genotype in which this gene is absent. Furthermore, the He genotype, which has two copies of ACS+, produced more cortisol than the Hu, which has only one copy of ACS+. However, the relative expression levels of CYP17 in the adrenal gland of the different genotypes have yet to be determined. Johansson et al. (1993) previously demonstrated that CYP2D6 gene duplication results in an increased metabolic capacity for drugs such as debrisoquine. The influence of copy number can therefore not be ignored and may be a contributing factor towards the increased cortisol production in He and Hu goats.

 

PRODUCTION DATA

The discussion presented thus far provides strong evidence that the CYP17 ACS- gene is responsible for the observed hypocortisolism in the South African Angora goat. Genotyping for this gene may therefore be a useful tool for farmers to employ when selecting rams to breed with in order to reduce stock losses due to cold stress. However, before this can be employed in the industry, the possible negative effects of selecting for better cortisol producing goats need to be considered.  Previously Van Rensburg (1971) suggested that the low level of adrenal function in the Angora may be advantageous for hair production, as it would remove the inhibitory effects of cortisol. Furthermore, Herselman & Pieterse (1992) demonstrated that regular intravenous injection of cortisol in Angora goats significantly reduced the greasy fleece weight produced and concurrently resulted in a greater internal fat weight and back fat depth. Production data were therefore gathered from fine hair goats kept at the Jansenville Experimental Station born between 2000 and 2008 and are summarised in Table 2. Statistical analysis revealed that there is no significant difference in the production data among the genotypes. Therefore, selection for more hardy goats based on the CYP17 genotype should not adversely affect the quality or quantity of mohair produced.

 

In addition to its importance in adrenal steroidogenesis, CYP17 is also vital to the production of sex steroids by the gonads (Payne & Hales, 2004). It is therefore possible that the differences in the activity of the CYP17 isoforms may affect the steroid output by the gonads. As these hormones play a critical role in determining the reproductive characteristics of the animal, the effect of the CYP17 genes on reproductive fitness will be investigated in a future study.   

 

 

Table 2. Production data for Jansenville fine hair goats collected from 2000 to 2008

Trait

Genotype

Ho

(n=62)

Hu

(n=119)

He

(n=117)

Body weight (kg)

2nd shearing

17.5 ± 0.4

17.7 ± 0.3

17.2 ± 0.3

3rd shearing

19.9 ± 0.6

20.5 ± 0.4

20.1 ± 0.4

4th shearing

24.0 ± 0.7

23.9 ± 0.5

22.8 ± 0.4

Raw fleece weight (kg)

2nd shearing

0.84 ± 0.02

0.91 ± 0.02

0.88 ± 0.02

3rd shearing

0.68 ± 0.02

0.68 ± 0.02

0.71 ± 0.02

4th shearing

1.03 ± 0.02

1.05 ± 0.02

1.07 ± 0.02

Staple length (cm)

2nd shearing

11.6 ± 0.3

12.2 a ± 0.2

11.6 a ± 0.2

3rd shearing

10.2 ± 0.3

10.4 ± 0.2

10.3 ± 0.1

4th shearing

9.0 ± 0.2

9.2 ± 0.1

9.0 ± 0.2

Clean yield (%)

2nd shearing

75.5 ± 0.6

76.0 ± 0.4

74.5 ± 0.4

3rd shearing

73.8 ± 0.5

74.1 ± 0.4

75.3 ± 0.4

4th shearing

76.0 ± 0.7

76.8 ± 0.5

76.6 ± 0.5

Clean fleece weight (kg)

2nd shearing

0.64 ± 0.02

0.69 ± 0.02

0.66 ± 0.02

3rd shearing

0.50 ± 0.01

0.51 ± 0.01

0.54 ± 0.01

4th shearing

0.78 ± 0.02

0.80 ± 0.02

0.82 ± 0.02

Fibre diameter (µm)

2nd shearing

23.2 ± 0.3

23.7 ± 0.2

23.5 ± 0.2

3rd shearing

23.0 ± 0.3

23.5 ± 0.2

23.7 ± 0.2

4th shearing

26.8 ± 0.4

26.9 ± 0.5

27.8 ± 0.3

a Values with the same superscripts differ significantly (P<0.01) among genotypes.

 

CONCLUSION

The results presented in this review have clearly implicated CYP17 as the primary cause of hypocortisolism in the South African Angora goat. A real-time PCR based genotyping method has been developed to distinguish between the three identified genotypes: Ho, Hu and He. Goats from the He genotype have been shown to produce more cortisol in vivo, while still producing the same quantity and quality of mohair as the other genotypes. The CYP17 gene is therefore a strong candidate to be used by farmers to aid in the selection of more hardy goats.

 

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Grootfontein Agric 11 (2): 24-37